Download lecture 15 - ecological speciation - Cal State LA

Ring Species
the salamander E. eschscholtzii
spread south from Oregon
along both sides of Sierra
Nevada mountains that divide
coastal and inland California
- over millions of years, the
populations gradually diverged
hybrid zone
- gene flow connects adjacent populations, but where the ends
of the “ring” meet, populations do not hybridize well

Sympatric speciation: divergence without isolation?
The idea that species could form in the absence of prolonged
isolation has been hotly debated for decades
Sympatric speciation occurs when two populations become
reproductively isolated “within cruising range” of each other
For this to occur, assortative mating has to arise despite gene
flow, and without relying on genetic drift
Scientists have long theorized that disruptive selection on any
trait could potentially lead to sympatric speciation, by splitting
one population of generalists into two species of specialists
Disruptive selection and assortative mating
Thoday and Gibson (1962)
- started with 4 wild flies that had different # of
bristles on their bodies
- their offspring had a
normal distribution
of bristle #’s
Disruptive selection and assortative mating
Thoday and Gibson (1962)
- started with 4 wild flies that had different # of
bristles on their bodies
- every generation, took the 8 flies with the
most bristles, and the 8 with the least bristles
- let them interbreed to form the next generation
- after only 12 generations, there were no
intermediate flies: all offspring had either lots
of bristles (white), or very few bristles (black)
Disruptive selection and assortative mating
What occurred?
- disruptive selection: individuals with high or
low bristle # survived to reproduce; average
number = no reproduction (you were selected
against)
- over time, no hybrid offspring were produced
(no flies with intermediate # of bristles)
- assortative mating resulted from selection
against hybrids: hairy flies only mated with
other hairy flies, and hairless with hairless
- effectively became different species in 12
generations (reproductively isolated)
Disruptive selection and assortative mating
Controversy!.. no one could reproduce the results of Thoday
and Gibson -- including themselves (got lucky the 1st time?)
It was argued that normally, selection and recombination have
opposing effects -- selection: builds up disequilibrium between trait (bristle #)
and mating preference for that trait
- recombination: removes disequilibrium between a trait and
mating preference for that trait
In a sexual population, recombination will prevent disruptive
selection from promoting assortative mating and speciation
Disruptive selection and assortative mating
hairy prefer
hairy
mates
selection favors linkage disequilibrium
between these 2 traits, since that will
prevent hybrids from forming
hairy
recombination removes disequilibrium
between these 2 traits, as crossing over
events will keep on separating them
prefer
hairy
hairless prefer
hairless
Disruptive selection on habitat choice
Rice (1987) and others demonstrated that you could get around
this problem if the trait under selection caused assortative
mating as a by-product (basically, coincidentally)
-i.e., when assortative mating was a correlated character,
instead of a separate trait controlled by other genes
Example: habitat choice
- if individuals mate only in their preferred habitat, then traits
controlling habitat choice indirectly control mating preference
- recombination can’t tear down this association, since it’s not
due to linkage of alleles controlling two different traits
Disruptive selection on habitat choice
The best examples of sympatric speciation are cases of hostswitching in specialized arthropods such as insects
(herbivores or parasites)
Following the introduction of a new host plant by agriculture,
some individuals of an insect species will switch onto the new
host
Adults are most likely to encounter other adults that prefer the
same host plant -- results in assortative mating
If host preference is heritable, their offspring will repeat the
pattern -- and a new host race is born
Case study: the apple maggot fly
The best-studied example is the apple maggot fly, Rhagoletis
pomonella
This species originally used the hawthorn tree as its host plant
Following the introduction of apples into the U.S. around 1850,
some individuals switched from hawthorns onto apples
In only 150 years, hawthorn and apple races became highly
genetically differentiated
- but, gene flow still occurs at ~ 6% a year, due to adults that
are not perfectly loyal to their original host tree
Case study: the apple maggot fly
Despite persistent gene flow, the two host races are genetically
differentiated and appear destined to speciate
Natural selection maintains 94% reproductive isolation, mainly
resulting from different fruiting times of the two trees
- apples mature 3 weeks earlier than hawthorn fruit
- adults of the apple race hatch earlier to take advantage of
new apples, and thus do not overlap with most hawthorn flies
This represents a case of incipient speciation...
Sequence for speciation by host shift
Steps in sympatric speciation
Result
(1) mate on preferred host
promotes assortative mating,
so decreases gene flow
(2) host-specific adaptations
build up over time
selects against hybrids, so
reinforces assortative mating
(3) other adaptations arise
that increase pre-zygotic
isolation, like host fidelity
and other mating traits
complete the speciation
process
Parasites also host shift
Genetically distinct host races of Rhagoletis flies known from
hawthorn (original), apples, snowberries and blueberries
Subsequently shown that specialized parasitic wasps that lay
eggs in fly larvae also exist as distinct races, one per fly race
- fly races hatch in the same order as their fruit matures:
blueberry > apples > hawthorn fruit
- wasps hatch in same order
As 20% of insects may be
parasitic wasps, there is
huge potential for speciation
by host shift on top of
speciation by host shift!
Forbes et al. (2009) Science 323: 776
Sympatric speciation by natural + sexual selection
A recent modelling study reported that sympatric speciation can
evolve when disruptive natural selection on any ecological trait
was paired with a connected male display trait, and sexual
selection in the form of female preference for the male display
- disruptive selection only: population kept average trait value
(like beak size)
- add sexual selection: population evolved into 2 distinct forms
disruptive selection only
VanDoorn et al. (2009) Science 326: 1704
disruptive + sexual selection
Sympatric speciation by natural + sexual selection
simulated males evolved to invest in display, and females
evolved to prefer the male display, because display was
connected to male’s ability to use either resource extreme
New paradigm: Ecological speciation
The concept of speciation was introduced by Darwin, in his
book On the Origin of Species by Natural Selection
Ironically, for most of the 20th century, little attention was paid to
the role of natural selection as a force driving speciation
- focus was on geographical isolation and genetic drift
New studies have shown that natural selection in different
environments can be a much more powerful evolutionary force
than isolation + time (i.e., genetic drift)
- the debate is now shifting from allopatry vs. sympatry to
ecological speciation (due to natural selection) vs. drift
Case study: 3-spined sticklebacks
Schluter and colleagues have studied stickleback fish in postglacial freshwater lakes
- receding glaciers left behind young lakes initially lacking fish
- marine ancestors of the stickleback colonized many such
lakes, independently but in parallel
- their descendants diversified into 2 forms in each of 6 lakes:
(1) limnetic, a small and sleek form that hunts in mid-water
(2) benthic, which is larger and hangs out on the bottom
Parallel speciation in 3-spined sticklebacks
Ecological differences result in size differences:
- benthic fish are big + slow; hunt invertebrates on bottom
- limnetic fish are smaller, streamlined hunters of plankton
limnetic
marine
ancestor
benthic
Parallel speciation in 3-spined sticklebacks
In mating trials, benthic fish mated with other benthic fish;
limnetic fish mated with other limnetic fish
Similar fish mated, no
matter which lakes
they came from
in other words,
regardless of genetic
similarity – fish from
different lakes are
not related
Rundle et al. 2000
Parallel speciation in 3-spined sticklebacks
In mating trials, benthic fish mated with other benthic fish;
limnetic fish mated with other limnetic fish
Benthic x limnetic
didn’t produce many
successful matings
Parallel speciation in 3-spined sticklebacks
Size-assortative mating  reproductive isolation
- big fish like to mate with big fish; small fish with small fish
In this case, preference genes don’t appear to be linked to
size-determining genes; fish can just have the allele for
“I find my own size to be sexy”, whatever that size is
Thus, no gene flow between the two ecotypes of stickleback,
although they are close relatives that diverged only a few
thousand years ago in each lake
Ecotype = different forms of one species that have not
yet evolved full reproductive isolation (not species, yet)