Howard D. Rundle 1 and Patrik Nosil 2

ECOLOGICAL SPECIATION

1Department of Zoology andEntomology, University ofQueensland, Brisbane,Queensland 4072, Australia

2Department of BiologicalSciences, Simon FraserUniversity, Burnaby, BC V5A1S6, Canada

3/30/2014 1

KASHMEERA N.A.PHD142004IISER-TVM

ECOLOGICAL SPECIATION

• Process by which barriers to gene

flow evolve between populations as a

result of ecologically-based

divergent selection (Rundle and

Nosil 2005).

3/30/2014 2

• When it arises as a consequence of the

interaction of individuals with their

environment

3/30/2014

•When it acts in contrasting directions in the two

populations.

•Disruptive selection.

3

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.

3/30/2014 4

COMPONENTS OF ECOLOGICAL

SPECIATION

A source of divergent selection.

A form of reproductive isolation.

A genetic mechanism linking the two.

3/30/2014 5

DIVERGENT SELECTION

3/30/2014 6

A form of divergent selection is required , where

selection is divergent when it acts in contrasting

directions in two populations

Divergent selection

Environmental differences

Sexual selection

Ecological interactions

3/30/2014

Ecological sources of divergent

selection

7

Environmental differences

• Difference between environments (eg: habitat structure, climate and resource availability) → divergent selection.

• Common cause of divergent selection.

3/30/2014 8

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

Ecological differences result in size differences:

- benthic fish are big + slow; hunt invertebrates on bottom

- limnetic fish are smaller, streamlined hunters of plankton

marineancestor

limnetic

benthic

Ecological interactions

• 1) Competition for shared resources.

• In 3 spine sticklebacks, resource competition →

morphological divergence of limnetics and

benthics.

3/30/2014 11

Reinforcement

• Interbreeding (hybridization) → ecological speciation via reinforcement.

• Reinforcement occurs when hybrids have reduced fitness such that selection favoursparental individuals that are less likely to hybridize.

• It has been implicated in ecological speciation of limnetic and benthic 3 spinesticklebacks.

3/30/2014 12

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.

3/30/2014 13

Dewlap spectral traits ofAnolis 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.

The major difference in dewlap design was in total

reflectance and transmittance

BRIGHTERDARKER

Premating isolation

FORMS OF REPRODUCTIVE

ISOLATION

Premating

• Occur before mating

• Eg:Habitat & temporal isolation,immigrantinviability,sexual isolation.

postmating• Occur after mating

3/30/2014 16

Premating isolation

• Arise when populations are separated inhabitat or time.

• Habitat isolation

• Occurs when populations exhibitgenetically based preferences forseparate habitats reducing the likelihoodof between population encounters & thusof interbreeding

• variation in host plant choice reducesgene flow between clover- and alfalfa-adapted populations of Acyrthosiphonpisum pea aphids

3/30/2014 17

Temporal isolation

• Occurs when populations exhibitdivergent developmental schedules suchthat mating happens at different times ineach.

• The apple-feeding host race of thetephritid fruit fly (Rhagoletis pomonella)differs from the hawthorn-feeding race inthat the apple race emerges earlier in theyear, and each host race preferentiallychooses to rest, lay eggs and mate on itsown host plant .

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).3/30/2014 19

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.

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.

3/30/2014 21

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”

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).

3/30/2014 23

• 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).

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.

3/30/2014 25

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.

3/30/2014 26

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)

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.

3/30/2014 28

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.

• Conducting studies using a diversity of taxa &

examining wide range of divergence from

incipient to established sps. will help.

THANK YOU