The geography of speciation
• most species
form when
geographic
barriers block
genetic
exchange
Allopatric speciation
• arguably the most common way species form
• evidence for allopatric speciation is common in
biogeography:
– related species often occupy nearby, non-overlapping
ranges
Allopatric speciation
• Isthmus of Panama
closed ~ 3.1 MYA
• Split ~150
“geminate”
(twin)
species
Evidence for allopatric speciation in snapping shrimps
Knowlton et al.(1993) created a
phylogeny of Pacific (P) and
Carribean (C) species pairs of
Alpheus
In 6 out of 7 cases, the closest
relative of a species was on the
other side of the Isthmus
The geography of speciation
Allopatric speciation through
“dispersal and colonization”
Evidence from phylogeny of
Hawaiian Drosophila
D. heteroneura
D. silvestris
Founder effect
speciation
• divergence of a small population isolated on periphery of range
• thought to explain species radiations on islands
Founder effect
speciation
• the genetic “founder effect” of small population size itself is thought
to drive speciation
Sympatric speciation
• no geographic barrier
• much rarer
• but shows how ecology can drive
speciation by selecting for
assortative mating
from Barluenga et al. (2006)
Nature 439: 719-723
Sympatric speciation
• valid cases:
– cichlids in crater lakes
– host races of Rhagoletis
(apple maggot fly)
from Barluenga et al. (2006)
Nature 439: 719-723
Lake Apoyo: an
isolated volcanic
crater lake,
homogeneous
habitat < 23,000
years old
mtDNA sequences are
monophyletic: no Lake
Apoyo sequences are
found in sister species
outside the Lake
This shows that all
Lake Apoyo fish
evolved in sympatry
The two sympatric species are
reproductively isolated
Morphological and ecological
differences evolved sympatrically
benthic
limnetic
Sympatric speciation in action
• Rhagoletis pomonella: the apple maggot fly
• Larvae feed on natural host: hawthorn fruits
• A “host race” infesting apple trees appeared in North
America ~150 years ago
hawthorns
apples
apples
hawthorns
Speciation in progress
• the apple race and hawthorn races
have evolved reproductive barriers
Adults mate and oviposit on their host trees
This leads to limited
interbreeding
Speciation in progress
• the apple race and hawthorn races
have evolved reproductive barriers
Adults mate and oviposit on their host trees
Genetic differences
between the races mark
the early stages of
speciation
Genetic differences between
apple and hawthorn races
• the two races show genetic
differences at six allozyme loci
• included is an aconitase locus
called Acon-2
hawthorns
citrate
aconitate
isocitrate
apples
Genetic differences between
apple and hawthorn races
• the frequency of the
Acon-2 95 allele is
much higher in the
hawthorn race
hawthorns
apples
The role of natural selection
in divergence of Rhagoletis
• Feder and coworkers predicted that natural selection
opposes migration and creates allozyme differences
• One hypothesis: hawthorns ripen fruits 3-4 weeks
later than apples
– Apple fly larvae experience longer periods of warm
temperatures prior to and while pupating
– This must select for divergent genes
Their experiment...
• Collect hawthorn pupae and expose to warm
temperatures for a varying duration
• Follow with freezing temperatures (“winter”), then
by warming (“spring”)
• Collect emerging adults and assay allozymes
hawthorn pupae exposed to longer periods of “pre-winter”
warmth produced adults with allele frequencies
approaching those of the apple race
The classic “three stage”
model of allopatric speciation
• 1st stage: a geographic barrier creates
isolation between two or more portions of
a population
• 2nd stage: the descendant populations
diverge genetically (due to drift and selection)
[Rhagoletis has
skipped the 1st and
is in the 2nd stage]
“Ecological speciation” in sticklebacks:
more evidence for natural selection
during the 2nd stage
• Dolph Schluter and coworkers have studied
Gasterosteus aculeatus in rivers and lakes of British
Columbia for many years
• A marine ancestor colonized rivers at least 3 times,
independently
“Ecological speciation” in sticklebacks
shows evidence for natural selection
during the 2nd stage
• Several lake colonizations have led to independent
cases of “ecological speciation”
“Ecological speciation” in sticklebacks
shows evidence for natural selection
during the 2nd stage
• In several lakes, two morphs that
show differences in diet,
morphology, and behavior have
diverged:
– A smaller limnetic (open water) form
– A larger benthic (bottom dwelling) form
• Benthic and limnetic forms mate
like-with-like
Evidence for a role for sexual
selection during the
“divergence” stage
Sexual selection on head width in D. heteroneura
Males with wider
heads are
chosen by
females on leks
And they win
contests with
males for
territories
This means that sexual
selection is likely to be
responsible for the
differences in head shape
between D. heteroneura
and D. silvestris
And that it served a role in
their speciation??
The classic “three stage”
model of allopatric speciation
• 3rd stage: reproductive isolation is completed,
or perfected
– this occurs after “secondary contact” between
allopatric populations
– a crucial step, why?
• secondary contact is common
• without complete reproductive barriers, species will re-fuse
Secondary contact after Pleistocene
glaciations: bird hybrid zones
from Futuyma (1998), p. 258
Secondary contact: hybrid zones
in blue mussels
Hilbish et al. 2000
Pacific mussels reinvaded the North Atlantic during
warm, high sea level period about 15-20,000 ya
from Riginos and Cunningham 2005
The classic “three stage”
model of allopatric speciation
• Reproductive isolation can be completed
in two distinct ways
– As a byproduct of the divergence process,
through drift and selection (unrelated to
interbreeding)
– Via selection against hybridization
(reinforcement)
Coyne and Orr’s (1997) survey
of Drosophila species pairs
• Reviewed data on over 150 species pairs
– Whether the species are allopatric or sympatric
– Genetic distance between the species pair (based
on allozymes) as an estimate of age
– The amount of postzygotic and prezygotic isolation
Their results...
Prezygotic isolation increases with genetic distance
Genes for prezygotic isolation diverge over time, just
like the rest of the genome
Full isolation evolves in allopatry
Secondary contact is not necessary to complete the
speciation process
Coyne and Orr’s (1997) survey supports reinforcement
Prezygotic isolation evolves faster in sympatry
Selection against hybridization drives more rapid
evolution
This is not “sympatric
speciation!”
why not?