Chapter 17
Origin of Species
Figure CO: Mimulus Flower
© LubaShi/ShutterStock, Inc.
Overview
• A few of Darwin’s predecessors had broken
with western civilization’s and the Christian
religion’s tradition of seeing species as
unchanging and having originated from
separate creations
• For Darwin and Wallace, speciation was the
process by which new forms arose from
ancestors, and natural selection was the main
means by which speciation occurred
Overview
• Darwin and his contemporaries amassed
evidence that adaptive radiations occurred
when a species moved to a new environment,
or the current environment changed, so that
new niches and their resources could be
utilized
• Darwin also demonstrated that evolutionary
change could be driven by sexual selection,
even when sexually selected phenotypic traits
seemed maladaptive to general survival
Overview
• Proponents of the Modern Synthesis
established that:
– the Hardy-Weinberg Principle and its underlying
assumptions provided the model for identifying
causes of evolutionary changes in natural
populations
– natural selection had three modes: stabilizing,
directional and disruptive
– reproductive modes and sexual behaviors contribute
to changes in genotype and phenotype
– some evolutionary changes in natural populations
occur in the absence of natural or sexual selection
Adaptation and Differentiation
• Adaptation to a changing environment, in the
simplest case, will be driven by directional
selection, and if enough change in genotypes
and phenotypes occurs, an ancestral species will
be transformed into a new descendant species
• Adaptive radiation signifies the rapid evolution
of one or a few forms into many different
species occupying a variety of habitats within a
new geographical area
What Interests Us About Speciation?
• Speciation provides evidence that evolution
occurs
• Speciation provides important insights into
the mechanisms of evolution
• Patterns of speciation provide insight into the
distribution patterns of extinct and living
organisms
• Speciation explains patterns in the ecology
and reproductive biology of organisms
What Interests Us About Speciation?
• What are the causes of speciation?
• What are the rates of speciation and do they
differ among different taxa?
Speciation
• The evolutionary formation of new species in
space or time, usually by the division of a single
species into two or more genetically distinct
ones
• Species share the same gene pool, or the sum of
all genetic codes possessed by individual
members of that species
• Speciation both marks and crosses the
boundary between microevolution and
macroevolution
The Species Problem
• Although most biologists accept the species as a real
biological entity, just as they do the individual
organism, it has not been easy to define the species
or to indentify the boundaries between species
– often we still lack adequate data
• Anti-evolutionists seize on these difficulties to claim
evolution does not occur
• They miss the point; the nature of reality is difficult
to pin down, even for scientists!
– Physicists, chemists and geologists would agree
Species Definitions
• Morphological / Typological Species: a set of organisms
sharing structural similarities between members and
discontinuities in structure between different species
• Mayr’s Biological Species: groups of interbreeding natural
populations that are reproductively isolated from other such
groups
• Ecological Species: a set of organisms adapted to a particular
set of resources, called a niche, in the environment
• Genetic species: A set of organisms exhibiting similarity of DNA
Species Definitions
• Agamospecies: a set of organisms in which sexual reproduction
does not occur, represented typically as a collection of clones
• Chronospecies / Paleospecies*: a set of extinct organisms which
changes in morphology, genetics, and/or ecology over time on an
evolutionary scale such that the originating species and the
species it becomes could not be classified as the same species
had they existed at the same point in time
– [*Note: experts establish fine distinctions between chronospecies and paleospecies]
• Phylogenetic (Cladistic) / Evolutionary Species: a set of
organisms that shares a common ancestor and maintains its
integrity with respect to other lineages through both time and
space
• Ring Species: a set of generally hybridizing species with a
geographic distribution that forms a ring and overlaps without
hybridization at the ends
An Important Reminder
• Regardless of species definitions, a species, to be a
biological entity, must exist in an ecological niche
• Can a population of organisms which no longer has a
niche still be a biological entity?
• Only 400-500 Siberian tigers still exist within their range
Problems Defining Species Through Time
a) Morphospecies:
Viewed today, at
one moment in
time, species A, C,
and E are clearly
distinct species,
demarcated by
current natural
discontinuities
between them
Problems Defining Species Through Time
b) Paleospecies
(chronospecies):
Viewed historically,
through time,
discovered fossil
intermediates (B and
D) fill in the missing
gaps above, giving us
a more or less
continuous series
with no obvious
morphological
discontinuities
between them [still
an oversimplification]
The Process of Species Formation
1. In the beginning, there is a single population with a
common shared gene pool
2. A discontinuity develops among some demes
3. Changes in allele frequencies develop at various loci in
the gene pools (and, usually, changes in phenotypes) of
the demes
4. Separate evolution of demes continues until one or more
have diverged to the point that each deme now meets
one of the definitions of a species concept
Species Can Change Without
Speciation Being Initiated
• Local adaptations may occur among the
populations and demes of a species
• Even when a species consists of many
subpopulations, gene flow between
subpopulations may slow or even inhibit local
specializations
Species Can Change Without
Speciation Being Initiated
• Gene flow promotes continuance and stability
of the species
Speciation
• Branching speciation
and adaptive radiations
can be initiated when
genetic exchange within
or among populations is
impeded
• Directional selection
produces anagenesis
• Disruptive selection
produces cladogenesis
Speciation
• Four different processes have been defined to
describe different modes of speciation
(cladogenesis) in sexually reproducing
species, most of which satisfy the definition
of Mayr’s Biological Species Concept
• These same four processes may also explain
speciation in many asexual organisms, but
isolating mechanisms will be different
• The processes differ in how the evolving
demes are distributed geographically
Spatial Aspects of Speciation
• Allopatric speciation – a physical barrier
divides a continuous population
• Peripatric speciation – a small founding
population enters a new or isolated niche
• Parapatric speciation – a new niche found
adjacent to the original niche
• Sympatric speciation - speciation occurs
without physical separation inside a
continuous population
Spatial Aspects of Speciation
As you can see in this
diagram, peripatric
and parapatric are
very similar
Peripatric does
involve crossing
some sort of barrier
to find favorable
habitat
Speciation Initiated
by Geographical Isolation
• three steps
– geographical isolation
– local adaptation
– reproductive isolation
• three forms of geographical isolation
sometimes called a porous
or permeable barrier
Figure 01C:
Parapatric
Figure 01A: Allopatric
Figure 01B: Peripatric
Allopatric Speciation
• Allopatric speciation: when an ancestral species population is
divided because of a natural physical barrier or because
intervening geographical populations become extinct
• This produces vicariance distributions
Allopatric Speciation
• Since smaller habitats generally are somewhat
different from one another, natural selection
will alter the demes on the opposite sides of
the barrier over time to improve their
adaptations to the different habitats
Allopatric Speciation
• Mechanisms that prevent interbreeding may be:
• ecological:
• seasonal breeding, migration, etc.
• habitat preference
• differing abiotic factors, etc.
• behavioral
• activity times
• food acquisition, etc.
• physiological
• reproductive biology
• fertilization
• embryonic development
[More on this later in the chapter.]
Allopatric Speciation
Four steps lead to speciation:
1.
2.
3.
4.
A single species is an
interbreeding reproductive
community
A barrier develops, or a
dispersal event occurs, dividing
the species
Separated into different
habitats, the divided
populations diverge through
the accumulation of gene and
trait differences
The separate populations
become so different that, if
and when the barrier
disappears and they overlap
again, interbreeding does not
occur
mountains and deserts are classic examples of new barriers
Allopatric Speciation
• The populations of Tamarin
monkeys (family Callitrichidae)
are separated on the sides of
the Amazon River
• Where the river tributary is
wide and individuals on
opposite banks do not
interbreed, the populations
are diverging toward separate
species
• Where the river tributary is
narrow, the individuals still
interbreed
Allopatric Speciation
• Vicariance events
provide complete
geographic isolation for
two robber fly genera,
Tillobroma and
Hypenetes
Allopatric Speciation
American
dogwood,
Cornus florida
Chinese
dogwood,
Cornus kousa
We mentioned a similar vicariant distribution for Catawba trees in CH 16
Allopatric Speciation
American alligator, Alligator mississippiensis
Chinese alligator, Alligator sinensis
Allopatric Speciation
American hellbender,
Cryptobranchus alleganiensis
A. japonicus
Andrias davidianus
Chinese giant salamander , Andrias davidianus and Japanese giant
salamander Andrias japonicus
Incomplete or Peripheral Isolation
• Ernst Mayr and Theodosius Dobzhansky,
speaking for the Modern Synthesis,
emphasized that most speciation was
allopatric, i.e., geographical isolation required
• Two modifications or variants have been
proposed:
– Peripatric speciation – a small population enters a
new or isolated niche
• [originally proposed by Mayr, and related to the
founder effect and genetic drift altering the isolate’s
gene pool]
– Parapatric speciation – a new niche found
adjacent to the original niche
Incomplete or Peripheral Isolation
Incomplete or Peripheral Isolation
• Peripatric speciation: when a population is
divided because of the budding off of a small
completely isolated founder colony from a
larger population so that gene flow is minimal
• Parapatric speciation: when a population at
the periphery of a species adapts to a
different environment but remains contiguous
with its parent so that gene flow is possible
between them
Peripatric vs. Parapatric speciation
• Peripatric speciation is caused
by being at the edge of the
range and almost isolated
geographically ↔ geographic
isolation leads to genetic
isolation
• Parapatric speciation is by
becoming genetically isolated
which leads the population to
become geographically
isolated ↔ genetic isolation
leads to geographic isolation
Both are less common and more difficult to demonstrate since
small niche and habitat differences rarely have fossil records
Peripatric and Parapatric Speciation
• Speciation triggered by
partial isolation (peripatric
and parapatric) are now
argued as being as or more
important in explaining
speciation events than
classic allopatric speciation
• Eldridge and Gould’s
Theory of Punctuated
Equilibria is just one
example of advocating for
their importance
dispersal
or
vicariance
Systematists and paleontologists need more
data to resolve that technical debate
Peripatric Speciation
• sweepstakes dispersal
events provide complete
geographic isolation and
reduce migration to
near zero
Peripatric Speciation
Northern Chestnut-tailed Antbird
Myrmeciza castanea
dispersal
• Potential peripatric speciation triggered by partial isolation in a
Bolivian natural forest island isolated 3000 years from the larger
continuous forest habitat during a dry period
• Divergence in song and certain alleles frequencies between the two
populations (reproductive isolation) suggest that incipient speciation
is under way (Southern Chestnut-tailed Antbird, Myrmeciza
hemimelaena and Northern Chestnut-tailed Antbird, Myrmeciza
castanea)
Peripatric
Speciation
Two male hammer-headed picture-wing
Drosophila battle for mating territory.
Figure B01A: “Picture-winged" group of Drosophila
Figure B01B: “Picture-winged" group of Drosophila
Figure B01C: “Picture-winged" group of Drosophila
Modified from Edwards K.A, Doescher L.T., Kaneshiro K.Y., Yamamoto
D., (2007) A Database of Wing Diversity in the Hawaiian Drosophila.
PLoS ONE 2(5): e487. doi:10.1371/journal.pone.0000487. Courtesy of
Kevin Edwards , Illinois State University.
Figure B02: “Picture-winged" group of Drosophila
Adapted from Carson, H.L. Drosophila Inversion Polymorpism. CRC Press, 1992.
Parapatric Speciation
dispersal
• Potential parapatric speciation in sweet vernal
grass/buffalo grass, Anthoxanthum odoratum,
triggered by adaptation to heavy metal contaminated
soils in many locations globally
• Divergence in flowering times (reproductive
isolation) between the two populations suggests that
incipient speciation is under way
Parapatric Speciation
Adjacent populations evolve into distinct species
while maintaining contact along a common border
BULLOCK’S
ORIOLE
HYBRID
ZONE
BALTIMORE
ORIOLE
Ring Species—Salamanders
• The ensatina salamander
(Ensatina eschscholtzii) occurs
from Canada to Southern
California with interbreeding
between adjacent populations
through this range
• The Central Valley—a dry, hot
lowland area—is divided into a
coastal arm and inland arm
• However, where these two
arms of the species meet again
in Southern California,
interbreeding does not occur
• Ring species are often
considered examples of
parapatric speciation
Ring Species Herring Gulls
•
•
•
•
•
As glaciers retreated, herring gulls (Larus
argentatus) were released out of a north
Pacific refugia spreading one way across
North America and into western Europe; and
spreading in the other direction across Alaska
into Siberia
From Siberia, as the herring gull now
extended its range further across Asia, it
tended to differentiate, producing a
subspecies (or species by some
ornithologists) such as the vega gull (Larus
vegae) and farther west the lesser
blackbacked gull (Larus fuscus)
Eventually its current circumpolar distribution
became established (dashed lines)
Adjacent subspecies interbreed (solid
arrows), but where the ends of the circular
range of the herring gull meet and overlap in
Europe, there is very little interbreeding
(dotted lines). (Simplified originally from
Mayr, 1963)
Ring species are often considered examples
of parapatric speciation
Speciation Without Geographical Isolation
• An important debate in evolutionary biology has
been whether speciation can be initiated
sympatricly by mechanisms that reduce gene
flow within a population in the absence of
initiating geographical isolation
• Can a deme experiencing disruptive selection
accumulate gene pool change without the allele
frequency changes being swamped by migration
into the deme from neighboring demes?
Speciation Without Geographical Isolation
• Recall that the other modes of speciation require three
steps:
1. geographical isolation
2. local adaptation
3. reproductive isolation
• Is it possible to skip the first step and have speciation
without any geographical isolation within the
continuous ancestral population?
• The biometricians thought so . . .
Sympatric Speciation
• Sympatric speciation is the
process through which new
species evolve from a single
ancestral species while
inhabiting the same geographic
region
• there is no geographic constraint
to interbreeding
• the term was invented by the
British entomologist Sir Edward
Bagnall Poulton (1856-1943) in
1904
Sympatric Speciation
• The first model was put forward by John
(1920 – 2004)
Maynard Smith in 1966
• Maynard Smith suggested that homozygotes
(AA and aa) might, under particular
environmental conditions, have a greater
fitness than heterozygotes (Aa) for a certain
trait
• Because of disruptive selection, therefore,
homozygotes would be favored over
heterozygotes, eventually leading to speciation
Sympatric Speciation
• Ernst Mayr completely rejected sympatry
• Debate continues on how important and widespread
sympatric evolution may be
• But well documented empirical evidence for sympatric
evolution exists, and sophisticated theories incorporating
multilocus genetics have been developed
• Australian biologist Michael J.D. White (1910-1983) wrote
one of the first books to document the evidence: Modes of
Speciation (1978)
Sympatric Speciation
• Sympatric divergence could also result from
sexual selection
• Reproductive isolation may develop through
changes in behavior, microhabitat, seasonality
of breeding, or chromosomal mutation or
ploidy events
• Sympatric speciation is common in plants; less
common in animals
• It is often difficult to confirm sympatric origin
Sympatric
Speciation
4N
4N
• The composites, salsify plants, from eastern Washington
include a tetraploid hybrid derived from two diploid species
• The many polyploid hybrids species such as these are the
best examples of sympatric speciation, including a few
animal polyploid species
Sympatric Speciation
• Crater lakes and oceanic islands provide optimal locations for
studying sympatric speciation because differentiation between sister
taxa found at these locations is likely to have occurred in situ
• Clockwise from top left, Amphilophus citrenellus, A. zaliosus [cichlid
fish], Howea forsteriana, H. belmoreana [palms], Lord Howe Island, S.
Pacific, Craterlake Apoyo, Nicaraugua
Sympatric Speciation
• Sympatric African Indigobirds are host specific nest
parasites
• Their hosts rear their young but their young do not destroy
the host’s young, as cuckoos often do
Sympatric Speciation
Heliconius cydno alithea
• Polymorphic mimicry in Heliconius
cydno alithea in western Ecuador,
where the white form (middle
left) mimics the white species
Heliconius sapho (top left) and the
yellow form (middle right) mimics
the yellow species Heliconius
eleuchia (bottom right).
• Sympatric Neotropical Heliconius butterfly species are Mullerian
mimics
• Their common toxicity is cyanide derived from cyanoglucosides in
various Passiflora , passion flower vines, host plants eaten by the
larvae
• The two H. cydno alithea color morphs exhibit assortative mating
Cichlid Fish, Family Cichlidae
• To the right are cladograms
for the cichlids of Africa
(blue), the Neotropics
(green), Madagascar (pink)
and East Asia (yellow) to
the level of genus
• The upper cladogram uses
the fragmenting of
Gondwana as its basis
• The lower cladogram uses
the fossil record of the
cichlids
East African Crater Lake Cichlid Fish
Figure 02A: A cichlid Malawi (Maylandia estherae)
© Wawritto/Dreamstime.com
Figure 02B: A red terror (Cichlasoma festae) cichlid
© Savone/Dreamstime.com
Figure 02C: A blue cichlid (Cyphotilapia frontosa)
© Showkontor/Dreamstime.com
East African
Crater Lake
Cichlid Fish,
members of the
globally
distributed
tropical family
Cichlidae,
provide some of
the better
studied cases of
sympatric
speciation in
nature
East African Crater Lake Cichlid Fish
Figure 02D: A blue cichlid
(Pseudotrophues demasoni)
Figure B03: Head of a cichlid fish
© Moori/Dreamstime.com
Figure 02E: A red cichlid (Hemichromis sp.
Bangui)
© Schoor/Dreamstime.com
Figure 02F: A purple or kribensis cichlid
(Pelvicachromis pulcher)
© Ecophoto/Dreamstime.com
Fish with pharyngeal jaws can adapt their oral jaws to prey
capture or food acquisition while the pharyngeal jaws can
process the ingested food matter
East African Crater Lake Cichlid Fish
lower pharyngeal jaws and teeth of cichlid fish
Figure B04A: Lower pharyngeal
jaws and teeth of cichlid fish
Figure B04B: East African cichlid
Figure B04E: East African cichlid
Figure B04F: East African cichlid
Figure B04C: East African cichlid
Photos a, b and e, courtesy of Dr. Axel Meyer,
University of Konstanz and photos c, d and f, courtesy
of courtesy of Dr. Ann Huysseune, Ghent University
Figure B04D: East African cichlid
B and D show pharyngeal teeth
adapted to cutting or chopping while
the rest are adapted to grinding
East African Crater Lake Cichlid Fish
DNA and other characters confirm monophyly
Figure 03B: Cichlid species - cladogram
Adapted from Kocher, T. D., J. A. Conroy, K. R. McKaye,
and J. R. Stauffer, Mol. Phylog. Evol., 2 (1993): 158-165.
Figure 03A: Cichlid species from Lakes
Tanganyika and Malawi (Nyassa)
Above are two clades from two east African
lakes, Lakes Tanganyika and Malawi
(Nyassa)
There are many pairs of ecological
equivalents present
We presume they arose from a few
founders and then speciated without any
geographical isolation
Their speciation was likely driven by
ecological specialization, niche
partitioning and by assortative mating
Speciation Types:
Summary
• A population with common
gene pool
• Discontinuity develops among
subpopulations
• Different selection pressures
applied in different niches drive
evolutionary change
• Reproductive isolation develops
as the new species evolve
peripatric
Speciation Types: Summary
Recall that non-sympatric
speciation requires three steps:
geographical isolation
local adaptation
reproductive isolation
Notice that sympatric speciation
may end up producing the same
species distribution patterns as
allopatric speciation
Figure 04a-e: Two modes of speciation
Adapted from Strickberger, M. W. Genetics, Third edition. Macmillan, 1985.
This is one reason it has been
difficult to demonstrate sympatric
speciation
Speciation Types: Summary
All four modes of geographical speciation assume that
local adaptation and reproductive isolation will occur
Individual cases may show local adaptation to precede
reproductive isolation; others may show reproductive
isolation to precede local adaptation (assortative
mating, changes in ploidy, etc.); and still others may
show local adaptation and reproductive isolation to
occur simultaneously
Another accomplishment of the Modern Synthesis was to
characterize the mechanisms of reproductive isolation
Reproductive Isolating
Mechanisms (RIMs)
• Different types of mechanisms that can prevent reproduction
between individuals of different species
• RIMs are also referred to as prezygotic or premating, versus
fertilization or mating, versus postzygotic or postmating
mechanisms
• prezygotic or premating mechanisms
– geographic, ecological, behavioral, and temporal isolation
• (mating mechanisms) still prezygotic
– mechanical mechanisms and gamete incompatibility
• postzygotic or postmating mechanisms
– zygotic mortality, hybrid inviability, hybrid sterility, and
hybrid breakdown
Geographical (Reproductive) Isolation
Iguana iguana
Conolophus subcristatus
Amblyrhynchus cristatus
• The two Galapagos iguana genera are, themselves, ecologically
isolated from each other
Ecological (Reproductive) Isolation
Agkistrodon piscivorus
Agkistrodon contortrix
• Water or cotton-mouth
moccasin is semiaquatic, feeds on
aquatic vertebrates, and
is aggressive
• Copperhead is
terrestrial, feeds on
terrestrial vertebrates,
and is less aggressive
Behavioral (Reproductive) Isolation
Anolis garmani
Anolis opalinus
Anolis trinitatis
• Members of the genus Anolis
on Jamaica chose different
perches and use different
patterns of head bobbing to
attract female anoles
• They also have separate
ecological niches
Temporal (Reproductive) Isolation
13 year cicada
17 year cicada
• Members of the genus
Magicicada, exist in
temporaly separated
populations, three species of
17 year cicadas, and four
species of 13 year cicadas
• There is also some
geographical isolation within
the 17 year cicadas in the
northeastern US and the 13
year cicadas in the
southeastern US
Mechanical (Reproductive) Isolation
• Members of the genus
Parafontaria, Japanese
millipedes, differ in body size
and in the size and shape of
their reproductive gonopodia
Mechanisms Facilitating Reproductive
Isolation
• Prezygotic mechanisms may be somewhat more
advantageous to the species when they develop,
because they prevent individuals from mating with
members of the other species – this saves energy
and prevents the waste of gametes
• Postzygotic mechanisms may be less advantageous
from that perspective, but, on the other hand, may
allow some alleles to pass from one species to the
other, a form of horizontal gene transfer often called
introgression
Introgression
• A 2010 study by Svante Pääbo and his colleagues confirms that
genetic “introgression” occurred in the human lineage, and
that between 1% and 4% of the DNA (SNPs) of Europeans,
Asians, and Papua New Guineans is attributable to
hybridization between “modern” humans and Neanderthals.
Mechanisms Facilitating
Reproductive Isolation
• On Isla Daphne in the Galapagos, hybrids formed between the
medium ground finch and the cactus finch
• The hybrids were intermediate in phenotype for bill size and
inferior competitors in normal years
• After a very rainy season, however, their numbers increased
because their intermediate sized bill was best adapted to the
plants that flourished in the wet
• Because female hybrids tended to mate with non-hybrid
males with distinctive species specific mating songs, genes
passed from one species to the other
• This horizontal gene transfer is probably beneficial to both
species, even though its occurrence is accidental
Reproductive Isolating
Mechanisms (RIMs)
• Prezygotic mechanisms: Factors which prevent individuals from
mating
– Geographical isolation already discussed
– Temporal isolation; Ecological isolation; Behavioral isolation; Mechanical
isolation ―
– Gametic incompatibility: Sperm transfer takes place, but the egg is not
fertilized
• Postzygotic isolating mechanisms: Genomic incompatibility,
hybrid inviability or sterility
–
–
–
–
Zygotic mortality: The egg is fertilized, but the zygote does not develop
Hybrid inviability: Hybrid embryo forms, but is not viable
Hybrid sterility: Hybrid is viable, but the resulting adult is sterile
Hybrid breakdown: First generation (F1) hybrids are viable and fertile, but
further hybrid generations (F2 and backcrosses) are inviable or sterile
Zygotic Mortality
In sheep and goat crosses fertilisation takes place but the hybrid
embryos die in the early developmental stages (Dobzhansky, Ayala, Stebbins
and Valentine, 1977. Evolution. W. H. Freeman)
Hybrid Inviability
Drosophila simulans
Drosophila melanogaster
Drosophila melanogaster and D. simulans have
incompatible nuclear pore protein alleles
Hybrid
Inviability
• The four groups of leopard frogs (Rana sp.) resemble one another closely
in their external appearance
• But early tests of interbreeding produced defective embryos (hybrid
inviability) in some combinations, leading biologists to suspect that these
might be different subspecies or even different species
• Research on males’ mating calls indicates that the various groups differ
substantially, and that such prezygotic behavioral isolation separates and
reproductively isolates members of each group, producing four species:
(1) Rana pipiens; (2) Rana blairi; (3) Rana utricularia; (4) Rana berlandieri
Hybrid Sterility
mule
hinny
liger
tigon
These hybrids have reduced, if not absent, fertility, though they are often otherwise healthy
Hybrid Breakdown
• In the parasitoid wasp genus Nasonia,
F2 hybrid males of Nasonia vitripennis
and Nasonia giraulti experience an
increased larval mortality rate relative
to the parental species. Previous
studies indicated that this increase of
mortality is a consequence of
incompatibilities between multiple
nuclear loci and cytoplasmic factors of
the parental species, but could only
explain ∼40% of the mortality rate in
hybrids with N. giraulti cytoplasm.
Hybrid larvae that carry the
incompatible allele on chromosome 5
halt growth early in their
development and ∼98% die before
they reach adulthood.
Nasonia giraulti
Reproductive Isolating
Mechanisms (RIMs)
• Over time, as a pair of sibling
species diverge, reproductive
isolation may increase by the
development of multiple forms
of both premating and
postmating isolating
mechanisms, as illustrated
here with two hypothetical
species of salamander
Reproductive Isolating Mechanisms (RIMs)
• normally, the barriers
separating species are not
caused by a single isolating
mechanism
–
–
–
–
seasonal
habitat
behavioral
mating
• isolating mechanisms have
been the most thoroughly
studied in Drosophila and in a
a variety of plants
Sexual Isolation in Sympatric and
Allopatric Populations
• Here is Drosophila data which
demonstrates that reproductive
isolating mechanisms are more
important when species arise in
sympatry
Figure 05: Sexual isolation for pairs of
allopatric and sympatric Drosophila
Adapted from Coyne, J. A., N. H. Barton and M. Turelli, Evolution 51 (1997): 643-671.
• The X axis, genetic distance for
species pairs (sister species)
indicates that allopatric
(geographically isolated) pairs may
have almost no sexual isolation
when their genetic distance is low,
but sympatric pairs have much
higher degrees of sexual isolation
Reproductive Isolating
Mechanisms (RIMs)
• Genetic and ecological isolation
may be occurring at the same time,
or before or after reproductive
isolating mechanisms form
• Not all RIMs are required for any
particular speciation event
• The sequence in which RIMs
develops is also unique to each
species
Patterns of Speciation
• Regardless of species definitions, a species, to be a
biological entity, must exist in an ecological niche
• Sometimes, the abiotic factors important in a
species’ niche vary in a regular fashion across the
range of the species
• If so, we can map those abiotic factors and then,
sometimes, find patterns within the species itself,
tracking the patterns in the abiotic factors of the
niche
Biotic Clines
• A biotic cline, in reference to population biology, is a
gradual change of phenotype (trait, character or feature)
and underlying gene pool allele frequencies in a species
over a geographical area, often as a result of
environmental heterogeneity
• This meaning of "cline" was introduced by Sir Julian
Huxley
Many speciation events seem to track or
parallel these environmental discontinuities
Clinal Variation
• In the leopard frog (Rana pipiens), tadpoles exhibit a range
of temperature tolerances, generally enduring colder
temperatures in higher (northern) latitudes and warm
temperatures at lower (southern) latitudes
Reproductive Success
• In a study by J. Moore in 1949 of the
leopard frog (Rana pipiens), eggs from
females in the north were fertilized with
sperm from males progressively farther to
the south
• The degree of embryo or tadpole
abnormalities was scored, from A (normal
young) through progressively more
abnormalities to F (high death rate)
• This study and others prompted biologists
to divide leopard frogs into several
different species
egg
mass
Biological Clines
• The flowering time of a plant may tend to be
later at higher altitudes (an altitudinal cline)
• In species in which the gene flow between
adjacent populations is high, the cline is
typically smooth, whereas in populations with
restricted gene flow the cline usually occurs as
a series of relatively abrupt changes from one
group to the next
Biological Clines
• We can also establish clines for polymorphic
phenotypes in a single species
Speciation and Hybridization Zones
• Hybrid populations becoming independent
species is more common in plants
Helianthus anomalus,
the hybrid offspring
formed less than 60
generations ago
© Colin D. Young/ShutterStock, Inc.
Helianthus annuus
© Cousin Avi/ShutterStock, Inc.
Helianthus petiolaris
Courtesy of Clarence A.
Rechenthin and USDA NRCS
Texas State Office
Figure 06: Western United States species show rapid evolution of a hybrid
Frequency and Impact of
Hybridization in Nature
• Hybridization is not particularly common in nature
and difficult for scientists to identify
• Statistically, most hybrids will be inferior, especially in
animals
• Nonetheless, episodic hybridization may provide for
introgressions and this horizontal gene transfer
increases the genetic variation on which selection
can later act
• In some circumstances, hybrids are superior, persist
in nature, and may give rise to new species,
especially in plants
Hybridization in Agriculture
Most agricultural crops
were artificially selected
from more than one
species which were
hybridized as a part of
the selection process
Figure 07B: Corn
© ailenn/ShutterStock, Inc.
Figure 07A: Teosinte
Photo by John Doebley
A Phylogeny of Mimulus cardinalis,
Mimulus lewisii and Kin
Monkey-flowers
Mimulus
cardinalis
Mimulus
bicolor
Mimulus lewisii
Monkey flowers, Mimulus sp.
• Mimulus cardinalis and Mimulus lewisii are
sister species (share the most recent common
ancestor)
• Both species live in the California mountains
• They may be successfully cross-fertilized in the
lab
• They do not hybridize in nature because they
rely on different pollinators
• Bee pollination is the ancestral state
Hummingbird pollination evolved twice within
the larger clade; that is an example of parallelism
Mimulus cardinalis and M. lewisii
Mimulus lewisii
Figure 08: Monkey flowers
Photos courtesy Toby Bradshaw (University of Washington)
and Douglas Schemske (Michigan State University)
Mimulus cardinalis
• M. lewisii is pollinated by bees
which do not see red well but do
need a landing platform
• M. cardinalis is pollinated by
hummingbirds which prefer red
flowers and do not need a landing
platform
The Mimulus Experiment
• Investigators tested individual phenotypic
traits against the preferences of pollinators
• Hummingbirds prefer nectar-rich flowers
with much purple pigment
• Bees prefer large flowers with minimum
yellow pigment
• This suggests that disruptive (sympatric?)
selection shaped flower phenotypes which
led to speciation
Speciation for Sexual Species
• If species reproduce asexually, reproductive
isolation is inherent in their formation;
offspring form asexual clones, but may acquire
new alleles through horizontal gene transfer
• If species reproduce sexually, the degree to
which species may hybridize varies greatly
• The ability to hybridize does not necessarily
contradict the reality of species distinction
• Some sister species never have the
opportunity to reproduce across populations
or form hybrids in nature
Genes and Speciation
• General statements about the evolution of
new species:
• Adaptive and Sexual traits first undergo
disruptive selection for differences between
populations during speciation
• Adaptive and Sexual traits later undergo
stabilizing selection for uniformity within
sister species after attaining speciation
• Hybridization is limited by Reproductive
Isolating mechanisms
Darwin’s Particular Genius
Over his career, Darwin wrote about 3 kinds of selection:
(1) Natural selection
- makes a species better adapted to its environment
- increases survival
(2) Sexual selection
- makes one sex more appealing to the other
- increases reproduction
(3) Artificial selection
- humans choose the desired traits and amplify them through
selective breeding of domestic organisms
In the 150 years since, scientists have added group and kin selection and
species selection as additional modes of speciation
Darwin’s Particular Genius
Darwin defined 3 kinds of selection. However, you will hear
biochemists use the term Directed Evolution: “A laboratory process
used on isolated molecules or microbes to cause mutations and
identify subsequent adaptations to novel environments.”
A typical directed evolution experiment involves three steps:
Diversification: The gene encoding the protein of interest is mutated and/or recombined at random to create a large
library of gene variants. Techniques commonly used in this step are error-prone PCR and DNA shuffling.
Selection: The library is tested for the presence of mutants (variants) possessing the desired property using a screen or
selection. Screens enable the researcher to identify and isolate high-performing mutants by hand, while selections
automatically eliminate all nonfunctional mutants.
Amplification: The variants identified in the selection or screen are replicated manyfold, enabling researchers to
sequence their DNA in order to understand what mutations have occurred.
Together, these three steps are termed a "round" of directed evolution. Most experiments will perform more than one
round. In these experiments, the "winners" of the previous round are diversified in the next round to create a new
library. At the end of the experiment, all evolved protein or RNA mutants are characterized using biochemical methods.
Biology Students: This is nothing but Darwin’s Artificial Selection using molecular
tools. The biochemists are just showing their ignorance of the history of science.
Summary of
Geographic Modes of Speciation
• Allopatric speciation – a physical barrier
divides a continuous population
• Peripatric speciation – a small founding
population enters a new or isolated niche
• Parapatric speciation – a new niche found
adjacent to the original niche
• Sympatric speciation - speciation occurs
without physical separation inside a
continuous population
Summary of Reproductive
Isolating Mechanisms (RIMs)
• Prezygotic mechanisms: Factors which prevent individuals from
mating
– Temporal isolation; Ecological isolation; Behavioral isolation; Mechanical
isolation
– Gametic incompatibility: Sperm transfer takes place, but the egg is not
fertilized
• Postzygotic isolating mechanisms: Genomic incompatibility,
hybrid inviability or sterility
–
–
–
–
Zygotic mortality: The egg is fertilized, but the zygote does not develop
Hybrid inviability: Hybrid embryo forms, but is not viable
Hybrid sterility: Hybrid is viable, but the resulting adult is sterile
Hybrid breakdown: First generation (F1) hybrids are viable and fertile, but
further hybrid generations (F2 and backcrosses) are inviable or sterile
Chapter 17
End
Small Population Effects