Lecture 21: Speciation II

• Speciation processes– Isolation: Dispersal, Vicariance

– Divergence: Drift, NaturalSelection, Sexual Selection

– Secondary contact: post-zygotic and pre-zygoticisolation

– How to avoid hybridization:sympatric speciation, sexualselection

Cichlid fish diversity:

1000’s of speciesAllopatric Speciation

1. initial isolation

2. divergence

3. secondary contact & “reinforcement”

Allopatric Speciation

IsolationColonization

Vicariance

DivergenceDrift

Natural selection

Sexual selection

Secondary contactHybrid offspring

unfit (postzygotic

isolation)

Selection against

hybridization(prezygotic

isolation)

Reinforcement

(?)

Allopatric Speciation

1. initial isolation

2. divergence

1. Genetic drift

2. Natural selection (Ecology)

3. Sexual selection (courtship, mating)

What Causes Divergence?

What Causes Divergence?

1. Genetic drift

• Dispersal and colonization: Small

populations

• Allele frequencies diverge

What Causes Divergence?

2. Natural selection (Ecology)

• diverge in traits, exploitation of

resources in new environment

Trait value

Fre

quenc

yF

itness

Divergence: Natural selection

• Three-spined stickleback

• Lakes in B.C.

Divergence: Natural selection

• Open water,planktonfeeders

• Bottomfeeders benthic

limnetic

Divergence: Drift and N.S.

Does divergence lead to evolutionary

independence (reproductive isolation)?

What Causes Divergence?

3. Sexual selection

• divergence in secondary sexual

traits affecting mate choice

Isolation

Mating

preferences

change

Genetic

divergence

Knight et al. (2004) Proc Roy Soc B 271: 675-680

Cichlid fishes: Lake Malawi• Geographic

populations

• Differ in malecolor morphs:secondarysexual traits

• Speciation bysexualselection?

Knight et al. (2004) Proc Roy Soc B 271: 675-680

Mate choice experiment

• Males hold territories

• Females spawn, then brood larvae in mouth

• Experiment: Partial partition mate choice– Females, Males from 5 geographic populations

– Number of spawnings

Females choose male with local color

morph

81100RW

426101EM

12908NB

22291MR

5111119CH

RWEMNBMRCHMale/

Female

Hawaiian crickets

• Divergence in secondary

sexual traits: male

courtship song– No ecological differences

• Rapid: 4 species/million

years

Allopatric Speciation

1. initial isolation

2. divergence

3. secondary contact & “reinforcement”

Secondary contact

• Fitness of hybrids?

Hybrids

Secondary contact

Hybrids fit:

Introgression

Hybrids unfit:

Speciation

1. Genetic incompatibility

2. Sexual selection against hybrids

3. Ecological inferiority of hybrids (Nat. Sel.)

Low hybrid fitness? Sticklebacks

Growth slower inboth environments

Low fitnessHybrids

Post-zygoticisolation

• Reinforcement: Selection for reducedhybrid matings, e.g.– Assortative mating– Timing of breeding

• Produces prezygotic reproductive isolation

Hybrids

• Rhagoletis pomonella

• Eggs, larva develop onHawthorne fruit

• Adults return to larval fruitfor mating

How to avoid hybrid mating 1

Hawthorne

winter

larvae pupae adultsadults

Rhagoletis pomonella

• Colonized Apple in 1800s

• Mating, habitat choice arelinked

• Hybrid matings (Hawthornevs. apple flies) rare

How to avoid hybrid mating 1

Apples

winter

larvae pupae adultsadults

Apples

winter

larvae pupae adultsadults

Hawthorne

winter

larvae pupae adultsadults

Linn et al. (2004) PNAS 101: 17753-17758

Sympatric Speciation How to avoid hybrid mating 2?

Divergence by N.S., drift

Isolation Divergence

Mating

preferences

change

reinforcement

How to avoid hybrid mating 2?

Divergence by Sexual selection: Prezygotic

isolation is a consequence of divergence

IsolationMating

preferences

change

Divergence

No reinfocement needed