What does evolution explain ?
Extending the evolutionary theory.
Philippe Huneman
IHPST (CNRS/ Université Paris I
Sorbonne)
• EXTENDED evolutionary theory.
Dawkins 1982; Turner 2001; Sterelny et al. 1996;
Odling-Smee et al. 2003; Pigliucci 2007; Gould
2002 (« expansion »)
Extending the explananda ? Extending the
explanantia ?
I. THE EXPLANANDA – AND
ESPECIALLY DIVERSITY
Evolutionary theory explains (Lewontin) :
Adaptation
Diversity
Transformations
Evolutionary theory explains (Lewontin) :
Adaptation
-> explained (defined ?) by natural selection
Diversity
Adaptive radiation, « principle of divergence »
Evolution
-> several causes, the major one being natural selection
Diversity
• Variety, but not much novelty (Darwin)
-> unity through diversity
•
Finally, then, although in many cases it is most difficult even to conjecture by what
transitions organs could have arrived at their present state; yet, considering how
small the proportion of living and known forms is to the extinct and unknown, I
have been astonished how rarely an organ can be named, towards which no
transitional grade is known to lead. It is certainly true, that new organs appearing
as if created for some special purpose rarely or never appear in any being; as
indeed is shown by that old, but somewhat exaggerated, canon in natural history
of "Natura non facit saltum." We meet with this admission in the writings of
almost every experienced naturalist; or, as Milne Edwards has well expressed it,
"Nature is prodigal in variety, but niggard in innovation." Why, on the theory of
Creation, should there be so much variety and so little real novelty? Why should all
the parts and organs of many independent beings, each supposed to have been
separately created for its own proper place in nature, be so commonly linked
together by graduated steps? Why should not Nature take a sudden leap from
structure to structure? On the theory of natural selection, we can clearly
understand why she should not; for natural selection acts only by taking advantage
of slight successive variations; she can never take a great and sudden leap, but
must advance by the short and sure, though slow steps.
Origin of species (6th ed.) ch. 6.
Similarity, through difference : The concept of
homology: ‘the same organ in different
animals under every variety of form and
function’. (Owen 1843)
Distinguishing homology and analogy (adaptive
convergence)
Diversity
Idea of a morphospace
Diversity
Idea of a morphospace
Generalised morphospace
Issue : The clustering within the morphospace
The Clustering question : Why ?
Diversity, 2
The pattern of branching
What is the explanans ????
5 Organisms (taxa, specimens) A, B, C, D, E
Possible comparisons -> (ab) (cde)), (abc (de)), (a
(bcde)) (ab ((cd)e)) etc.
• (A (B (CD))
• (A (CB) D)
• (A (BC) D)
• (A (D (BC)) = (A (DBC)) + (A D (BC))
• Suppose you have (A (DB) C) also … : then no
possible branching pattern
The Branching question
• The fact is that we have something like a tree !
Why ??
-> Common descent (by any means ??).
Plant cladogram
Vertebrates cladogram
Partial cladograms
Dinosaurs
• Evolution = two questions about rare facts in
spaces of possible spaces
• Are there equal ? Or is an answer to the
Clustering question not enough to answer the
Branching question ?
• Is there any process likely to answer the two
questions ?
II. DARWINIAN EVOLUTIONARY
THEORY EXPLAINING DIVERSITY
Darwin’s view
• Unity through diversity ?
The Conditions of existence and the Unity of
type
(E.S. Russell Form and function, 1916; Cuvier vs.
Geoffroy St Hilaire 1830)
Forme
(Function)
Geoffroy St Hilaire
Unity of type
Cuvier
Conditions of existence
Common descent
Natural selection
• It is generally acknowledged that all organic beings have been formed on
two great laws Unity of Type, and the Conditions of Existence. By unity of
type is meant that fundamental agreement in structure, which we see in
organic beings of the same class, and which is quite independent of their
habits of life. On my theory, unity of type is explained by unity of descent.
The expression of conditions of existence, so often insisted on by the
illustrious Cuvier, is fully embraced by the principle of natural selection.
For natural selection acts by either now adapting the varying parts of each
being to its organic and inorganic conditions of life; or by having adapted
them during long-past periods of time: the adaptations being aided in
some cases by use and disuse, being slightly affected by the direct action
of the external conditions of life, and being in all cases subjected to the
several laws of growth. Hence, in fact, the law of the Conditions of
Existence is the higher law; as it includes, through the inheritance of
former adaptations, that of Unity of Type.
• Chapter VI. Origin of species
• Darwin’s thesis : « unity of type » is subsumed
under natural selection
• What is « common descent » for two traits at
a same phylogenetic level
….resorts to « natural selection » when
considering the first stages (plesiomorphic
states) of the trait…
• Thus, we can hardly believe that the webbed feet of the upland goose or
of the frigate-bird are of special use to these birds; we cannot believe that
the same bones in the arm of the monkey, in the fore leg of the horse, in
the wing of the bat, and in the flipper of the seal, are of special use to
these animals. We may safely attribute these structures to inheritance.
But to the progenitor of the upland goose and of the frigate-bird, webbed
feet no doubt were as useful as they now are to the most aquatic of
existing birds. So we may believe that the progenitor of the seal had not a
flipper, but a foot with five toes fitted for walking or grasping; and we may
further venture to believe that the several bones in the limbs of the
monkey, horse, and bat, which have been inherited from a common
progenitor, were formerly of more special use to that progenitor, or its
progenitors, than they now are to these animals having such widely
diversified habits. Therefore we may infer that these several bones might
have been acquired through natural selection, subjected formerly, as now,
to the several laws of inheritance, reversion, correlation of growth, &c.
• Consequences : micro and macroevolution
• Darwin’s gradualism
• The scale-free diagram – zoom in (processes)
and zoom out (branching pattern, homologies)
The Modern Synthesis view
Change in allelic frequencies as the core of evolution
« Population thinking » (Mayr) and population genetics: genes
and (alleles) populations are the two main explanatory
levels.
Irrelevance of development to evolution (what counts is the
phenotype’s effect on the replication rates of the
genotypes)
Natural selection is the main cause of the departures from
gene frequencies equilibria
About processes: macroevolution as
extrapolation from microevolution (Mayr,
Simpson)
About patterns : Gradualism
III. WHAT WOULD BE AN
ALTERNATIVE VIEW ?
Population thinking and typology
• Amundson’s classification (« Homology and
homoplasy : a philosophical perspective » 2004):
typology = homology more important than
natural selection
« Typologists were united not by metaphysical or anti-evolutionary
commitments, but by a belief in the importance of homology over
adaptations”
• Micro vs macro evolution = is adaptation different
than « novelty »? (Muller and Newman 2003)
Should novelty need another explanans ?
Forme
Function
Geoffroy St Hilaire
Unity of type
Cuvier
Conditions of existence
Common descent
Natural selection
Homology
Unity
Typological thinking
Adaptive radiation
Diversity
Population thinking
• MS : explains diversity through adaptation
(radiations…)
• Explains unity by common descent (and in fine
natural selection…)
• Typologist : explains unity (in diversity) by
commonalities of structure (developmental)
The role of development
• MS view : natural selection explains the
branching (through common descent) and
(through both homologies and analogies) the
clustering
• (Developmental) typologist view : Common
developmental processes underpin
homologies (hence the branching) and
clustering
4. WHAT IS EMPIRICAL AND WHAT IS
« PURELY » CONCEPTUAL IN THOSE
CHALLENGES TO THE MODERN
SYNTHESIS ?
Extending vs rebuilding evolutionary
theory ?
Integrating :
• Development (Gilbert, Opitz & Raff 1996)
• Ecology (Odling Smee et al. 2003; Hubbell
2001)
• Both(Eco evo devo, Gilbert 2008)
Several cleavages
• Genes / epigenetics (Gilbert; Kirschner & Gerhardt 2005)
• Genes / developmental systems (DST)
• Genes / Genomics (Fox Keller 2000, Stolz, etc.): what if
« genes » don’t exist any more in molecular genetics ?
• Genes / organisms (West Eberhardt 2003; Walsh 2008;
Odling Smee et al. (2003))
(Gould & Lewontin 1979)
• Forms / Genes (Pigliucci 2007)
• Structure/ Function (adaptation) (Amundson 2005) or
Externalism vs. Internalism (Godfrey Smith 1996)
The problem
Are all those « challenges » to the modern
synthesis likely to be synthesised ?
What hangs upon empirical findings in those
question ?
A common thread: Against « gene
centrism »
1. In defining evolution
• Evolution is not « change in allelic frequencies » (i)
(a definition already controversial among MS writers : “Evolution is not
a change in gene frequencies, as is claimed so often, but the
maintenance (or improvement) of adaptedness and the origin of
diversity. Changes in gene frequency are a result of such evolution,
not its cause.” (Mayr 1998, 2093)).
But
• Change in developmental pathways (Caroll 2005) (ii)
• Change in bodyplans (iii)
Etc.
(No more assuming that i entails ii and iii)
A consequence :
• Macro evolution is not immediately derived
from microevolution (vs. Mayr, Simpson,
Dobzhanski)
• The challenges are in the same time about the
patterns of evolution
(here, emphasis on the processes; see Gayon
2008)
Against gene centrism
2. In explaining evolution
a. Development is as important as genotypes
b. (Evolution is not led by genes)
c. Organism is a fundamental level of evolution;
About c. : Darwin vs. Neodarwinism (Ariew 2008;
Pigliucci 2007)
d.Evolutionary genes don’t match genes from
genomics/molecular genetics (pb. for a theory of
variation and inheritance ?)
Two options:
1) explain other explananda (e. g. 1.a Form, the unity of
forms ; or a synthesis of allelic evolution and form, or
adaptation and form) (e.g. Amundson 2005; Pigliucci
2007;
1.b, novelty (Newman and Muller 2003; Watson 2005))
2) explain differently (some of) the same explananda
• 2a. Another explanans than natural selection (Kauffmann;
walsh; odling smee et al. ?; Gould ?; Jablonka & Lamb ?;
• 2b. Natural selection differently understood is an explanans
(DST; Caroll 2005; West Eberhardt ?)
• (1) calls for a synthesis of MS and something
else in order to build a general evolutionary
biology
• (2a) needs to extend MS
• (2b) means a re-writing of MS
What may be the targets of criticism ?
The 2 controversial gene-related theses in the Modern
Synthesis :
Darwinism (natural selection as a major cause of evolution,
adaptation and diversity)
Mendelism (Particular inheritance)
-> genes are the substrate of inheritance
Weissmanism (germen soma distinction)
-> development does not impinge on evolution
(p)
(q)
The typological developmentalist attacks q.
But :
Possibly : very sophisticated developmental
phenomena (involving genes, epigenetics etc.)
without making development relevant for
evolution
-> the argument from genotype phenotype
maps
Gspace, Pspace
Gspace, Pspace, with Dspace
Dspace is evolutionary irrelevant
Genotype space
Phenotype
space
Developmental
space
Gspace, Pspace, with Dspace
Dspace is evolutionary relevant
CONCLUSION
- Plurality of alternatives to MS (is there one
typologism?)
- Not all have the same explananda (even when it
comes to diversity)
- The emphasis on development as explanation for
evolution and unity-through-diversity is
contingent upon empirical facts concerning P-D-G
spaces mappings.