Ecological speciation - ppt based on review of Howard D. Rundle1* and Patrik Nosil2

1. Howard D. Rundle 1 and Patrik Nosil 2
ECOLOGICAL SPECIATION
1Department of Zoology and
Entomology, University of
Queensland, Brisbane,
Queensland 4072, Australia
2Department of Biological
Sciences, Simon Fraser
University, Burnaby, BC V5A
1S6, Canada
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KASHMEERA N.A.
PHD142004
IISER-TVM

2. ECOLOGICAL SPECIATION
• Process by which barriers to gene
flow evolve between populations as a
result of ecologically-based
divergent selection (Rundle and
Nosil 2005).
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3. • When it arises as a consequence of the
interaction of individuals with their
environment
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•When it acts in contrasting directions in the two
populations.
•Disruptive selection.
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4. NON-ECOLOGICAL SPECIATION
• Processes of speciation that do not involve
divergent selection between environments.
• Chance events dominate the initiation of the
evolution of reproductive isolation.
• Mechanisms include genetic drift, founder
events/ population bottle necks, hybridization,
polyploidization.
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5. COMPONENTS OF ECOLOGICAL
SPECIATION
A source of divergent selection.
A form of reproductive isolation.
A genetic mechanism linking the two.
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6. DIVERGENT SELECTION
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A form of divergent selection is required , where
selection is divergent when it acts in contrasting
directions in two populations

7. Divergent
selection
Environmental
differences
Sexual
selection
Ecological
interactions
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Ecological sources of divergent
selection
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8. Environmental differences
• Difference between environments (eg: habitat
structure, climate and resource availability) →
divergent selection.
• Common cause of divergent selection.
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9. Schluter and colleagues have studied stickleback fish in post-
glacial freshwater lakes
- 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
3-spined sticklebacks

10. Ecological differences result in size differences:
- benthic fish are big + slow; hunt invertebrates on bottom
- limnetic fish are smaller, streamlined hunters of plankton
marine
ancestor
limnetic
benthic

11. Ecological interactions
• 1) Competition for shared resources.
• In 3 spine sticklebacks, resource competition →
morphological divergence of limnetics and
benthics.
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12. Reinforcement
• Interbreeding (hybridization) → ecological
speciation via reinforcement.
• Reinforcement occurs when hybrids have
reduced fitness such that selection favours
parental individuals that are less likely to
hybridize.
• It has been implicated in ecological speciation
of limnetic and benthic 3 spinesticklebacks.
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13. SEXUAL SELECTION
• Divergent sexual selection arises when mate
preferences differ between populatons.
• considered as component of ecological
speciation when it is initiated by divergent
selection between environments.
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14. Dewlap spectral traits of
Anolis in mesic & xeric
habitats
(Manuel Leal and Leo J. Fleishman,2004)
• Mesic and xeric conditions – light conditions
differ.
• Dewlap spectral traits differ →↑signal
detectability in native habitat.

15. The major difference in dewlap design was in total
reflectance and transmittance
BRIGHTER
DARKER
Premating isolation

16. FORMS OF REPRODUCTIVE
ISOLATION
Premating
• Occur before mating
• Eg:Habitat & temporal isolation,immigrant
inviability,sexual isolation.
postmating
• Occur after mating
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17. Premating isolation
• Arise when populations are separated in
habitat or time.
• Habitat isolation
• Occurs when populations exhibit
genetically based preferences for
separate habitats reducing the likelihood
of between population encounters & thus
of interbreeding
• variation in host plant choice reduces
gene flow between clover- and alfalfa-
adapted populations of Acyrthosiphon
pisum pea aphids
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18. Temporal isolation
• Occurs when populations exhibit
divergent developmental schedules such
that mating happens at different times in
each.
• The apple-feeding host race of the
tephritid fruit fly (Rhagoletis pomonella)
differs from the hawthorn-feeding race in
that the apple race emerges earlier in the
year, and each host race preferentially
chooses to rest, lay eggs and mate on its
own host plant .

19. Immigrant inviability
• Individuals from foreign, ecologically divergent habitats are
less likely to survive, can restrict gene flow among diverging
populations and result in speciation.
• The Shortfin Molly (Poecilia mexicana) is a small fish that
lives in the Sulfur Caves of Mexico.
• Two distinct populations of mollies—the dark interior fish and
the bright surface water fish—are becoming more genetically
divergent.
• Common predator - large water bug (Belostoma spp).
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20. Tobler collected the bug and both types of mollies, placed them in
large plastic bottles, and put them back in the cave.
After a day, it was found that, in the light, the cave-adapted fish
endured the most damage.
In the dark, the situation was the opposite.
The mollies’ senses can detect a predator’s threat in their own
habitats, but not in the other ones.

21. Sexual isolation
• Arise before mating
• Due to difference between populations in
mating signals & preferences.
• Main component of rep. isolation between
recently evolved taxa.
• In stickle back fish adaptation to different
habitats causes divergence in body size &
because mate choice is assortative w.r.t. size
sexual isolation arises as a byproduct.
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22. 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
“I find my
own size to be
sexy”

23. Post mating isolation
• Arise when hybrid fitness is reduced because
of ecological mismatch between intermediate
hybrid phenotypes and environment.
• Eg: ecologically dependent reduction of hybrid
fitness in limnetic-benthic stickle backs.
• . Hybrids are less likely to be selected as mates
than conspecific forms (Hatfield and Schluter
1996; Vamosi and Schluter 1999).
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24. • The intermediate morphology of the hybrids
makes them less effective foragers than either
parental form, unable to efficiently exploit
limnetic or benthic resources (Schluter
1993; Schluter 1995; Schluter 1996).

25. Genetic mechanisms linking selection
and reproductive isolation
• Final component of ecological speciation.
• Selection of ecological traits is transmitted to
genes causing reproductive isolation.
• Two ways of genetic mechanisms – based on
relationship between genes under divergent
selection and those causing reproductive
isolation.
Pleiotropy & Direct selection.
Linkage Disequilibrium & Indirect selection.
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26. Pleiotropy and Direct selection
• Reproductive isolation evolve by direct
selection.
• Alleles responsible for reproductive isolation
are themselves under selection.
• Eg: Habitat isolation evolves as a direct
consequence of selection on genes affecting
habitat choice.
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27. Sexual isolation in Mimulus monkey flowers
( Bradshaw,H.D & Schemske,D.W 2003).
• Divergent natural selection
acts on a flower colour gene
in Mimulus monkey flowers
via the effects of colour on
attractiveness to pollinators.
• Adaptation to different
pollinators via divergence in
flowercolour gene directly
affects probability of cross
pollination( i.e.
hybridization) a form of
sexual isolation.
Mimulus cardinalis (red)and M. lewisii (pink)

28. Linkage disequilibrium and
indirect selection
• Genes under divergent selection are physically different
from those causing reproductive isolation.
• Selection acts on genes causing reproductive isolation
which are nonrandomly associated (linkage
disequlibrium) with the genes directly under selection
• And cause correlated evolutionary response in latter
genes.
• Indirect selection is less effective than direct in
evolution of reproductive isolation
• Because genetic association between two gene sets
(linkage disequilibrium) is not perfect →↓strength of
selection transmitted to reproductive isolation genes.
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29. REMAINING QUESTIONS
• Ininsufficient attention to understanding
contribution of ecological interactions &
sexual selection to evolution of reproductive
isolation.
• Relative importance of various forms of
rep.isolation & likelihood that each evolves via
divergent selection are not well resolved.
• Direct tests of genetic link between traits under
selection& rep.isolation are lacking.

30. • Conducting studies using a diversity of taxa &
examining wide range of divergence from
incipient to established sps. will help.

31. THANK YOU