PHASMATODAE
Stick Insect Phasmida
Stick insects are among the best camouflaged of all creatures, with a body shape that mimics the
branches of their home.
Photograph by Robert Sisson
http://animals.nationalgeographic.com/animals/bugs/stick-insect/
Seksloze wandelende takken gaan al een miljoen
jaar mee
Sommige soorten onder de wandelende takken bestaan al meer dan één miljoen jaar zonder
seksuele voortplanting, zo blijkt uit Canadees onderzoek. Het is bekend dat sommige soorten
wandelende takken van het geslacht Timema zich voortplanten zonder enige vorm van seks.
Deze insecten, die leven aan de Amerikaanse Westkust, produceren genetische klonen van
zichzelf.
Wetenschappers uit Canada probeerden aan de hand van de mutatiesnelheid van de genen te
achterhalen hoe lang deze soorten reeds een seksloos bestaan leiden. Uit dat onderzoek is nu
gebleken dat met name de Timema tahoe en de Timema genevieva al minstens één miljoen
jaar overleven zonder seks.
"Ons onderzoek levert nieuw bewijs dat aseksualiteit niet noodzakelijk resulteert in het snelle
verdwijnen van soorten", zo verklaarde vorser Tanja Schwander van de Simon Fraser
University in Canada aan de BBC.
In het verleden ging men ervan uit dat soorten met een aseksuele reproductie geen lang leven
was beschoren. Ze planten zich weliswaar sneller voort, maar zouden evolutionair op een
dood spoor zitten. Ze kunnen zich immers minder aanpassen aan veranderende
leefomstandigheden.
De Timema zijn de oudste insecten waarvan bewijzen bestaan dat ze voor zeer lange periodes
seksloos kunnen overleven. Hoe ze dat precies flikken, ondanks die darwinistische
onheilstijdingen, moet volgens Schwander verder onderzocht worden. (belga/jv)
20/07/11
http://en.wikipedia.org/wiki/Timema
http://bugguide.net/node/view/14905
Order Phasmatodea
Family Timematidae
Genus Timema
Species unknown.
Meestervermommer uit ver verleden
http://www.geo.uu.nl/ngv/geonieuws/geonieuwsart.php?artikelnr=772
In Eocene afzettingen is een prachtig bewaard gebleven meestervermommer gevonden. Het betreft
een fossiel insect dat tot de wandelende bladeren (Phyliinae) moet worden gerekend. Deze groep
behoort samen met de wandelende takken tot de orde van de Phasmatodea, waarvan recent zo’n
3000 soorten bekend zijn, vooral uit de tropen en subtropen. Wandelende bladeren beschermen zich
tegen hun vijanden door hun gelijkenis met bladeren. Niet voor niets heeft een van de bekendste
recente geslachten dan ook de naam Phyllium (= blad). Het nu gevonden exemplaar van 47 miljoen
jaar oud heeft met dit geslacht betrekkelijk grote gelijkenis, en is daarom Eophyllium (= vroeg blad)
genoemd. Zijn soortnaam, messelensis, is gegeven naar de vindplaats, de groeve Messel in de
nabijheid van de Duitse plaats Darmstadt.
Eophyllium
Photo (A) and line drawing (B) of holotype of fossil leaf insect E. messelensis gen. et sp. nov. from the
Eocene Messel Pit, Germany (MeI 12560). a3–a10, abdominal segments 3–10; ant, antennae; cer,
cerci; fl, foreleg; fw, forewing; hl, hindleg; hw, hindwing; int, intestinal tract; ml, midleg; vom, vomer.
E. messelensis gen. et sp. nov. in evolutionary and biogeographical context. (A) Simplified cladogram
with a partial geochronologic scale showing the phylogenetic position of E. messelensis and the
temporal sequence of character evolution. Oldest fossil records of determined adult representatives of
Timematodea and Euphasmatodea are depicted. M, Messel fossil site; B, Baltic Amber. Dating of
splitting events of crown-group Phasmatodea is unknown. Euphasmatodea represent an unknown
number of lineages. Figures are not to scale. (B) Distribution of extant and fossil leaf insects.
http://scienceblogs.com/pharyngula/2007/01/eophyllium_messelensis.php
Het fossiele exemplaar van Eophyllium
messelensis
(foto Georg Oleschinski).
Phyllium celebicum, een recente verwant van het
fossiele wandelende blad (foto Georg
Oleschinski)
Het fossiel dat nu beschreven is, en dat in 2005 werd gevonden, is zeer goed bewaarde gebleven. Het
is, samen met soortgelijke fossielen die eerder in dezelfde groeve werden gevonden, veruit het oudste
fossiel van een dier dat door zogeheten mimicry (de aanname in kleur en vorm van de natuurlijke
omgeving) zijn overlevingskansen vergrootte. Dat was kennelijk een uitstekende tactiek, gezien het
feit dat nazaten van tientallen miljoenen later deze tactiek nog steeds (en met succes) toepassen.
Tot de tactiek behoort uiteraard ook dat deze dieren zich niet door hun beweging onderscheiden van
hun omgeving. Overdag blijven ze daarom bewegingsloos zitten; pas ‘s nachts worden ze actief.
Wanneer ze overdag of “s nachts worden gestoord, bewegen ze zich op een manier die op de
beweging van een blad in de wind lijkt. Ook dat maakt (en maakte) het moeilijk voor hun belagers om
hen als prooi te herkennen. De belagers van Eophyllium messelensis moeten, zoals uit de fossiele
fauna daar blijkt, vooral hebben bestaan uit vogels, primitieve primaten en vleermuizen. Op z’n minst
een belangrijk deel van die jagers moet bij de jacht op zijn ogen hebben vertrouwd; anders had
mimicry immers geen zin.
De omgeving van groeve Messel nabij Darmstadt
Bij de recente wandelende bladeren zien mannetjes en vrouwtjes er heel verschillend uit, waarbij de
mimicry bij de vrouwtjes nog beter is ontwikkeld dan bij de mannetjes. Daaruit zou kunnen worden
opgemaakt dat het nu beschreven exemplaar, ondanks zijn duidelijk verbrede lijf, een mannetje moet
zijn. De onderzoekers sluiten overigens niet uit dat er in het verleden ook vrouwtjesexemplaren zijn
gevonden, die door de vinders als fossiele bladeren zijn beschouwd.
De vondst heeft ook paleobiogeografische consequenties, want het verbreidingsgebied van de
wandelende bladeren moet in het Eoceen veel groter zijn geweest dan nu. Van wandelende bladeren
zijn recent 37 soorten bekend, die alle in zuidoost-Aziê en de direct aangrenzende gebieden leven.
Referenties:

Wedmann, S., Bradler, S. & Rust, J., 2007. The first fossil leaf insect: 47 million years of
specialized cryptic morphology and behavior. Proceedings of the National Academy of
Sciences of the United States of America 104, p. 565-569.
http://fossilinsects.net/pdfs/wedmann_etal_2007_PNAS_FirstLeafInsect47My.pdf
Foto’s welwillend ter beschikking gesteld door Sonja Wedmann, Institut für Paläontologie,
Rheinische Friedrich-Wilhelms-Universität, Bonn (Duitsland).
http://www.physorg.com/news90157844.html
http://discovermagazine.com/2008/jan/first-fossil-of-a-leaf-insectfound/?searchterm=2007%20paleontology
http://www.wandelendetakken.be/specials/Eophyllium_messelensis.htm
http://en.wikipedia.org/wiki/Eophyllium_messelensis
Eophyllium messelensis
Een wandelend blad van 47 miljoen jaar oud.
Fossielen van wandelende bladeren en wandelende takken zijn extreem zeldzaam. In de Messel groeve nabij de
Duitse stad Darmstadt heeft men bij opgravingen in 2005 een perfect bewaard fossiel gevonden van de
Eophyllium messelensis.
Eophyllium messelensis dankt zijn naam aan de tijd waarin het gevonden is (Eoceen), de genus (Phyllium) en
aan de vindplaats (Messel).
Mogelijk heeft men in het verleden wel eens fossielen van wandelende bladeren gemist omdat men
veronderstelde dat het fossiele bladeren waren.
Eerdere fossiele vondsten van de wandelende takken zoals de Bacilllus en de Leptynia worden gedateerd op 23
miljoen jaar oud waarmee de huidige vondst werkelijk een missing link is voor deze Phyllium groep.
Mogelijk zijn de eerste wandelende bladeren ontstaan zo’n 125-90 miljoen jaar geleden wanneer de roofdieren
hun visuele kenmerken begonnen te gebruiken om te jagen.
Het gevonden fossiel betreft een mannelijk wandelend blad dat opmerkelijke vergelijkingen vertoont met de nog
levende wandelende bladeren zoals de Phyllium celebicum.
Deze vondst toont aan dat de zogeheten mimicry (de aanpassing in kleur en vorm van de natuurlijke omgeving)
zijn overlevingskansen vergrootte.
Deze tactiek blijkt zeer doeltreffend te zijn gezien het feit dat de nu levende wandelende bladeren, zo’n 47
miljoen jaar later, deze tactiek nog steeds toepassen.
De belagers van Eophyllium messelensis moeten, zoals uit de fossiele fauna die ook in de buurt gevonden zijn,
vooral hebben bestaan uit vogels, primitieve primaten (oa apen) en vleermuizen, die voornamelijk jagen
met hun zicht waardoor de mimicry tot zijn recht komt. De tactiek past in het hele plaatje: zowel kleur als vorm
zijn aangepast aan de omgeving, maar ook de dieren zelf blijven overdag onbeweeglijk en wachten op de
nacht en wat wind om zich te verplaatsen en te eten.
Uit deze vondst blijkt ook dat het verspreidingsgebied van de Phylliinae veel groter was in het eoceen dan nu.
Dit komt voornamelijk omdat de temperatuur miljoenen jaren geleden veel hoger was dan nu en de
voedselplanten voor de wandelende bladeren ook hier te vinden waren.
De meeste wandelende bladeren zijn nu enkel te vinden in zuidoost-Azië en de aangrenzende gebieden. Eerdere
fossiele vondsten van de wandelende takken zoals de Bacilllus en de Leptynia
Het gevonden fossiel bevindt zich nu in het Senckenberg museum in Frankfurt-am-Main in Duitsland.
Foto’s ter beschikking gesteld door Sonja Wedmann, Institut für Paläontologie, Rheinische Friedrich-WilhelmsUniversität, Bonn
Bron: www.pnas.org
en het artikel dat door Sonja Wedmann ter beschikking gesteld werd.
cladogram. Phasmatodea ;
;het fossiel is Europees ;er zijn geen extante inheemse Europese phasmida (bekend )
Wandelende tak kreeg vleugels terug/Weg is niet
helemaal weg
Jacqueline de Vree
http://noorderlicht.vpro.nl/artikelen/10268384/
zie ook -->
When Parsimony Goes Wrong: The Wings of Stick Insects
The stick insect Sipyloidea sipylus opening its wings. Photo by Drägüs.
Timema dorotheae, a member of the basalmost genus of Phasmatodea. Photo by David Maddison.
One of the most useful features in characterising insect wings is their venation. A generalised diagram
of insect wing venation is given above, but different orders of insects have significantly different wing
venation, enough so that relationships can be recognised for fossil insects known from wings alone.
Comparisons between wing venation of different orders can also be very useful in establishing their
relationships.
http://catalogue-of-organisms.blogspot.com/2008/12/when-parsimony-goes-wrong-wings-of.html
Links



Webpagina's van het laboratorium van Michael Whiting, Brigham Young Universiteit in
Provo, Utah
Webpagina's over wandelende takken en bladeren van het Australian Museum Online
Takkezooi: Nederlandstalige website over het houden en verzorgen van wandelende
takken. Mogelijkheid om eitjes te bestellen van verschillende soorten.
Tenminste vier keer in de evolutie hebben wandelende takken het vermogen om te vliegen
verworven. En voor een groot deel weer verloren, overigens. Een dergelijke herhaling van de
evolutie was geheel in strijd met sommige 20 eeuwse opvattingen binnen de evolutietheorie.(1)
Weg is weg.
Eenmaal uit de stamlijn verdwenen eigenschappen keren nooit meer terug. Dit principe staat bekend
als WET VAN DOLLO , begin vorige eeuw opgesteld door de Belgische bioloog en dinosaurusjager
Louis Dollo.
Zo krijgen kippen nooit meer de tanden terug die ze in hun evolutie van dinosaurus tot pluimvee
hebben verloren, en krijgen mensen nooit meer een staart, ons rudimentaire staartbeentje ten spijt.
Michael Whiting met een van de 37 verschillende soorten wandelende takken ,gebruikt in zijn
onderzoek
FIGURE 1. Examples of wing features in stick insects, a, Example of a fully winged
(macropterous) female phasmid (Phasma gigas) with enlarged hindwings and thickened
forewings.
b, Wing venation of male Phyllium celebicum with major veins labelled, demonstrating homology with
other insect wing veins. A, anal vein; C, costa vein; Cu, cubitus vein; M, medial vein; R, radius vein;
Rs, radial sector vein; Sc, subcosta vein. c, Example of a partially winged (brachypterous) female
phasmid (Extatosoma popa) with reduced hindwings. d, Example of a wingless (apterous) female
phasmid (Leprocaulinus sp.) with wings entirely absent.
Onderzoek van de Amerikaanse bioloog/entomoloog Michael Whiting, verbonden aan de Brigham
Young Universiteit in Provo, Utah, tart deze ‘use it or lose it’-(2)opvatting in de hedendaagse
biologie.
Tenminste vier keer in de evolutie hebben wandelende takken terug functionele vleugels ontwikkeld ,
zo schrijft hij in een artikel in het tijdschrift Nature.
<="" a="">
http://www.newscientist.com/news/news.jsp?id=ns99993269
The graphic is only phasmid (ingroup) taxa.
Since Whiting's hypothesis is that the basal phasmid was wingless the basal node carries the
wingless state.
But the ancestor to the Phasmida is winged (not depicted in the graphic) so the phasmids lost
flight (synapomorphy for phasmida on
Whiting tree), then regained it four times(red =wing gain ), with two reversals(green=wing loss .)
This partial graphic only shows four separate origins of wings and two subsequent losses but no
regaining of wings after they were lost
Here is are the complete graphics from the article =
Character mapping of wing types on phasmid phylogeny
Parsimony optimization (ACCTRAN) of winged (blue) and wingless (red) states for male
phasmids on the optimization alignment topology. This reconstruction requires seven steps
with four wing gains and three losses; DELTRAN optimization requires five wing gains and two
losses. Maximum likelihood reconstruction produces similar results (see Supplementary
Information).
Phylogeny of Phasmatodea on the basis of molecular data.
Shown is the single optimization alignment tree based on 18S rDNA, 28S rDNA and histone 3.
Nonparametric bootstrap supports are given above each node and partitioned Bremer
supports are below each node in the order 18S/28S/H3. This topology is congruent with the
maximum likelihood and bayesian topology (see Supplementary Information). Boxes at the end
of nodes represent wing character states for males and females, respectively. n. sp., new
species; spec. indet., undetermined species.
Supplementary Figure 1: This is the ingroup portion of the Bayesian tree, with posterior
probabilities given above nodes, and the probability that the ancestral state was wingless
given below the node. This reconstruction requires 5 independent wing gains and 2 wing
losses.
- Download PDF file (61 KB)
Supplementary Information: This file provides additional details concerning taxon selection,
optimization alignment methodology, incongruence length difference metrics, parsimony tree
reconstruction, likelihood tree reconstruction, Bayesian analysis, congruence of molecular
results with known morphological characters, parsimony character mapping, and likelihood
character mapping.
- Download Word file (168 KB)
Nooit eerder is een dergelijk staaltje re-evolutie aangetoond, reden voor het tijdschrift om Whitings
onderzoek op de cover te zetten.
Whiting was er helemaal niet op uit de evolutietheorie de duimschroeven aan te draaien. Hij wilde een
moleculaire phylogenetische stamboom maken van de Phasmida ( Phasmatodea / Phasmatodea
)gebaseerd op vergelijkend DNA onderzoek tussen de wandelende takken /en de daaraan verwante
wandelende bladeren/ Phylliidae , want dat was nog niet eerder gedaan. Voor zijn onderzoek reisde
hij de hele wereld af op zoek naar nieuwe exemplaren.
Op basis van hun DNA stelde Whiting een evolutionaire stamboom op van 37( voornamelijk soorten
uit Nieuw guinea ) verschillende extante soorten Phasmiden.
Uit het DNA-onderzoek concludeerde Whiting dat de eerste wandelende tak -Timema knulli - 275
miljoen jaar geleden is ontstaan( volgens de berekeningen met de moleculaire klok ) .
Deze " oudste/primitiefste " wandelende tak had tot zijn verbazing geen vleugels. Veertig procent van
de wandelende takken gaat gevleugeld door het leven, en de verwachting was dat de oer-tak dus ook
vleugels zou moeten hebben. (3)
Vijftig tot honderd miljoen jaar later doken er in Whitings vertakkende stamboom wandelende takken
op met vleugels.
En enige tijd later weer. ..En weer... En weer.
Kan niet, volgens de evolutionaire opvattingen. Als een complexe eigenschap eenmaal uit een
soort is verdwenen, muteren de erbij horende genen in rap tempo, (4) zodat die eigenschap
nooit meer terug kan komen. Weg is weg.
Kan wél, zeg Whiting.
“Deze insekten zijn miljoenen jaren vleugelloos door het leven gegaan. Maar de genetische
blauwdruk ervoor bleef behouden.” (A)
Whiting opperde dat de genetische instructies voor het maken van vleugels ook nauw betrokken zijn
aan die voor het aanmaken van ledematen zoals de poten . Iets dergelijks is bijvoorbeeld aangetoond
bij het fruitvliegje – Drosophila melanogaster.
Het is daarom niet verwonderlijk dat de samenwerkende set genen (5)grotendeels ongeschonden
(6)miljoenen jaren evolutie heeft doorstaan, schrijft Whiting in Nature.
De terugkeer van een verloren(7) gewaande eigenschap zou wel eens een grotere rol kunnen
spelen in de evolutie dan tot nu toe werd aangenomen.
Whiting vermoed dat iets dergelijks ook het geval is bij kakkerlakken, en bij halfvleugeligen zoals
wantsen en schaatsenrijders.
Michael F. Whiting et. al.: Loss and recovery of wings in stick insects. In: Nature, vol. 421, p.
264 (16 januari 2003).
http://www.tungate.com/whiting.htm
http://www.nature.com/nature/journal/v421/n6920/extref/nature01313-s2.doc
http://www.ncbi.nlm.nih.gov/pubmed/1252964
(A)
Poten regeneratie maakt kreupele vleugels
http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1634783
http://dbs.umt.edu/research_labs/emlenlab/Lab/Maginnis/Maginnis%20Royal%20Society%202006.pdf
Overigens verliezen veel insekten tijdens hun levensloop( of als vertegenwoordigers van een kaste )
hun ( dure ) vleugels --->mieren , bladluizen
De( HOX ? ) genen die de vleugelontwikkeling aansturen zijn waarschijnlijk gemakkelijk "aan "of
"uitzetbaar " ( sociohormonen ? honger , vitamine-tekorten voedingstoffen /spoorelementen schaarste
e.d .
Links :
classificatie
http://tol.tolweb.org/Euphasmida
http://zipcodezoo.com/Key/Phasmida_Order.asp
http://www.newscientist.com/news/news.jsp?id=ns99993269
http://phasmida.speciesfile.org/.
Noten
(1)dat werd en wordt , dankbaar aangegrepen door allerlei creationisten
http://www.evcforum.net/dm.php?control=msg&t=9438
Ondertussen heeft EVC deze ( ondertussen volledig aanvaarde ) kennis over de evolutie van
wandelende takken -vleugels opgenomen in haar archief en geplaats in een bredere evolutiecontext in een afzonderlijk - educatief / documentatie- artikel
http://razd.evcforum.net/WingWalkingstick.htm
Zie verder over de spartelingen van die GUTOB creationist ---->
http://www.volkskrantblog.nl/blog/5720 ( zie de reacties van PeterMudde )
http://www.volkskrantblog.nl/bericht/243296
(creationist ) PB:
"De geinformeerde bioloog weet dat re-evolutie van vleugels in phasmieden (wandelden
takken) Darwin's theorie volledig falsifieerde."
Peter mudde
Een( oudbollig ) stukje (door Darwin-volgelingen ) =
" Darwin's theorie"(?) of bedoelt is misschien veeleer het idee dat als een complex ontwerp
verdwijnt, het niet weer( weggelaten door de meeste creationisten = de moderne nuancering =
op dezelfde manier ) verschijnt ? .(= De wet van Dollo )
De idee "eenmaal ( een bepaalde eigenschap ) weg , altijd weg " dateert van voordat de kennis
van genetica het huidige nivo bereikte.
Nu weten we, dat de aanzet tot de ontwikkeling van iets als vleugels uitgeschakeld kan worden, maar
dat de rest van het programma niettemin aanwezig kan blijven.
Als het op de een of andere manier weer aangezet kan worden krijg je de situatie zoals bij de
Phasmida. Daar is niks antidarwinistisch aan...
ATAVISME <
(2) ....het principe is afkomstig van JB de Lamarck
(3)Volgens de gebruikelijke consensus onder entomologen is het bezit van vleugels een basale
en noodzakelijke eigenschap voor het fittere / gemakkelijkere overleven van de orthoptera ...
Het stelt deze dieren in staat gemakkelijker te ontsnappen aan predatoren , partners op te
zoekn en nieuwe (tijdelijke)voedselbronnen(--> bladluizen levenscyclus/ galwespen ) aan te
boren De voorvaderlijke aftakkingen die leide tot de phasmida wordt verondersteld een
gevleugeld insect te zijn ....
Uitgaande van Whiting & all kan men dus nog steeds de mogelijkheid open houden dat het
eerste voorouderlijke lid van de phasmida( niet noodzakelijk T.Knulli ) inderdaad vleugels bezat
... Waarbij T.knulli de meest primitieve extante soort is ( die de vleugels al heeft kwijt gespeeld )
zie ook( A )
The Phylogeny of the Extant Hexapod Orders
http://research.amnh.org/scicomp/pdfs/wheeler/Wheeler_etal2001.pdf
(4) Genetische erosie : genen waar geen selectiedruk op staat verdwijnen uit de genenpool van
de populatie (en de vertikale verspreiding binnen de stamlijn )
(5) HOX genen /HOX
GENES LINKS <
(6) staat er geen selectie druk(meer) op de vleugels ( =ze ontbrekenof zijn vestigaal ) dan blijft
er nog wel conserverende en stabiliserende druk bestaan op poten -bezit
(7) " uitgeschakeld "( bijvoorbeeld door een back mutation mechanisme) is niet hetzelfde als
daadwerkelijk verlies (= degeneratie ) aan genetische info
UPDATES :
* University of California - San Diego (2006, January 24). UCSD Biologists Find New
Evidence For One-way Evolution. ScienceDaily. Retrieved January 4, 2010, from
http://www.sciencedaily.com/releases/2006/01/060123172341.htm
Adapted from materials provided by University of California - San Diego.
Neutrale mutaties beletten dat evolutie in eiwit
omkeert
September 2009
Gedane zaken nemen geen keer. Dat geldt ook voor de geschiedenis van het leven op aarde.
Voor het eerst is op moleculair niveau bewijs gevonden dat het evolutieproces (op moleculair
niveau ) onomkeerbaar is. Het onderzoek staat vandaag in Nature.
Evolutie zorgt ervoor dat de biologische diversiteit voortdurend toeneemt. Naarmate de miljoenen
jaren verstrijken, ontstaan er nieuwe soorten en gaan organismen steeds minder op elkaar lijken.
Die trend gaat er onder meer vanuit dat de tijd niet kan worden omgedraaid. Maar geldt dit ook voor
het evolutieproces?
Kan bij sommige organismen de evolutie omkeren – door een nieuwe ijstijd, bijvoorbeeld – waardoor
een dier of plant opeens weer in een vroeger evolutiestadium terechtkomt?
Amerikaanse wetenschappers van de University of Oregon hebben ontdekt dat de(moleculaire) basis
voor zo’n omgekeerde evolutie(minstens in dit speciale geval/onderzoek ) ontbreekt. Ze vonden dat
mutaties aan een eiwit een welbepaalde functie geven, niet zomaar kunnen worden ‘uitgezet’.
Daarvoor zitten andere mutaties, die ontsnappen aan natuurlijke selectie, onvermijdelijk in de weg.
De onderzoekers keken naar het receptoreiwit GR dat in de cellen van gewervelde dieren het
hormoon cortisol bindt. Dit eiwit stamt af van een oereiwit dat 400 miljoen jaar geleden behalve cortisol
ook het hormoon aldosteron kon binden. Het team kon eerder al de zeven mutaties identificeren die
samen voor de cortisolvoorkeur hebben gezorgd. Door deze zeven uit te schakelen, wilden ze GR
weer in haar vroegere evolutionaire stadium brengen. Het resultaat was verrassend. In de plaats van
een overactief eiwit dat zowel cortisol als aldosteron kon binden, zagen ze een inactief, dood eiwit.
Blijkbaar hadden andere, nog onbekende en oudere mutaties een succesvolle omkering van de
evolutie tegengehouden. Ondanks genetische manipulatie kon de oerfunctie van GR niet meer
worden aangezet.
De wetenschappers bekeken de atoomstructuur van de gemanipuleerde oereiwitten met behulp van
de röntgenstralen van een deeltjesversneller in Chicago. Ze vonden vijf willekeurige mutaties die op
een of andere manier samen als een ratelmechanisme werken, zodat het evolutieproces niet kan
worden opgekeerd.
‘Stel dat je je bed verzet, en op de vrijgekomen plaats zet je vervolgens je kleerkast. Als je dan
je bed terug zou willen zetten, zal je dus eerst ook je kast weer moeten verzetten’, legt Joe
Thornton, hoofdauteur van de studie, metaforisch uit.
‘Pas wanneer we deze vijf blokkerende mutaties hadden uitgeschakeld, kon de evolutie worden
omgedraaid en kon GR behalve cortisol ook aldosteron binden.’
Maar omdat de vijf ontdekte mutaties geen of weinig effect op de werking van het eiwit hebben, is dit
in de natuur uitgesloten.
‘De natuurlijke selectie heeft er geen vat op. Als de oude eiwitfunctie – mét aldosteronbinding –
door veranderende omstandigheden opnieuw de meest optimale zou worden, dan nog kan de
natuur niet om deze vijf mutaties heen.’
*Een uitgebreide heldere uiteenzetting komt zoals gewoonlijk van
http://www.nytimes.com/2009/09/29/science/29evol.html?_r=1
Carl Zimmer
Van belang is
1.- Het begin van het artikel:
"The Belgian biologist Louis Dollo was the first scientist to ponder reverse evolution.
“An organism never returns to its former state,” he declared in 1905, a statement later dubbed Dollo’s
law.
To see if he was right, biologists have reconstructed evolutionary history.
In 2003, for example, a team of scientists studied wings (on the many species )of stick insects.
They found that the insects’ common ancestor had wings, but some of its descendants lost them.
Later, some of those flightless insects evolved wings again.
Yet this study did not necessarily refute Dollo’s law.
The stick insects may indeed have evolved a new set of wings,
but it is not clear whether this change appeared as reverse evolution at the MOLECULAR level.
= Did the insects go back to the EXACT original biochemistry for building wings, or find a NEW
route, essentially evolving new proteins?
Daarbij moet dus worden onderstreept dat het in het onderzoek van Thornton e.all ,gaat om
"teruggedraaide "evolutie op moleculair niveau van een ontwikkelings- stamlijn van een
welbepaaldde functie(s)van een eiwit
Daaruit sensationele "conclusies " trekken zoals de "mogelijke " evolutie van , bijvoorbeeld ,
"walvissen die terug kieuwen gaan ontwikkelen" is eigenlijk journalisten -onzin die kan leiden tot
stroman-constructies die gemakkelijk passen in de trukendoos van creationisten ...
2.- Maar belangrijker nog is de laatste paragraaf :
"....For now it is an open question whether other proteins have an equally hard time evolving
backward.
But Dr. Thornton suspects they do.
“I would never say evolution is never reversible,” Dr. Thornton said.
But he thinks it can only go backward when the evolution of the trait is simple, like when a single
mutation is involved.
When new traits are produced by several mutations that influence one another, he argues, that
complexity shuts off reverse evolution.
“We know that kind of complexity is very common,” he said.(*zie noot1)
If this molecular Dollo’s law holds up, Dr. Thornton believes it says something important about the
course of evolutionary history.
Natural selection can achieve many things, but it is hemmed in.
Even harmless, random mutations can block its path.
“The biology (= evolutionary outcome) we ended up with ( in this study )was not inevitable,” he said.
“It was just one roll of the evolutionary dice.”
Noot 1 ;
Bijvoorbeeld :
Ook de E.Colli stam in de experimenten van Lenski, die uiteindelijk citraat+ kon doorlaten door
zijn moleculaire membraamporieen,gebruikte een opeenvolgende reeks mutaties om die
nieuwe eigenschap te verwerven
(Pierra) Het is algemeen bekend dat evolutie voortkomt uit meerdere (opeenvolgende) mutaties. In
het lab worden binnen kort tijdsbestek voortdurend mutanten gekweekt van virussen, bacterien of
celkweken die vaak erg afwijken van het wild-type wat betreft hun DNA-sequentie en eventuele
celfuncties.
***Als men het over (on)omkeerbare evolutie wilt hebben kan je eigenlijk niet buiten S.J. GOULD( de
rol van de contigency ) en zijn britse tegenpool CONWAY MORRISS ( over "onvermijdelijke
"convergente evolutie en de mogelijke teleologische ( en dus ooit voorspelbare ) richting van de
evolutieprocessen ? )
Lees vooral het blog van Zimmer en de commentaren die daar ongetwijfeld zullen komen ...:
blogs.discovermagazine.com/loo...
Belangrijke update :
Joe Thornton dient ID-coryfee M. Behe van antwoord :
Het is een bijna even doorwegend antwoord als dat van Lenski aan die kerel van conservapedia ....
blogs.discovermagazine.com/loo...
(Carl Zimmer)
en vergeet de website van Thornton niet, met alle relevante pdf documenten
www.uoregon.edu/~joet/pubs.htm
waaronder het Nature artikel
www.uoregon.edu/~joet/PDF/brid...
En er is ook nog de website van biochemicus Larry Moran
die het ook al over dit onderzoek van Thornton( en over Carl Ziummer ) heeft ...
sandwalk.blogspot.com/2009/09/...
PIERRA
http://www.vkblog.nl/bericht/279265/Terugwaartse_evolutie
Terugwaartse evolutie vrijdag 25 september 2009
Een onderzoeksteam aan de Universiteit van Oregon laat zien dat moleculaire evolutie ( in de
natuur ) niet teruggedraaid kan worden.
Ze onderzochten twee receptors voor cortisol; een voorouderlijke versie van zo’n 400 miljoen
jaar geleden en een modernere versie die gedurende de daaropvolgende 40 miljoen jaar
evolueerde.
De mutaties, die de huidige versie zijn specifieke taak verschaften, werden gevolgd door
‘permissieve’ mutaties, die van geen belang zijn voor de functie van de receptor, maar die wel
verhinderden om de huidige receptor zijn voorouderlijke functie terug te geven.
Er bestaat veel discussie rond de vraag of evolutie wel dan niet omkeerbaar zou zijn.
Er wordt bijvoorbeeld aangenomen dat walvissen en dolfijnen niet meer terug kunnen( op natuurlijke
wijze ) naar het gebruik van kiewen; hun evolutionaire weg is te lang geweest om dit mogelijk te
maken.
Tot nu toe is het nooit mogelijk geweest de (on)omkeerbarheid van de evolutie te toetsen.
De onderzoekers bestudeerden daartoe de structuur van een enkel eiwit: de glucocorticoidreceptor
(GR) die zich bindt aan het hormoon cortisol, een stresshormoon dat o.a. belangrijk is voor het
functioneren van het afweersysteem.
GR is een eiwit en dus opgebouwd uit aminozuren. Door de volgorde van deze aminozuren te
vergelijken in verschillende huidige diersoorten, zijn de onderzoekers erin geslaagd een ‘stamboom’
van dit eiwit te maken en de voorouderlijke vorm te reconstrueren.
Het blijkt dat de GR zo’n 450 miljoen jaar geleden (toen
de kraakbeenvissen zoals haaien zich afsplitsten van
de beenvissen) zowel affiniteit had voor cortisol als
aldosterone, een ander hormoon. Zo’n 40 miljoen jaar
later (toen de eerste viervoeters aan land gingen), werd
deze receptor specifiek voor cortisol alleen.
Gedurende die 40 miljoen jaar zijn er 37 aminozuren
veranderd (door mutatie). Slechts twee veranderingen
(de X mutaties) waren nodig om de functie te
veranderen ofwel om specifiek te worden voor
cortisol: de één zorgde dat de proteïne (het eiwit
GR) zich vouwde, waardoor het zijn affiniteit voor
beide hormonen verloor, en de tweede zorgde
ervoor dat de receptor specifiek werd voor cortisol
alleen. ( noot A)
De onderzoekers vroegen zich af of het mogelijk was
de latere vorm van GR terug te brengen in de
voorouderlijke vorm waarin het zowel cortisol als
aldosterone kan herkennen, door de mutaties van
deze twee aminozuren terug te draaien.
Ze melden dat het niet mogelijk was de ancestrale
functie terug te krijgen en dat bovendien de
receptor geen enkel hormoon meer herkende.
Het blijkt nu dat er van de 37 mutaties vijf zijn die zich
na de twee eerder genoemde (X-mutaties) voordeden;
deze waren willekeurig en hadden geen effect op de
verandering in de moleculaire functie van de proteïne.
Zodra de onderzoekers probeerden de twee
hoofdmutaties terug te draaien, bleken deze vijf
willekeurige mutaties verantwoordelijk voor het in
elkaar storten van de structuur van de proteïne.
Van het internet: protein structure
Eerst moesten dus de( andere dan de
hormoonproducerende ) effecten van deze vijf (
historisch plaats gevonden ) mutaties omgekeerd worden(= back mutations ); Dat kan misschien wel
artificieel bewerkstelligd/opgelost worden,;
Maar , in de toekomstige natuurlijke loop van de evolutieprocessen (= in de werkelijke(natuurlijke )
wereld )bestaat er geen selectieve druk om deze mutaties terug te draaien;
Deze " back- mutations " hebben immers geen (direct) voordeel- effect op welk hormoon de
proteïne herkent ....( natuurlijk wél op de "leefbaarheid" / de structuur van het eiwit )
Dergelijke "neutrale" mutaties ( = eventueel aanwezige mutaties waar geen (selectie)druk wordt
op uitgeoefend wat betreft de moleculaire hoofdfunctie van het eiwit )spelen geen rol in
adaptationistische (schema's aangaande die functies) ...
Maar ze " verdwijnen" daarom niet ...
*1.- Ze lijken slechts "neutraal "want ze spelen wel degelijk een rol : ze bepalen namelijk of het
eiwit structureel nog "levend" is
Het gaat er in dit onderoek om dat de 5 neutrale mutaties geen betere of slechtere receptor maken.
Ze zijn dus neutraal.( wat dat betreft )
(Pierra ) ....De vijf permissieve ofwel neutrale mutaties (die geen effect hebben op de functie van de
receptor) deden zich voor NA de twee essentiele mutaties die de receptor specifiek maakten voor
cortisol.
-Deze neutrale mutaties zijn niet onderhevig aan de wetten van het uitsluitend adaptationistische
evolutiemodel , ofwel het zogenaamde natuurlijke selectie mechanisme op uitsluitend
adaptionalistische leest , en vormen daarom een blokkeerpen waardoor het rad van de evolutie niet
teruggedraaid kan worden.
De selectie vindt dus ook niet plaats op die laatste mutatie(s)!!
Dat is nu juist het probleem. wat verder onderzoek stimuleerde
Je zou kunnen stellen ( in de trant van een theistische evolutionist)dat deze neutrale, onnuttige
mutaties de functie hebben evolutie 'vooruit'te laten gaan en die zouden daarom als teleologisch
'nuttig' gezien kunnen worden
Natuurlijke selectie heeft er geen vat op neutrale mutaties : het omdraaien, al was het maar van één
van deze vijf mutaties, levert eveneens geen voordeel of nadeel op.
Dus mocht dit toevallig toch gebeuren (zie hieronder kansberekening), iets wat heel
onwaarschijnlijk is, dan zijn er nog steeds 4 van dat soort mutaties die teruggedraaid moeten worden.
Voor elk van deze 4 is de kans opnieuw minimaal dat ze terugmuteren( op natuurlijke wijze ) en
dan ook nog eens in de populatie blijven bestaan.
* Kansberekening :De kans om twee maal zes te gooien is 1/6x1/6.
De kans om een zes en daarna een willekeurig ander getal te gooien is 1/6x5/6.
Als je dit doorvoert naar het genoom (stel 1 miljard basen), en je wilt twee keer dezelfde base
muteren, dan kom je op ongeveer 1
(de kans om een willekeurige base te muteren) x 1 miljardste om diezelfde base weer te muteren (x1/4
om dezelfde base ervoor terug te krijgen),
tegenover 1 miljard -1miljardste / 1 miljard voor een willekeurige mutatie elders in het genoom.
Dit natuurlijk afgezien van de muatiesnelheid in het algemeen en dus van de kans op mutatie zelf.
Gezien de kans zo laag is kun je je erover verbazen dat een dergelijk onderzoek als dat van Joe
Thorton gedaan wordt oftewel nodig is.
Dus al met al is de natuurlijke omkering van de vijf permissieve mutaties een zeer onwaarschiinlijke /
quasi onmogelijke gebeurtenis. Daarom worden deze 5 mutaties gezien als een pal die voorkomt
dat het (natuurlijke ) evolutierad terugdraait.
*2.- (Neutrale en andere/zelfs(bijvoorbeeld) homozygotisch schadelijke ) mutaties kunnen
evengoed door historisch toeval ( genetische drift /contingency ) non-adaptationistisch worden
behouden ....
Over adaptationisme en genetische drift ;
http://bioinfo.med.utoronto.ca/Evolution_by_Accident/Evolution_by_Accident.html
Evolution by Accident
v1.43 ©2006 Laurence A. Moran
http://bioinfo.med.utoronto.ca/Evolution_by_Accident/Evolution_by_Accident.h
tml
Modern concepts of evolutionary change are frequently attacked by those who find the notions of
randomness, chance, and accident to be highly distasteful. Some of these critics are intelligent design
creationists. Their objections have been refuted elsewhere. In this essay I'm more concerned about
my fellow evolutionists who go to great lengths to eliminate chance and accident from all discussions
about the fundamental causes of evolution. This is my attempt to convince them that evolution is not
as predictable as they claim. I was originally stimulated to put my ideas down on paper when I read
essays by John Wilkins [Evolution and Chance] and Loren Haarsma [Chance from a Theistic
Perspective] on the TalkOrigins Archive.
The main conclusion of this essay is that a large part of ongoing evolution is determined by stochastic
events that might as well be called "chance" or "random." Furthermore, a good deal of the past history
of life on Earth was the product of chance events, or accidents, that could not have been predicted.
When I say "evolution by accident" I'm referring to all these events. This phrase is intended solely to
distinguish "accidental" evolution from that which is determined by non-random natural selection. I will
argue that evolution is fundamentally a random process, although this should not be interpreted to
mean that all of evolution is entirely due to chance or accident. The end result of evolution by accident
is modern species that do not look designed.
Random Genetic Drift
Most of us are familiar with natural selection as one of the mechanisms of evolution. Another wellknown mechanism is random genetic drift. This is a mechanism that results in fixation (or loss) of
alleles by purely random processes. The topic is thoroughly covered in all the major textbooks—it is
not controversial. Unless you deny the existence of random genetic drift, you must agree that some of
evolution is entirely due to chance events.
The controversy is over how much of evolution is due to drift and how much is due to natural selection.
Excellent arguments have been advanced to prove that most of evolution is due to random genetic
drift and that's the position I take. Thus, in a discussion about the role of chance and accident in
evolution I would say that most of evolution is accidental because of the frequency of drift vs.
selection. Note that this says nothing about the perceived importance of these mechanisms. That's a
value judgement. Some evolutionists think that adaptation, or evolution by natural selection, is the only
interesting part of evolution. These evolutionists don't deny that random genetic drift occurs; instead,
they simply relegate it to the category of uninteresting phenomena. Others, like me, think that random
genetic drift is far more interesting than natural selection because drift is responsible for junk DNA,
molecular phylogenies, molecular clocks, and DNA fingerprinting.
Whenever you hear someone denying the role of chance in evolution you can be certain they are
ignoring random genetic drift. In some cases this is because they don't even know about drift. Those
people are easy to spot because they usually reveal their ignorance of evolution in other ways. In
other cases the chance-deniers are well aware of the existence of random genetic drift but they
choose to define it out of evolution. Sometimes they specifically say that evolution by drift isn't really
evolution. More often, they will use terms like "Darwinism" to describe evolution.
Technically, Darwinism can be construed to mean only evolution by natural selection so this is an
acceptable way of avoiding the topic of drift. However, if you read closely, you'll see that these writers
are often very sloppy about using ""Darwinism" to describe their interests. The term often fills in for all
of evolution in a sort of rhetorical sleight of hand. Thus, this group of chance-deniers tends to eliminate
chance from evolution by re-defining evolution so that it only applies to natural selection. As you might
expect, those who choose to eliminate chance by redefinition are usually the same people that are
only interested in natural selection (see above).
If adaptationists were being really honest, they would take the time to make their point very clear. I
think they should say something like, "If we ignore random genetic drift, for the reasons that we have
just given, then evolution is not a random process." Or perhaps they could say, "Much of evolution is
truly random but I'm only interested in the part that isn't."
The Meaning of Chance
Before continuing, we have to address another semantic issue. Philosophers will argue that there is no
such thing as chance, randomness, and accident. They will point out, quite correctly, that almost
everything has a cause. For example, if the allele for O-type blood became fixed in some native North
American populations by random genetic drift—as it did—then this is not really a "random" event.
Each and every step in the process had a cause even though it may have been as subtle as a tribe
that had a favorable corn crop or a single tuberculosis bacterium that killed off a small child. The net
result of millions of caused events was that the A and B alleles were eliminated from the population
and everyone has O-type blood.
The point about every event having an ultimate cause is true. Taken to its logical conclusion, this is the
line of argument that leads to the denial of free will and the triumph of determinism. While it's fun to
argue these points over a few beers—and I don't deny that I've engaged in those arguments—it
doesn't really impinge on the real world that we are dealing with here. After all, if we are going to deny
that anything is random then we have to stop talking about the outcomes of coin flips and the spin of
the roulette wheel. But that would be silly. We all know what we mean when we talk about chance
events or accidents. We mean that such events are not predictable by any means at our disposal. We
are contrasing such events with those, such as natural selection, that have an obvious cause and a
(mostly) predictable outcome.
Accidents and Contingency
Evolution is more than just a change in the frequency of alleles in a population [What Is Evolution?].
It's also the history of life on Earth from the earliest beginnings almost four billion years ago to the
present day. When I say that evolution is by accident I'm referring as much to this historical event as to
short-term changes within a population. I assume that the opponents of chance are excluding
randomness from history as well as from population dyamics but this isn't always clear.
There are many random events that took place in the past and these had a profound influence on the
outcome of evolution. One of the easiest to understand is mass extinction, although there are many
more including some very mundane things like ice ages and continental drift. We don't need to draw
up a list in order to make the point. Here's how Freeman and Heron put it in their book "Evolutionary
Analysis."
Because they are responsible for such thorough turnover in dominant life forms, mass extinctions are
an important force in explaining the diversification of life through time. It is doubtful whether many of
the major changes observed over the past 250 million years would have taken place had mass
extinctions not occurred. ... It is important to recognize, though, that these defining moments were
largely random events. Just as mutation and drift introduce a strong random component into the
process of adaptation, mass extinctions introduce chance into the process of diversification. This is
because mass extinctions are a sampling process analogous to genetic drift. Instead of sampling allele
frequencies, mass extinctions sample species and lineages. ... The punchline? Chance plays a large
role in the processes responsible for adaptation and diversity.
Scott Freeman and Jon C. Herron (1998) p. 520
I'm not sure how the chance-deniers deal with mass extinctions and all the other things that happened.
Perhaps they think mass extinctions are really non-random events but they just forget to tell us why?
Perhaps they don't think this is part of evolution so they can deny that chance plays a role in evolution
by ignoring history?
Most likely, the chance-deniers just never think about the significance of the fossil record.
The word "contingency" often comes up in these discussions. Contingency is not the same as chance
or randomness. Contingency refers to historical events where each step is dependent upon, or
contingent upon, preceeding events. The final result of any long historical process is the product of a
large number of distinct circumstances any one of which might have been different. If all these
circumstances were severely constrained, then the end result might be highly predictable, like the
outcome of a computer algorithm. On the other hand, if most of the circumstances were the result of
random events, like lucky accidents, then the end result would be unpredictable. When I refer to
evolution by accident I'm referring, in part, to a history of evolution that includes a huge number of
chance and random events all of which are contingent upon everything that preceeded them. Thus,
modern species have taken an unpredicatble path through time.
Chance and Necessity
Natural selection is an important mechanism of evolution. In negative selection, an unfavorable allele
is eliminated from a population because it confers a phenotype that is detrimental to the individual.
Individuals carrying this unfavorable allele will tend to have fewer offspring than those with a more
beneficial allele. In positive selection, the individuals carrying the beneficial allele will prosper and
eventually the beneficial allele will become fixed in the population so that everyone enjoys its
phenotypic benefits.
New alleles arise because of mutation. If the new allele is detrimental it stands an excellent chance of
being eliminated—this is negative, or purifying, selection. If the new allele is beneficial it has a
signifcant chance of becoming fixed in the population by positive natural selection. The probablitiy
depends on just how beneficial the allele is, or on how fit the individual carrying it is relative to the rest
of the population. The probabilty also depends on the size of the population. The numbers have been
worked out by population geneticists.
It turns out that for large populations (>1000) the size of the population can be ignored and the
probability of fixation of any beneficial allele is P = 2s. This means that if a new mutation happens to
produce an allele that confers a 5% (s = 0.05) advantage, then there's a 10% (2s = 2 × 0.05 = 0.10)
chance that it will be fixed in the population by natural selection. This probability is much higher than
the probability that a neutral allele (s = 0) will be fixed by random genetic drift. This is why natural
selection is not random.
Natural selection is a non-random process because there is a preferred outcome but mutation is, to all
intents and purposes, random. This is what Jacques Monod means when he refers to evolution as a
combination of chance and necessity. The "chance" is the randomness of mutation and mutations
supply the raw material for evolution. The "necesssity" is the non-random process of natural selection.
Monod points out that the underlying cause of evolution depends entirely on chance mutations This
means that evolution is fundamentally random. In the following passage from his book "Chance and
Necessity" he begins by describing the various classes of mutation; substitutions, deletions/insertions,
and "scrambling" of chromosomes. He goes on to state clearly what this means for an understanding
of evolution.
We call these events accidental; we say that they are random occurrences. And since they constitute
the only possible source of modifications in the genetic text, itself the sole repository of the organism's
hereditary structures, it necessarily follows that chance alone is at the source of every innovation, of all
creation in the biosphere. Pure chance, absolutely free but blind, at the very root of the stupendious
edifice of evolution: this central concept of modern biology is no longer one among other possible or
even conceivable hypotheses. It is today the sole conceivable hypothesis, the only one that squares
with observed and tested fact.
Jacques Monod (1971)
As you can see, Jacques Monod is a strong supporter of evolution by accident. Note that Monod is not
even referring to random genetic drift or to accidental events during the history of life. He has focused
entirely on the relationship between mutation and natural selection and comes down on the side of
"pure chance, absolutely free but blind." He's not the only one who takes this position; the mutationists
also favor a prime role for random mutation (see below).
The Attack of the Adaptationists
Richard Dawkins disagrees with Monod. According to Dawkins, natural selection is the exact opposite
of chance. Recall that under positive selection there is a signifcant probabilty that a beneficial allele
will become fixed in the population by natural selection. This probabilty (e.g., 10%) is much higher than
the probability of fixing a neutral allele by random genetic drift (e.g., 0.001%). It follows that luck and
chance are ruled out if you focus your attention on adaptation. The emphasis in all of Dawkins' writing
is on evolution by natural selection, or Darwinian theory, or Darwinism.
The whole rationale of Darwin's theory was, and is, that adaptive complexity comes about by slow and
gradual degrees, step by step, no single step making too large a demand on blind chance as
explanation. The Darwinian theory, by rationing chance to the small steps needed to supply variation
for selection, provides the only realistic escape from sheer luck as the explanation for life.
Richard Dawkins (2004) p. 465
This is the adaptationist perspective with a peculiar Dawkins flavor. Dawkins believes that all living
things look designed and the illusion of "design" is due to natural selection. According to Dawkins,
evolution cannot be haphazard, random, or accidental, else it could never achieve the design
attributed to it.
I don't think anyone disagrees with the fact that natural selection is non-random. The disagreement is
about whether the observed result is due more to the particular mutations that arose or to the process
by which they become fixed in the population. (There is also disagreement about the importance of
other mechanisms, such as random genetic drift, but Dawkins ignores that part because he focuses on
Darwinism in these discussions. The point he wants to make is that adaptation is not random. It's the
only part of evolution that he cares about.)
What about Monod's argument that evolution is pure chance because mutations are random? Doesn't
this mean that the end result of evolution is largely due to those mutations that just happened to
occur? How does Dawkins deal with that? Well, for one thing, he focuses on the particular pathway
that was followed and not on all possible pathways that could have been followed. Here's how
Dawkins explains it in "The Blind Watchmaker."
We have seen that living things are too improbable and too beautifully "designed" to have come into
existence by chance. How, then, did they come into existence? The answer, Darwin's answer, is by
gradual, step-by-step transformations from simple beginnings, from primordial entities sufficiently
simple to have come into existence by chance. Each successive change in the gradual evolutionary
process was simple enough, relative to its precursor, to have arisen by chance. But the whole
sequence of cumulative steps constitutes anything but a chance process, when you consider the
complexity of the final end-product relative to the original starting point. The cumulative process is
directed by nonrandom survival.
Richard Dawkins (1986) p. 43
One could hardly disagree with this statement as long as you keep your eye on the process and
ignore the mutations. Of course natural selection is non-random. That's so trivial that intelligent people
wonder why Dawkins spends so much time on it. (It's because some of his readers are Creationists
who need to hear it over and over again.) But, does the fact that natural selection is non-random mean
that there's no signifcant random component to evolution? No, it doesn't. Dawkins is interested in
explaining the "design" he sees in nature. In order to get here from there he reminds us that nonrandom natural selection plays an important role.
Fair enough, but what other possible pathways could have been followed? If the actual pathway is
only one of several million possibilities then why was that one particular design selected? Is there no
possibility that it could have been accidental and selected? Isn't it possible that the actual end-product
is as much due to the random mutations that occurred as it is due to natural selection?
Dawkins seems to ignore this possibility when he draws attention to the non-randomness of evolution.
I think that's a mistake. We can easily agree that natural selection is non-random but it doesn't
necessarily follow that evolution now becomes predictable.
But perhaps Dawkins does favor some form of predictability? As a matter of fact, he does. He
frequently argues that species will become ideally adapted to their environment implying that there is
only one way to achieve this goal. He frequently uses metaphors like "Climbing Mt. Improbable" that
suggest a single goal for a species. If there's only one mountain in the adaptive landscape, and if the
goal is simply to get to the top of that mountain, and if all possible mutations that help achieve that
goal are bound to happen, then we can essentially ignore the role of mutation and concentrate on
natural selection. Perhaps that's what he's thinking. I dunno.
Mutationism
Mutationism is one of Dawkins' most feared bogeymen. The term isn't widely used these days
because it's tainted by association with saltationist ideas from the middle of the last century. It's time to
put aside that bias and think about the modern view of mutationism. Mutationists put even more
emphasis on the role of mutation than Jacque Monod did. In this sense they are about as far from the
Dawkins position as you can possibly be and this is why modern mutationists are feared by
adaptationists.
The idea behind mutationism is not difficult to grasp. It's not even radical. The basic concept is that the
path of evolution is steered by mutation pressure. One of the modern proponents of this concept is
Masatoshi Nei who writes in his book "Molecular Evolutionary Genetics" ...
At the DNA level, most new genes seem to have been produced by gene duplication and subsequent
nucleotide changes. In these cases, the mutational change of DNA (duplication and nucleotide
substitution) is clearly responsible for creating a new gene or character. Natural selection plays no
such role. The role of natural selection is to eliminate less fit genotypes and save a beneficial one
when there are many different genotypes in the same environment. Therefore, it seems clear that at
the molecular level evolution occurs primarily by mutation pressure, though positive selection certainly
speeds up gene substition in populations.
Masatoshi Nei (1987) p. 413
Mutations are random. If you believe Nei then much of evolutionary biology must be due to chance
events. The mutationists claim that mutation is the rate-limiting event in evolution and it's the source of
novelty. This contrasts dramatically with those who think that all required mutations will occur when
needed and the species will always get to the top of the fitness peak. They don't believe that mutations
limit evolution.
In addition to their emphasis on the importance of mutation, mutationists are also tolerant of other nonrandom events in evolution.
In this book, I have examined various aspects of molecular evolution and concluded that mutation is
the driving force of evolution at the molecular level. I have also extended this view to the level of
phenotypic evolution and speciation, though I do not deny the importance of natural selection in
evolution. I have challenged the prevailing view that a population or organisms contains virtually all
sorts of variation and that the only force necessary for a particular character to evolve is natural
selection. I have also emphasized the unpredictability of the evolutionary fate of organisms caused by
uncontrollable external factors such as rapid climactic changes, geological catastrophes, or even
asteroid impacts.
Masatoshi Nei (1987) p. 431
You don't need to agree with Nei in order to get the point. The point is that one can't dismiss the role of
mutation out-of-hand as the chance-deniers usually do. Those who deny the role of chance need to
make their case. It's not good enough to simply declare that natural selection trumps mutation.
The Case Against Mutation
So, what is the case against chance mutation as a major influence on the evolution of species? I
alluded to it earlier. The argument is that mutations do not seriously limit the rate of evolution and,
more importantly, they do not direct the course of evolution. In other words, mutations are so common
that they will always be available whenever they are needed. The only thing required is that natural
selection act on the pool of mutations and this is why natural selection is the most important force in
evolution.
A corollary to this idea is that modern species are perfectly adapted to their environment and the only
thing that will cause further evolution is a change in the environment. This is a necessary corollary
since over a period of time all mutations must have been available and must have become fixed in the
population.
Is this really what Dawkins and his colleagues are saying? Well, not exactly. Mostly they avoid stating
flat out that this is their belief but it's pretty clear that it is. How else can they ignore the importance of
chance mutations? Here's an example of belief in the perfection of adaptation from "The Blind
Watchmaker."
If the conditions in which a lineage of animals lives remain constant; say it is dry and hot and has been
so without a break for 100 generations, evolution in that lineage is likely to come to a halt, at least as
far as adaptations to temperature and humidity are concerned. The animals will become as well fitted
as they can to the local conditions. This doesn't mean that they couldn't be completely redesigned to
be even better. It does mean that they can't improve themselves by any small (and therefore likely)
evolutionary step .... Evolution will come to a standstill until something in the conditions changes: the
onset of an ice age, a change in the average rainfall of the area, a shift in the prevailing wind.
Richard Dawkins (1986) p.178-179
Do you get the point? Mutations aren't important. When new alleles are required they will be present in
the population. That's why species can become perfectly adapted to their local environment over a
short period of time. That's why natural selection is the important force and mutations only incidentally
are required to supply the raw material for evolution.
What Dawkins only implies, Ernst Mayr states explicitly.
According to their evolutionary significance, three kinds of mutations can be distinguished: beneficial,
neutral, or deleterious. Individuals with genotypes that contain a beneficial new mutation will be faored
by natural selection. However, since almost all conceivable beneficial mutations of a population
in a stable environment have already been selected in the recent past, the occurence of new
beneficial mutations is rather rare. ... Before the role of selection was fully understood, it was believed
by many evolutionists that some evolutionary changes were due to "mutation pressure." This is a
misconception. The frequency of a gene in a population is in the long run determined by natural
selection and stochastic processes, and not by the frequency of mutation. [my emphasis]
Ernst Mayr (2001) p.98
With drastic selection taking place in every generation, it is legitimate to ask why evolution is normally
so slow. The major reason is that owing to hundreds or thousands of generations that have undegone
preceeding selection, a natural population will be close to the optimal genotype. ... All the mutations
of which this geneotype is capable and that could lead to an improvement of this standard
phenotype have already been incorporated in previous generations. [my emphasis]
Ernst Mayr (2001) p.135
If you believe that most species are perfectly adapted to their environment then you will believe that
the probablity of a given mutation does not play a major role in evolution by natural selection. Thus,
the randomness of mutation isn't a significant feature of evolution but the non-randomness of natural
selection is. I don't buy it. I don't see perfection around me and, furthermore, I don't believe that
adaptation is all there is to evolution. I think that the path of evolution is influenced by the frequency
and randomness of mutations.
What Causes Speciation?
Speciation plays an important role in evolution. Let's consider branching speciation where a single
species splits into two species. This form of speciation usually begins when a small population
becomes isolated from the rest of the species. Over time, the two populations diverge because there is
no exchange of genes between them. After many generations of isolation they have diverged to the
point where they can no longer interbreed when they come back into contact. Two species, or two
taxa, have evolved.
The events that lead to an isolated population can be entirely accidental. For example, a small flock of
birds is blown to the Galapagos by a typhoon or the rising Isthmus of Panama splits a fish species into
two groups on the east and west sides. The genetic changes leading to speciation can also be random
because they result from fixation of neutral alleles by random genetic drift—this is thought to be the
main mechanism of speciation.
Thus, speciation—one of the most important events in evolution—is largely by accident. Ernst Mayr,
who was one of the world's leading experts on speciation, puts it in plain, simple language.
If evolutionists have learned anything from a detailed analysis of evolution, it is the lesson that the
origin of new taxa is largely a chance event. Ninety-nine out of 100 newly arising species probably
became extinct without giving rise to descendant taxa. And the characteristic of any new taxon is to a
large extent determined by such chance factors as the genetic composition of the founding population,
the special internal structure of its genotype, and the physical as well as biotic environment that
supplies the selection forces of the new species population.
Ernst Mayr (1985)
Replaying the Tape of Life
A the risk of forcing an analogy, it's useful to think of evolution as a tinkerer and not a designer. This is
the view originally espressed by François Jacob, winner of the Nobel Prize in 1965 along with Jacques
Monod. Jacob points out that the tinkerer takes whatever parts and pieces are lying around and
cobbles together something that works. That's what evolution does, whether the mechanism is natural
selection, random genetic drift, or anything else. The implications are profound. It means that the
modern product of all this tinkering comes from a long line of Rube-Goldberg-like ancestors with no
discernable plan or goal.
Here's how Jacob puts it,
It is hard to realize that the living world as we know it is just one among many possibilities; that its
actual structure results from the history of the earth. Yet living organisms are historical structures:
literally creations of history. They represent, not a perfect product of engineering, but a patchwork of
odd sets pieced together when and where opportunities arose. For the opportunism of natural
selection is not simply a matter of indifference to the structure and operation of its products. It reflects
the very nature of a historical process, full of contingency.
François Jacob (1977) p. 1166
This reminds us that historical processes are contingent and the individual events that occur are often
haphazard and unpredictable. The end result—in this case a modern species—is unpredictable
because at each step there were many different paths that could have been followed. This is evolution
by accident.
Stephen Jay Gould wrote a book about the role of chance in evolution. He called it "Wonderful Life."
On the surface it's a book about the Burgess Shale and the Cambrian explosion but there's a powerful
message as well. Gould is interested in why some species survive while others go extinct. Are the
survivors better adapted than the losers of is it a matter of luck? We could answer this question if we
could carry out an experiment.
I call this experiment "replaying life's tape." You press the rewind button and, making sure you
thoroughly erase everything that actually happens, go back to any time and place in the past—say, to
the seas of the Burgess Shale. Then let the tape run again and see if the repetition looks at all like the
original. If each replay strongly resembles life's actual pathway, then we must conclude that what
really happened pretty much had to occur. But suppose that the experimental versions all yield
sensible results strikingly different from the actual history of life? What could we then say about the
predictability of sefl-conscious intelligence? or of mammals? or of vertebrates? or of life on land? or
simply of multicellular persistence for 600 million years?
Stephen Jay Gould (1989) pp. 48-50
I'm one of those who think that the tape will be different every time it's replayed because there are so
many accidents and contingencies. This is Gould's point as well, recognizing of course that the
experiment can never be performed. Others disagree. Simon Conway Morris (2003), for example,
claims that something similar to humans will evolve whenever the tape is replayed. And Richard
Dawkins (2004) is, as you might imagine, sympathetic to the idea that many similar things will
reappear each time.
Chance Restored
I've tried to summarize all of the random and accidental things that can happen during evolution.
Mutations are chance events. Random genetic drift is, of course, random. Accidents and contingency
abound in the history of life. All this means that the tape of life will never replay the same way. Chance
events affect speciation. All these things seem obvious. So, what's the problem?
The "problem" is that writers like Richard Dawkins have made such a big deal about the nonrandomness of natural selection that they risk throwing out the baby with the bathwater. A superficial
reading of any Dawkins' book would lead you to the conclusion that evolution is an algorithmic process
and that chance and accident have been banished. That's not exactly what he says but it sure is the
dominant impression you take away from his work.
I said at the beginning of this essay that I was inspired to write it by reading John Wilkins' essay on
"Evolution and Chance." Wilkins denies, as does Dawkins, that there is any "deep improbability" in
evolution. In fact, his main conclusion is that "evolution is not fundamentally a random process." I beg
to differ. I think there's a lot of randomness and improbability in evolution and I hope I've convinced
you. I think the term "evolution by accident" is an accurate description of how evolution occurs.
http://sandwalk.blogspot.com/2009/09/naked-adaptationism.html
http://sandwalk.blogspot.com/2009/09/adaptationist-in-piazza-san-marco.html
Gould, S.J. and Lewontin, R.C. (1979) The Spandrels of San Marco
and the Panglossian paradigm: a critique of the adaptationist
programme. Proc. R. Soc. Lond. B 205:581-598.
http://ethomas.web.wesleyan.edu/wescourses/2004s/ees227/01/spa
ndrels.html
The Spandrels of San Marco and the Panglossian Paradigm: A Critique
of the Adaptationist Programme
Stephen Jay Gould and Richard C. Lewontin
Republished from the original with the kind permission of The Royal Society of London: Gould, S.
J. And Lewontin, R. C., "The Spandrels of San Marco and the Panglossian Paradigm: A Critique Of
The Adaptationist Programme," Proceedings Of The Royal Society of London, Series B, Vol. 205,
No. 1161 (1979), Pp. 581-598.
An adaptationist programme has dominated evolutionary thought in England and the United States
during the past forty years. It is based on faith in the power of natural selection as an optimizing
agent. It proceeds by breaking an organism into unitary "traits" and proposing an adaptive story for
each considered separately. Trade-offs among competing selective demands exert the only brake upon
perfection; non-optimality is thereby rendered as a result of adaptation as well. We criticize this
approach and attempt to reassert a competing notion (long popular in continental Europe) that
organisms must be analyzed as integrated wholes, with Baupläne so constrained by phyletic heritage,
pathways of development, and general architecture that the constraints themselves become more
interesting and more important in delimiting pathways of change than the selective force that may
mediate change when it occurs. We fault the adaptationist programme for its failure to distinguish
current utility from reasons for origin (male tyrannosaurs may have used their diminutive front legs
to titillate female partners, but this will not explain why they got so small); for its unwillingness to
consider alternatives to adaptive stories; for its reliance upon plausibility alone as a criterion for
accepting speculative tales; and for its failure to consider adequately such competing themes as
random fixation of alleles, production of non-adaptive structures by developmental correlation with
selected features (allometry, pleiotropy, material compensation, mechanically forced correlation), the
separability of adaptation and selection, multiple adaptive peaks, and current utility as an
epiphenomenon of nonadaptive structures. We support darwin's own pluralistic approach to
identifying the agents of evolutionary change.
1. Introduction
The great central dome of St. Mark's Cathedral in Venice presents in its mosaic design a detailed
iconography expressing the mainstays of Christian faith. Three circles of figures radiate out from a
central image of Christ: angels, disciples, and virtues. Each circle is divided into quadrants, even
though the dome itself is radially symmetrical in structure. Each quadrant meets one of the four
spandrels in the arches below the dome. Spandrels-the tapering triangular spaces formed by the
intersection of two rounded arches at right angles are necessary architectural byproducts of mounting
a dome on rounded arches. Each spandrel contains a design admirably fitted into its tapering space.
An evangelist sits in the upper part flanked by the heavenly cities. Below, a man representing one of
the four biblical rivers (Tigris, Euphrates, Indus, and Nile) pours water from a pitcher in the
narrowing space below his feet.
The design is so elaborate, harmonious, and purposeful that we are tempted to view it as the starting
point of any analysis, as the cause in some sense of the surrounding architecture. But this would invert
the proper path of analysis. The system begins with an architectural constraint: the necessary four
spandrels and their tapering triangular form. They provide a space in which the mosaicists worked;
they set the quadripartite symmetry of the dome above.
Such architectural constraints abound, and we find them easy to understand because we do not
impose our biological biases upon them. Every fan-vaulted ceiling must have a series of open spaces
along the midline of the vault, where the sides of the fans intersect between the pillars. Since the
spaces must exist, they are often used for ingenious ornamental effect. In King's College Chapel in
Cambridge, for example, the spaces contain bosses alternately embellished with the Tudor rose and
portcullis. In a sense, this design represents an "adaptation," but the architectural constraint is clearly
primary. The spaces arise as a necessary by-product of fan vaulting; their appropriate use is a
secondary effect. Anyone who tried to argue that the structure exists be-cause the alternation of rose
and portcullis makes so much sense in a Tudor chapel would be inviting the same ridicule that
Voltaire heaped on Dr. Pangloss: "Things cannot be other than they are... Everything is made for the
best purpose. Our noses were made to carry spectacles, so we have spectacles. Legs were clearly
intended for breeches, and we wear them." Yet evolutionary biologists, in their tendency to focus
exclusively on immediate adaptation to local conditions, do tend to ignore architectural constraints
and perform just such an inversion of explanation.
As a closer example, recently featured in some important biological literature on adaptation,
anthropologist Michael Harner has proposed (1977) that Aztec human sacrifice arose as a solution to
chronic shortage of meat (limbs of victims were often consumed, but only by people of high status). E.
O. Wilson (1978) has used this explanation as a primary illustration of an adaptive, genetic
predisposition for carnivory in humans. Harner and Wilson ask us to view an elaborate social system
and a complex set of explicit justifications involving myth, symbol, and tradition as mere
epiphenomena generated by the Aztecs as an unconscious rationalization masking the "real" reason
for it all: need for protein. But Sahlins (1978) has argued that human sacrifice represented just one part
of an elaborate cultural fabric that, in its entirety, not only represented the material expression of
Aztec cosmology, but also performed such utilitarian functions‚as the maintenance of social ranks‚and
systems of tribute among cities.
We strongly suspect that Aztec cannibalism was an "adaptation" much like evangelists and rivers in
spandrels, or ornamented bosses in ceiling spaces: a secondary epiphnomenon representing a fruitful
use of available parts, not a cause of the entire system. To put it crudely: a system developed for other
reasons generated an increasing number of fresh bodies; use might as well be made of them. Why
invert the whole system in such a curious fashion and view an entire culture as the epi-phenomenon
of an unusual way to beef up the meat supply. Spandrels do not exist to house the evangelists.
Moreover, as Sahlins argues, it is not even clear that human sacrifice was an adaptation at all. Human
cultural practices can be orthogenetic and drive toward extinction in ways that Darwinian processes,
based on genetic selection, cannot. Since each new monarch had to outdo his predecessor in even
more elaborate and copious sacrifice, the practice was beginning to stretch resources to the breaking
point. It would not have been the first time that a human culture did itself in. And, finally, many
experts doubt Harner's premise in the first place (Ortiz de Montellano, 1978). They argue that other
sources of protein were not in short supply, and that a practice awarding meat only to privileged
people who had enough anyway, and who used bodies so inefficiently (only the limbs were
consumed, and partially at that) represents a mighty poor way to run a butchery.
We deliberately chose non-biological examples in a sequence running from remote to more familiar:
architecture to anthropology. We did this because the primacy of architectural constraint and the
epiphenomenal nature of adaptation are not obscured by our biological prejudices in these examples.
But we trust that the message for biologists will not go unheeded: if these had been biological systems,
would we not, by force of habit, have regarded the epiphenomenal adaptation as primary and tried to
build the whole structural system from it?
2. The adaptationist programme
We wish to question a deeply engrained habit of thinking among students of evolution. We call it the
adaptationist programme, or the Panglossian paradigm. It is rooted in a notion popularized by A.R.
Wallace and A. Weismann, (but not, as we shall see, by Darwin) toward the end of the nineteenth
century: the near omnipotence of natural selection in forging organic design and fashioning the best
among possible worlds. This programme regards natural selection as so powerful and the constraints
upon it so few that direct production of adaptation through its operation becomes the primary cause
of nearly all organic form, function, and behavior. Constraints upon the pervasive power of natural
selection are recognized of course (phyletic inertia primarily among them, although immediate
architectural constraints, as discussed in the last section, are rarely acknowledged). But they are
usually dismissed as unimportant or else, and more frustratingly, simply acknowledged and then not
taken to heart and invoked.
Studies under the adaptationist programme generally proceed in two steps:
(1) An organism is atomized into "traits" and these traits are explained as structures optimally
designed by natural selection for their functions. For lack of space, we must omit an extended
discussion of the vital issue "What is a trait?" Some evolutionists may regard this as a trivial, or merely
a semantic problem. It is not. Organisms are integrated entities, not collections of discrete objects.
Evolutionists have often been led astray by inappropriate atomization, as D'Arcy Thompson (1942)
loved to point out. Our favorite example involves the human chin (Gould, 1977, pp. 381-382;
Lewontin, 1978). If we regard the chin as a "thing," rather than as a product of interaction between two
growth fields (alveolar and mandibular), then we are led to an interpretation of its origin
(recapitulatory) exactly opposite to the one now generally favored (neotenic).
(2) After the failure of part-by-part optimization, interaction is acknowledged via the dictum that an
organism cannot optimize each part without imposing expenses on others. The notion of "trade-off' is
introduced, and organisms are interpreted as best compromises among competing demands. Thus
interaction among parts is retained completely within the adaptationist programme. Any suboptimality of a part is explained as its contribution to the best possible design for the whole. The
notion that sub-optimality might represent anything other than the immediate work of natural
selection is usually not entertained. As Dr. Pangloss said in explaining to Candide why he suffered
from venereal disease: "It is indispensable in this best of worlds. For if Columbus, when visiting the
West Indies, had not caught this disease, which poisons the source of generation, which frequently
even hinders generation, and is clearly opposed to the great end of Nature, we should have neither
chocolate nor cochineal." The adaptationist programme is truly Panglossian. Our world may not be
good in an abstract sense, but it is the very best we could have. Each trait plays its part and must be as
it is.
At this point, some evolutionists will protest that we are caricaturing their view of adaptation. After
all, do they not admit genetic drift, allometry, and a variety of reasons for non-adaptive evolution?
They do, to be sure, but we make a different point. In natural history, all possible things happen
sometimes; you generally do not support your favored phenomenon by declaring rivals impossible in
theory. Rather, you acknowledge the rival but circumscribe its domain of action so narrowly that it
cannot have any importance in the affairs of nature. Then, you often congratulate yourself for being
such an undogmatic and ecumenical chap. We maintain that alternatives to selection for best overall
design have generally been relegated to unimportance by this mode of argument. Have we not all
heard the catechism about genetic drift: it can only be important in populations so small that they are
likely to become extinct before playing any sustained evolutionary role (but see Lande, 1976).
The admission of alternatives in principle does not imply their serious consideration in daily practice.
We all say that not everything is adaptive; yet, faced with an organism, we tend to break it into parts
and tell adaptive stories as if trade-offs among competing, well designed parts were the only
constraint upon perfection for each trait. It is an old habit. As Romanes complained about A.R.
Wallace in 1900: "Mr. Wallace does not expressly maintain the abstract impossibility of laws and
causes other than those of utility and natural selection... Nevertheless, as he nowhere recognizes any
other law or cause... he practically concludes that, on inductive or empirical grounds, there is no such
other law or cause to be entertained. The adaptationist programme can be traced through common
styles of argument. We illustrate just a few; we trust they will be recognized by all:
(1) If one adaptive argument fails, try another. Zig-zag commissures of clams and brachiopods, once
widely regarded as devices for strengthening the shell, become sieves for restricting particles above a
given size (Rudwick, 1964). A suite of external structures (horns, antlers, tusks) once viewed as
weapons against predators, become symbols of intra-specific competition among males (Davitashvili,
1961). The eskimo face, once depicted as "cold engineered" (Coon, et al., 1950), becomes an adaptation
to generate and withstand large masticatory forces (Shea, 1977). We do not attack these newer
interpretations; they may all be right. We do wonder, though, whether the failure of one adaptive
explanation should always simply inspire a search for another of the same general form, rather than a
consideration of alternatives to the proposition that each part is "for" some specific purpose.
(2) If one adaptive argument fails, assume that another must exist; a weaker version of the first
argument. Costa & Bisol (1978), for example, hoped to find a correlation between genetic
polymorphism and stability of environment in the deep sea, but they failed. They conclude (1978, pp.
132, 133): "The degree of genetic polymorphism found would seem to indicate absence of correlation
with the particular environmental factors which characterize the sampled area. The results suggest
that the adaptive strategies of organisms belonging to different phyla are different."
(3) In the absence of a good adaptive argument in the first place, attribute failure to imperfect
understanding of where an organism lives and what it does. This is again an old argument. Consider
Wallace on why all details of color and form in land snails must be adaptive, even if different animals
seem to inhabit the same environment (1899, p. 148): "The exact proportions of the various species of
plants, the numbers of each kind of insect or of bird, the peculiarities of more or less exposure to
sunshine or to wind at certain critical epochs, and other slight differences which to us are absolutely
immaterial and unrecognizable, may be of the highest significance to these humble creatures, and be
quite sufficient to require some slight adjustments of size, form, or color, which natural selection will
bring about."
(4) Emphasize immediate utility and exclude other attributes of form. Fully half the explanatory
information accompanying the full-scale Fiberglass Tyrannosaurus at Boston's Museum of Science
reads: "Front legs a puzzle: how Tyrannosaurus used its tiny front legs is a scientific puzzle; they were
too short even to reach the mouth. They may have been used to help the animal rise from a lying
position." (We purposely choose an example based on public impact of science to show how widely
habits of the adaptationist programme extend. We are not using glass beasts as straw men; similar
arguments and relative emphases, framed in different words, appear regularly in the professional
literature.) We don't doubt that Tyrannosaurus used its diminutive front legs for something. If they had
arisen de novo, we would encourage the search for some immediate adaptive reason. But they are,
after all, the reduced product of conventionally functional homologues in ancestors (longer limbs of
allosaurs, for example). As such, we do not need an explicitly adaptive explanation for the reduction
itself. It is likely to be a developmental correlate of allometric fields for relative increase in head and
hindlimb size. This non-adaptive hypothesis can be tested by conventional allometric methods
(Gould, 1974, in general; Lande, 1978, on limb reduction) and seems to us both more interesting and
fruitful than untestable speculations based on secondary utility in the best of possible worlds. One
must not confuse the fact that a structure is used in some way (consider again the spandrels, ceiling
spaces, and Aztec bodies) with the primary evolutionary reason for its existence and conformation.
3. Telling Stories
‘All this is a manifestation of the rightness of things, since if there is a volcano at Lisbon it could not be
anywhere else, for it is impossible for things not to be where they are, because everything is for the
best’. [Dr. Pangloss on the great Lisbon earthquake of 1755, in which up to 50,000 people lost their
lives].
We would not object so strenuously to the adaptationist programme if its invocation, in any particular
case, could lead in principle to its rejection for want of evidence. We might still view it as restrictive
and object to its status as an argument of first choice. But if it could be dismissed after failing some
explicit test, then alternatives would get their chance. Unfortunately, a common procedure among
evolutionists does not allow such definable rejection for two reasons. First, the rejection of one
adaptive story usually leads to its replacement by another, rather than to a suspicion that a different
kind of explanation might be required. Since the range of adaptive stories is as wide as our minds are
fertile, new stones can always be postulated. And if a story is not immediately available, one can
always plead temporary ignorance and trust that it will be forthcoming, as did Costa & Bisol (1978),
cited above. Secondly, the criteria for acceptance of a story are so loose that many pass without proper
confirmation. Often, evolutionists use consistency with natural selection as the sole criterion and
consider their work done when they concoct a plausible story. But plausible stories can always be told.
The key to historical research lies in devising criteria to identify proper explanations among the
substantial set of plausible pathways to any modern result.
We have, for example (Gould, 1978) criticized Barash's (1976) work on aggression in mountain
bluebirds for this reason. Barash mounted a stuffed male near the nests of two pairs of bluebirds while
the male was out foraging. He did this at the same nests on three occasions at ten-day intervals: the
first before eggs were laid, the last two afterwards. He then counted aggressive approaches of the
returning male toward the model and the female. At time one, aggression was high toward the model
and lower toward females but substantial in both nests. Aggression toward the model declined
steadily for times two and three and plummeted to near zero toward females. Barash reasoned that
this made evolutionary sense, since males would be more sensitive to intruders before eggs were laid
than afterward (when they can have some confidence that their genes are inside). Having devised this
plausible story, he considered his work as completed (1976, pp. 1099, 1100):
 The results are consistent with the expectations of evolutionary theory. Thus
aggression toward an intruding male (the model) would clearly be especially
advantageous early in the breeding season, when territories and nests are normally
defended... The initial aggressive response to the mated female is also adaptive in
that, given a situation suggesting a high probability of adultery (i.e., the presence of
the model near the female) and assuming that replacement females are available,
obtaining a new mate would enhance the fitness of males... The decline in malefemale aggressiveness during incubation and fledgling stages could be attributed to
the impossibility of being cuckolded after the eggs have been laid... The results are
consistent with an evolutionary interpretation.
They are indeed consistent, but what about an obvious alternative, dismissed without test by Barash?
Male returns at times two and three, approaches the model, tests it a bit, recognizes it as the same
phony he saw before, and doesn't bother his female. Why not at least perform the obvious test for this
alternative to a conventional adaptive story: expose a male to the model for the first time after the eggs
are laid?
After we criticized Barash's work, Morton et al. (1978) repeated it, with some variations (including the
introduction of a female model), in the closely related eastern bluebird Sialia sialis. "We hoped to
confirm", they wrote, that Barash's conclusions represent "a widespread evolutionary reality, at least
within the genus Sialia. Unfortunately, we were unable to do so." They found no "anti-cuckoldry"
behavior at all: males never approached their females aggressively after testing the model at any
nesting stage. Instead, females often approached the male model and, in any case, attacked female
models more than males attacked male models. "This violent response resulted in the near destruction
of the female model after presentations and its complete demise on the third, as a female flew off with
the model's head early in the experiment to lose it for us in the brush" (1978, p. 969). Yet, instead of
calling Barash's selected story into question, they merely devise one of their own to render both results
in the adaptationist mode. Perhaps, they conjecture, replacement females are scarce in their species
and abundant in Barash's. Since Barash's males can replace a potentially "unfaithful" female, they can
afford to be choosy and possessive. Eastern bluebird males are stuck with uncommon mates and had
best be respectful. They conclude: "If we did not support Barash's suggestion that male bluebirds
show anticuckoldry adaptations, we suggest that both studies still had 'results that are consistent with
the expectations of evolutionary theory' (Barash 1976, p. 1099), as we presume any careful study
would." But what good is a theory that cannot fail in careful study (since by 'evolutionary theory', they
clearly mean the action of natural selection applied to particular cases, rather than the fact of
transmutation itself)?
4. The Master's Voice Re-examined
Since Darwin has attained sainthood (if not divinity) among evolutionary biologists, and since all
sides invoke God's allegiance, Darwin has often been depicted as a radical selectionist at heart who
invoked other mechanisms only in retreat, and only as a result of his age's own lamented ignorance
about the mechanisms of heredity. This view is false. Although Darwin regarded selection as the most
important of evolutionary mechanisms (as do we), no argument from opponents angered him more
than the common attempt to caricature and trivialize his theory by stating that it relied exclusively
upon natural selection. In the last edition of the Origin, he wrote (1872, p. 395):
 As my conclusions have lately been much misrepresented, and it has been stated
that I attribute the modification of species exclusively to natural selection, I may be
permitted to remark that in the first edition of this work, and subsequently, I placed in
a most conspicuous position-namely at the close of the introduction-the following
words: "I am convinced that natural selection has been the main, but not the exclusive
means of modification." This has been of no avail. Great is the power of steady
misinterpretation.
Romanes, whose once famous essay (1900) on Darwin's pluralism versus the panselectionism of
Wallace and Weismann deserves a resurrection, noted of this passage (1900, p. 5): "In the whole range
of Darwin's writings there cannot be found a passage so strongly worded as this: it presents the only
note of bitterness in all the thousands of pages which he has published." Apparently, Romanes did not
know the letter Darwin wrote to Nature in 1880, in which he castigated Sir Wyville Thomson for
caricaturing his theory as panselectionist (1880, p. 32):
 I am sorry to find that Sir Wyville Thomson does not understand the principle of
natural selection... If he had done so, he could not have written the following sentence
in the Introduction to the Voyage of the Challenger: "The character of the abyssal
fauna refuses to give the least support to the theory which refers the evolution of
species to extreme variation guided only by natural selection." This is a standard of
criticism not uncommonly reached by theologians and metaphysicians when they
write on scientific subjects, but is something new as coming from a naturalist ... Can
Sir Wyville Thomson name any one who has said that the evolution of species
depends only on natural selection? As far as concerns myself, I believe that no one has
brought forward so many observations on the effects of the use and disuse of parts, as
I have done in my "Variation of Animals and Plants under Domestication"; and these
observations were made for this special object. I have likewise there adduced a
considerable body of facts, showing the direct action of external conditions on
organisms.
We do not now regard all of Darwin's subsidiary mechanisms as significant or even valid, though
many, including direct modification and correlation of growth, are very important. But we should
cherish his consistent attitude of pluralism in attempting to explain Nature's complexity.
5. A Partial Typology of Alternatives to the Adaptationist Programme
In Darwin's pluralistic spirit, we present an incomplete hierarchy of alternatives to immediate
adaptation for the explanation of form, function, and behavior.
(1) No adaptation and no selection at all. At present, population geneticists are sharply divided on the
question of how much genetic polymorphism within populations and how much of the genetic
differences between species is, in fact, the result of natural selection as opposed to purely random
factors. Populations are finite in size, and the isolated populations that form the first step in the
speciation process are often founded by a very small number of individuals. As a result of this
restriction in population size, frequencies of alleles change by genetic drift, a kind of random genetic
sampling error. The stochastic process of change in gene frequency by random genetic drift, including
the very strong sampling process that goes on when a new isolated population is formed from a few
immigrants, has several important consequences. First, populations and species will become
genetically differentiated, and even fixed for different alleles at a locus in the complete absence of any
selective force at all.
Secondly, alleles can become fixed in a population in spite of natural selection. Even if an allele is
favored by natural selection, some proportion of populations, depending upon the product of
population size N and selection intensity s, will become homozygous for the less fit allele because of
genetic drift. If N*s is large, this random fixation for unfavorable alleles is a rare phenomenon, but if
selection coefficients are on the order of the reciprocal of population size (N*s = 1) or smaller, fixation
for deleterious alleles is common. if many genes are involved in influencing a metric character like
shape, metabolism, or behavior, then the intensity of selection on each locus will be small and N*s per
locus may be small. as a result, many of the loci may be fixed for non-optimal alleles.
Thirdly, new mutations have a small chance of being incorporated into a population, even when
selectively favored. Genetic drift causes the immediate loss of most new mutations after their
introduction. With a selection intensity s, a new favorable mutation has a probability of only 2s of ever
being incorporated. Thus one cannot claim that, eventually, a new mutation of just the right sort for
some adaptive argument will occur and spread. "Eventually" becomes a very long time if only one in
1,000 or one in 10,000 of the "right" mutations that do occur ever get incorporated in a population.
(2) No adaptation and no selection on the part at issue; form of the part is a correlated consequence of
selection directed elsewhere. Under this important category, Darwin ranked his "mysterious" laws of
the "correlation of growth." Today, we speak of pleiotropy, allometry, "material
compensation"‚(Rensch, 1959, pp. 179-187) and‚ mechanically forced correlations in D'Arcy
Thompson's sense (1942; Gould 1971). Here we come face to face with organisms as integrated wholes,
fundamentally not decomposable into independent and separately optimized parts.
Although allometric patterns are as subject to selection as static morphology itself (Gould, 1966), some
regularities in relative growth are probably not under immediate adaptive control. For example, we
do not doubt that the famous 0.66 interspecific allometry of brain size in all major vertebrate groups
represents a selected "design criterion," though its significance remains elusive (Jerison, 1973). It is too
repeatable across too wide a taxonomic range to represent much else than a series of creatures
similarly well designed for their different sizes. But another common allometry, the 0.2 to 0.4 intraspecific scaling among homeothermic adults differing in body size, or among races within a species,
probably does not require a selectionist story, though many, including one of us, have tried to provide
one (Gould, 1974). R. Lande (personal communication) has used the experiments of Falconer (1973) to
show that selection upon body size alone yields a brain-body slope across generations of 0.35 in mice.
More compelling examples abound in the literature on selection for altering the timing of maturation
(Gould, 1977). At least three times in the evolution of arthropods (mites, flies, and beetles), the same
complex adaptation has evolved, apparently for rapid turnover of generations in strongly r-selected
feeders on super-abundant but ephemeral fungal resources: females reproduce as larvae and grow the
next generation within their bodies. Offspring eat their mother from inside and emerge from her
hollow shell, only to be devoured a few days later by their own progeny. It would be foolish to seek
adaptive significance in paedomorphic morphology per se; it is primarily a byproduct of selection for
rapid cycling of generations. In more interesting cases, selection for small size (as in animals of the
interstitial fauna) or rapid maturation (dwarf males of many crustaceans) has occurred by progenesis
(Gould, 1977, pp. 324-336), and descendant adults contain a mixture of ancestral juvenile and adult
features. Many biologists have been tempted to find primary adaptive meaning for the mixture, but it
probably arises as a by-product of truncated maturation, leaving some features "behind" in the larval
state, while allowing others, more strongly correlated with sexual maturation, to retain the adult
configuration of ancestors.
(3) The de-coupling of selection and adaptation.
(i) Selection without adaptation. Lewontin (1979) has presented the following hypothetical example: "A
mutation which doubles the fecundity of individuals will sweep through a population rapidly. If there
has been no change in efficiency of resource utilization, the individuals will leave no more offspring
than before, but simply lay twice as many eggs, the excess dying because of resource limitation. In
what sense are the individuals or the population as a whole better adapted than before? Indeed, if a
predator on immature stages is led to switch to the species now that immatures are more plentiful, the
population size may actually decrease as a consequence, yet natural selection at all times will favour
individuals with higher fecundity."
(ii) Adaptation without selection. Many sedentary marine organisms, sponges and corals in particular,
are well adapted to the flow regimes in which they live. A wide spectrum of "good design" may be
purely phenotypic in origin, largely induced by the current itself. (We may be sure of this in numerous
cases, when genetically identical individuals of a colony assume different shapes in different
microhabitats.) Larger patterns of geographic variation are often adaptive and purely phenotypic as
well. Sweeney and Vannote (1978), for example, showed that many hemi-metabolous aquatic insects
reach smaller adult size with reduced fecundity when they grow at temperatures above and below
their optima. Coherent, climatically correlated patterns in geographic distribution for these insects-so
often taken as a priori signs of genetic adaptation may simply reflect this phenotypic plasticity.
"Adaptation" -- the good fit of organisms to their environment -- can occur at three hierarchical levels
with different causes. It is unfortunate that our language has focused on the common result and called
all three phenomena "adaptation": the differences in process have been obscured, and evolutionists
have often been misled to extend the Darwinian mode to the other two levels as well. First, we have
what physiologists call "adaptation": the phenotypic plasticity that permits organisms to 'mold' their
form to prevailing circumstances during ontogeny. Human "adaptations" to high altitude fall into this
category (while others, like resistance of sickling heterozygotes to malaria, are genetic, and
Darwinian). Physiological adaptations are not heritable, though the capacity to develop them
presumably is. Secondly, we have a "heritable" form of non-Darwinian adaptation in humans (and, in
rudimentary ways, in a few other advanced social species): cultural adaptation (with heritability
imposed by learning). Much confused thinking in human sociobiology arises from a failure to
distinguish this mode from Darwinian adaptation based on genetic variation. Finally, we have
adaptation arising from the conventional Darwinian mechanism of selection upon genetic variation.
The mere existence of a good fit between organism and environment is insufficient for inferring the
action of natural selection.
(4) Adaptation and selection but no selective basis for differences among adaptations. Species of
related organisms, or sub-populations within a species, often develop different adaptations as
solutions to the same problem. When "multiple adaptive peaks" are occupied, we usually have no
basis for asserting that one solution is better than another. The solution followed in any spot is a result
of history; the first steps went in one direction, though others would have led to adequate prosperity
as well. Every naturalist has his favorite illustration. In the West Indian land snail Cerion, for example,
populations living on rocky and windy coasts almost always develop white, thick, and relatively
squat shells for conventional adaptive reasons. We can identify at least two different developmental
pathways to whiteness from the mottling of early whorls in all Cerion, two paths of thickened shells
and three styles of allometry leading to squat shells. All 12 combinations can be identified in
Bahamian populations, but would it be fruitful to ask why -- in the sense of optimal design rather than
historical contingency -- Cerion from eastern Long Island evolved one solution, and Cerion from
Acklins Island another?
(5) Adaptation and selection, but the adaptation is a secondary utilization of parts present for reasons
of architecture, development or history. We have already discussed this neglected subject in the first
section on spandrels, spaces, and cannibalism. If blushing turns out to be an adaptation affected by
sexual selection in humans, it will not help us to understand why blood is red. The immediate utility
of an organic structure often says nothing at all about the reason for its being.
6. Another, and Unfairly Maligned, Approach to Evolution
In continental Europe, evolutionists have never been much attracted to the Anglo-American penchant
for atomizing organisms into parts and trying to explain each as a direct adaptation. Their general
alternative exists in both a strong and a weak form. In the strong form, as advocated by such major
theorists as Schindewolf (1950), Remane (1971), and Grassé (1977), natural selection under the
adaptationist programme can explain superficial modifications of the Bauplan that fit structure to
environment: why moles are blind, giraffes have long necks, and ducks webbed feet, for example. But
the important steps of evolution, the construction of the Bauplan itself and the transition between
Baupläne, must involve some other unknown, and perhaps "internal," mechanism. We believe that
English biologists have been right in rejecting this strong form as close to an appeal to mysticism.
But the argument has a weaker -- and paradoxically powerful -- form that has not been appreciated,
but deserves to be. It also acknowledges conventional selection for superficial modifications of the
Bauplan. It also denies that the adaptationist programme (atomization plus optimizing selection on
parts) can do much to explain Baupläne and the transitions between them. But it does not therefore
resort to a fundamentally unknown process. It holds instead that the basic body plans of organisms
are so integrated and so replete with constraints upon adaptation (categories 2 and 5 of our typology)
that conventional styles of selective arguments can explain little of interest about them. It does not
deny that change, when it occurs, may be mediated by natural selection, but it holds that constraints
restrict possible paths and modes of change so strongly that the constraints themselves become much
the most interesting aspect of evolution.
Rupert Riedl, the Austrian zoologist who has tried to develop this thesis for English audiences (1977
and 1975, translated into English by R. Jeffries in 1978) writes:
 The living world happens to be crowded by universal patterns of organization
which, most obviously, find no direct explanation through environmental conditions
or adaptive radiation, but exist primarily through universal requirements which can
only be expected under the systems conditions of complex organization itself... This is
not self-evident, for the whole of the huge and profound thought collected in the field
of morphology, from Goethe to Remane, has virtually been cut off from modern
biology. It is not taught in most American universities. Even the teachers who could
teach it have disappeared.
Constraints upon evolutionary change may be ordered into at least two categories. All evolutionists
are familiar with phyletic constraints, as embodied in Gregory's classic distinction (1936) between
habitus and heritage. We acknowledge a kind of phyletic inertia in recognizing, for example, that
humans are not optimally designed for upright posture because so much of our Bauplan evolved for
quadrupedal life. We also invoke phyletic constraint in explaining why no mollusks fly in air and no
insects are as large as elephants.
Developmental constraints, a subcategory of phyletic restrictions, may hold the most powerful rein of
all over possible evolutionary pathways. In complex organisms, early stages of ontogeny are
remarkably refractory to evolutionary change, presumably because the differentiation of organ
systems and their integration into a functioning body is such a delicate process so easily derailed by
early errors with accumulating effects. Von Baer's fundamental embryological laws (1828) represent
little more than a recognition that early stages are both highly conservative and strongly restrictive of
later development. Haeckel's biogenetic law, the primary subject of late nineteenth century
evolutionary biology, rested upon a misreading of the same data (Gould, 1977). If development occurs
in integrated packages and cannot be pulled apart piece by piece in evolution, then the adaptationist
programme cannot explain the alteration of developmental programmes underlying nearly all
changes of Bauplan.
The German paleontologist A. Seilacher, whose work deserves far more attention than it has received,
has emphasized what he calls "bautechnischer," or architectural constraints (Seilacher, 1970). These
arise not from former adaptations retained in a new ecological setting (phyletic constraints as usually
understood), but as architectural restrictions that never were adaptations but rather were the
necessary consequences of materials and designs selected to build basic Baupläne. We devoted the
first section of this paper to nonbiological examples in this category. Spandrels must exist once a
blueprint specifies that a dome shall rest on rounded arches. Architectural constraints can exert a farranging influence upon organisms as well. The subject is full of potential insight because it has rarely
been acknowledged at all.
In a fascinating example, Seilacher (1972) has shown that the divaricate form of architecture occurs
again and again in all groups of mollusks, and in brachiopods as well. This basic form expresses itself
in a wide variety of structures: raised ornamental lines (not growth lines because they do not conform
to the mantle margin at any time), patterns of coloration, internal structures in the mineralization of
calcite and incised grooves. He does not know what generates this pattern and feels that traditional
and nearly exclusive focus on the adaptive value of each manifestation has diverted attention from
questions of its genesis in growth and also prevented its recognition as a general phenomenon. It must
arise from some characteristic pattern of inhomogeneity in the growing mantle, probably from the
generation of interference patterns around regularly spaced centers; simple computer simulations can
generate the form in this manner (Waddington and Cowe, 1969). The general pattern may not be a
direct adaptation at all.
Seilacher then argues that most manifestations of the pattern are probably non-adaptive. His reasons
vary but seem generally sound to us. Some are based on field observations: color patterns that remain
invisible because clams possessing them either live buried in sediments or remain covered with a
periostracum so thick that the colors cannot be seen. Others rely on more general principles: presence
only in odd and pathological individuals, rarity as a developmental anomaly, excessive variability
compared with much reduced variability when the same general structure assumes a form judged
functional on engineering grounds.
In a distinct minority of cases, the divaricate pattern becomes functional in each of the four categories.
Divaricate ribs may act as scoops and anchors in burrowing (Stanley, 1970), but they are not properly
arranged for such function in most clams. The color chevrons are mimetic in one species (Pteria zebra)
that lives on hydrozoan branches; here the variability is strongly reduced. The mineralization
chevrons are probably adaptive in only one remarkable creature, the peculiar bivalve Corculura
cardissa (in other species they either appear in odd specimens or only as post-mortem products of shell
erosion). This clam is uniquely flattened in an anterio-posterior direction. It lies on the substrate,
posterior up. Distributed over its rear end are divaricate triangles of mineralization. They are
translucent, while the rest of the shell is opaque. Under these windows dwell endosymbiotic algae!
All previous literature on divaricate structure has focused on its adaptive significance (and failed to
find any in most cases). But Seilacher is probably right in representing this case as the spandrels,
ceiling holes, and sacrificed bodies of our first section. The divaricate pattern is a fundamental
architectural constraint. Occasionally, since it is there, it is used to beneficial effect. But we cannot
understand the pattern or its evolutionary meaning by viewing these infrequent and secondary
adaptations as a reason for the pattern itself.
Galton (1909, p. 257) contrasted the adaptationist programme with a focus on constraints and modes
of development by citing a telling anecdote about Herbert Spencer's fingerprints:
 Much has been written, but the last word has not been said, on the rationale of
these curious papillary ridges; why in one man and in one finger they form whorls
and in another loops. I may mention a characteristic anecdote of Herbert Spencer in
connection with this. He asked me to show him my Laboratory and to take his prints,
which I did. Then I spoke of the failure to discover the origin of these patterns, and
how the fingers of unborn children had been dissected to ascertain their earliest
stages, and so forth. Spencer remarked that this was beginning in the wrong way; that
I ought to consider the purpose the ridges had to fulfill, and to work backwards.
Here, he said, it was obvious that the delicate mouths of the sudorific glands required
the protection given to them by the ridges on either side of them, and therefrom he
elaborated a consistent and ingenious hypothesis at great length. I replied that his
arguments were beautiful and deserved to be true, but it happened that the mouths of
the ducts did not run in the valleys between the crests, but along the crests of the
ridges themselves.
We feel that the potential rewards of abandoning exclusive focus on the adaptationist programme are
very great indeed. We do not offer a council of despair, as adaptationists have charged; for nonadaptive does not mean non-intelligible. We welcome the richness that a pluralistic approach, so akin
to Darwin's spirit, can provide. Under the adaptationist programme, the great historic themes of
developmental morphology and Bauplan were largely abandoned: for if selection can break any
correlation and optimize parts separately, then an organism's integration counts for little. Too often,
the adaptationist programme gave us an evolutionary biology of parts and genes, but not of
organisms. It assumed that all transitions could occur step by step and underrated the importance of
integrated developmental blocks and pervasive constraints of history and architecture. A pluralistic
view could put organisms, with all their recalcitrant yet intelligible complexity, back into evolutionary
theory.
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(Noot )
Als de GR nu opeens niet meer zou werken zou dat absoluut problemen opleveren in het
lichaam.
Het klopt dat cortisol ook kan binden aan de MR, maar die affiniteit is lager,
de MR zit op andere plaatsen in het lichaam en bovendien zet de MR intracellulair een andere
pathway ingang.
Kort gezegd, de functie van de MR is anders dan die van de GR.
Wat nou direct het grootste probleem zou opleveren bij het niet werken van de GR zou ik niet
precies weten.
Er zijn dus nog steeds in ons gestel receptoren aanwezig die én cortisol én aldosteron
herkentnen : de in het artikel beschreven cortisol-receptor stamt van een dergelijke maar dan
wel oudere versie af.
De mogelijkheid om aldosteron te binden werd ook niet verloren:
de oude stamreceptor-drager van beide , bracht twee nieuwe soorten receptors voort.
Later zag ik(=Pierra) dat deze auteur (Joe Thornton) ook de stamboom van deze andere
receptor gepubliceerd heeft, ik meen ook in Nature...
Bronnen:(Van Pierra ) New Scientist Nu.nl ScienceDaily
LORD HOWE STICK INSECT
http://whyevolutionistrue.wordpress.com/2012/03/01/a-giant-insect-saved-from-extinction/
http://vimeo.com/14413689
http://vimeo.com/6260138
http://en.wikipedia.org/wiki/
Dryococelus_australis
http://www.nhm.ac.uk/nature-online/species-of-the-day/biodiversity/endangered-species/dryococelusaustralis/index.html
http://www.arkive.org/lord-howe-island-stick-insect/dryococelus-australis/
http://www.npr.org/blogs/krulwich/2012/02/24/147367644/six-legged-giant-finds-secret-hideawayhides-for-80-years
Eurycantha calcarata Lucas.
http://bunyipco.blogspot.com/2010/09/interesting-dilemma.html
http://www.youtube.com/embed/Cs1Xs3Eheag
http://www.youtube.com/embed/5apWTc23mHU
Fossil GLADIATOR
http://lemondedesphasmes.free.fr/spip.php?article69
Raptophasma Kernegerri Zompro
2001 Baltic amber
Misverstanden rond "levende" fossielen
http://sandwalk.blogspot.com/2012/04/myth-of-living-fossils.html
The general public has been told time and time again that there exist among us certain species that
have not evolved for millions of years. These so-called "living fossils" have somehow managed to
avoid any changes in the frequencies of alleles in their evolving populations. This is, of course,
impossible by any reasonable definition of evolution, a conclusion that was promoted on talk.origins
two decades ago [Claim CB930:].
Yet the myth persists. It persists for three reasons:
It plays into the popular misconception that natural selection is synonymous with evolution. If a species
isn't adapting by obvious changes over time then it isn't evolving. Another way of saying this is that
some species can be so perfectly adapted to their environment that all changes are selected against
and negative selection prevents evolution.
External morphological changes are the only evidence of evolution.?
The so-called "living fossils" show no evidence of morphological change over millions of years when,
in fact, all of the popular examples show plenty of evidence of such change. In other words, the facts
are misrepresented