Species are evolutionary lineages….
Species are evolutionary lineages….
So what is speciation?
So what is speciation?
Speciation describes the process(es) through which new
lineages arise from ancestral lineages.
Speciation describes the process(es) through which new
lineages arise from ancestral lineages.
By definition, requires evolution of reproductive isolation: lineages
are not exchanging genes
By definition, requires evolution of reproductive isolation: lineages
are not exchanging genes
— may be due to extrinsic factors (e.g., geographical barriers to gene flow)
— may be due to extrinsic factors (e.g., geographical barriers to gene flow)
— may be due to intrinsic factors (e.g., genetic incompatibility)
— may be due to intrinsic factors (e.g., genetic incompatibility)
This reproductive isolation arises through the same
evolutionary processes as we’ve been studying all semester
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A traditional (and still useful) way of classifying different ways that speciation
can occur focuses on the geographical distributions of populations (E. Mayr):
Geographic modes of speciation
Gene flow (lack thereof)
Genetic Drift
Natural Selection/Sexual selection
Mutation  genetic incompatibility (e.g., chromosomal rearrangements,
polyploidy)
A) Allopatric Speciation
1. Vicariance: splitting of one continuous population into two (or more)
large groups that are geographically isolated from each other.
— Reproductive barriers evolve while populations are geographically isolated,
although the two species may subsequently become sympatric
TIME
• Allopatric – reproductive barriers arise between
populations that have become geographically separated
• Parapatric – neighboring populations, between which
there is moderate gene flow, diverge and become
reproductively isolated
• Sympatric – evolution of reproductive barriers within a
single, initially random-mating population
1) Initial geographic separation
-geology (e.g., mountains, rivers)
-climate change
-sea-level fluctuation
2) Divergence in different areas
-limited or no gene flow between areas
(allows divergence)
3) Reproductive isolation in
sympatry (if there’s
subsequent, secondary
contact)
-may interbreed in narrow hybrid
zone
-hybrids generally have reduced
fitness
Vicariance is widely accepted as an important speciation mechanism
1
Evidence for allopatric speciation: geographical distributions of different
species within a genus.
isolation between geographically isolated populations of a species
e.g.,vicariance in Holbrookia lizard species
Sp. 1
Evidence for allopatric speciation: look for incipient reproductive
Southwest US, northern Mexico:
Sp. 2
4
7
Sp. 3
e.g., dusky salamander
(Desmognathus
ochrophaeus), southern
Appalachians
3
Sp. 4
2
Sp. 5
Sp. 6
6
 Species clades
correspond to distinct
geographical regions
1
5
Futuyma Fig. 16.2
Greater geographical isolation corresponds to decreased
mating success, greater genetic distance.
Sp. 7
Evidence for allopatric speciation: look for haplotype divergence
among geographically distinct populations of a species
Allopatric Speciation
2. Peripatric
Examination of
geographical
distributions of
haplotypes
among
populations
Founder event from widespread species into new habitat, divergence in isolation
— genetic drift, natural selection both occurring
— expect new selective pressures in new environment
TIME
Localized
population
diverges
Range expansion may (or may
not) subsequently re-establish
sympatry
Controversial whether genetic drift alone would be sufficient for speciation
2
Possible example
of peripatric
speciation
Haplotype trees can reveal evidence of peripatric speciation
e.g., Drosophila simulans (widepread, Africa) was apparent
ancestor of D. sechellia, D. mauritiana (Indian Ocean islands)
Haplotype trees from 2 loci:
Uta lowei
D. sechellia, D. mauritiana
clades are nested within D.
simulans haplotypes for
multiple haplotype trees
Uta palmeri
The island species have
different resource uses:
suggests selection (rather
than drift alone) was important
in species divergence.
Uta nolascensis
Island endemics
Widespread species
Uta squamata
Morphologically and
ecologically distinct
from widespread
mainland species
Futuyma Fig. 16.13
Uta stansburiana
May 2007, PLOS-Biology Vol. 5, Iss. 5
How important is genetic drift in causing peripatric speciation?…
Lab experiments generally indicate that genetic drift alone does not lead
to reproductive isolation
e.g., 50 bottlenecked laboratory populations of D. melanogaster
Look for assortative mating: do individuals of a specific population selectively
mate with individuals from the same population?…
Most herbivorous insects specialize on one or a few host plants; understanding the
processes and genetics underlying this specialization has broad implications across
biology. Drosophila sechellia, a fruit fly endemic to the Seychelles, feeds
exclusively on the ripe fruit of Morinda citrifolia, a tropical plant commonly
known as Tahitian Noni. Although other fruit flies never approach this fruit
because of its toxins, D. sechellia is resistant and is actually attracted by
the same toxins. D. sechellia is a close relative of D. melanogaster, an established
model species of genetics. By comparing D. melanogaster and D. sechellia, we
revealed that two genes encoding odorant-binding proteins, Obp57d and
Obp57e, are not only involved in the fruit fly's taste perception, but can
also change the behavioral response of the flies to the toxins contained in
the fruit. By knowing how an insect's food preference is determined by its genes,
we can gain insight into how insect lifestyles evolve and investigate whether such
changes can lead to the formation of new species. We can also begin to understand
how to manipulate insects' behavior by changing their preference for particular
substances.
Number of experimental lineages
10
Author Summary
The average bottlenecked
lineage showed no assortative
mating
8
No sexual isolation from
ancestral population
6
4
2
0
-0.3
Negative
assortative
mating
-0.2
-0.1
0
No
assortative
mating
0.1
0.2
0.3
Positive
assortative
mating
Rundle (1998)
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B) Parapatric Speciation
Evidence for incipient parapatric speciation:
- Neighboring populations, between which there is moderate gene flow,
diverge and become reproductively isolated
- Moderate gene flow b/t populations
- Different selection pressures
in adjacent areas
Reproductive isolation has arisen between metal tolerant
and intolerant grass populations
(Recall: spatial
environmental
heterogeneity, grass
on mine tailings)
- Divergent selection pressures lead to
evolution of reproductive isolation
Anthoxanthum
odoratum
Shifts in flowering time, selffertilization have led to reproductive
isolation between metal tolerant and
intolerant populations
Environmental
transition
Reinforcement: natural
selection favors the evolution
of reproductive barriers
between populations
- Natural Selection overrides the diluting effects of gene flow
- May occur, but hard to distinguish retrospectively from allopatric
speciation that was followed by secondary contact
Parapatric Speciation in progress in Little Greenbuls
C) Sympatric Speciation
T.B. Smith et al. (2005): J. Evol. Biol.
— Evolution of reproductive barriers within a single, initially randommating population
— Requires evolution of reproductive isolation without any spatial
isolation among diverging lineages.
Randomly mating population
Andropadus virens
Reproductive isolation
begins to form
Completion of reproductive
isolation
 Sympatric speciation has been most controversial of the three geographical modes
 Phenotypic and genetic divergence is correlated with environment, not geography
despite gene flow between environments: Selection overwhelms gene flow
4
C) Sympatric Speciation
Assortative mating by resource use…
— Evolution of reproductive barriers within a single, initially randommating population
— Requires evolution of reproductive isolation without any spatial
isolation among diverging lineages.
Randomly mating population
Reproductive isolation
begins to form
Completion of reproductive
isolation
A mechanism by which sympatric speciation can occur:
If natural selection is favoring divergence in resource use (e.g.,
diversifying selection)
and
if mate choice is directly correlated with resource use…
Then diversifying (disruptive) selection for resource use will lead to
assortative mating by resource
Reproductive isolation with no spatial separation required
 Sympatric speciation has been most controversial of the three geographical modes
But, two ways it can occur:
• Genome doubling (polyploids — we’ll come back to this later)
• Assortative mating by resource use…
Sympatric Speciation in
Rhagoletis pomonella
(apple maggot flies)
Differences in timing of maturation of hawthorn vs apple fruits creates
reproductive isolation between hawthorn and apple ‘host races’
Apple host race
infests earlier
- Hawthorn: native host for Rhagoletis; flies
mate on fruit, lay eggs inside where larvae
develop
- Apples introduced to US ~300 years ago
- Expansion of host plant use to include
apples: apple ‘host race’ spread throughout
North America; host preference is heritable
-Fruits develop at different rates (apples 3-4
weeks before hawthorns)
and matures earlier
(<6% reproduction
between host races)
Reproductive isolation is
creating genetic differentiation
between host races
5
A test of whether speciation was likely allopatric or sympatric
Examine the extent of range overlap over time (i.e., genetic divergence)
A test of whether speciation was likely allopatric or sympatric
Examine the extent of range overlap over time (i.e., genetic divergence)
Allopatric speciation: range overlap can only increase from zero, so we
expect that over time range overlap will increase (positive slope)
Consistent with
allopatric
speciation
- Sympatric speciation: expect range overlap to remain the same or
decrease over time (level or negative slope)
(Assume
molecular clock, so
that greater
genetic divergence
= older speciation)
Allopatric
Range Overlap of species pairs (%)
Examine multiple
pairs of species in
a genus:
Sympatric
(i.e., Rhagoletis)
100
Potentially
consistent with
sympatric
speciation
0
Genetic Divergence (time)
A test of whether speciation was likely allopatric or sympatric
Kozak & Wiens (2006): Evolution 60:2604
-Positive slope suggests allopatric mode
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