Evolution Lecture 25

Chapter 16

Topics for today:

Four modes of geographic speciation

1. Genetic factors that lead to reproductive isolation
2. Evidence from:

• natural populations

• experimentation

EvoBeaker: Snails

Exercise 5

Mechanisms of speciation

Reproductive isolation is key

· Geographic factors reduce gene flow and allow reproductive isolation to evolve (4 ways)

· Genetic factors cause reproductive isolation

1. Genetic divergence due to:

• Ecological selection

• Sexual selection

2. Break up of positive epistasis (Dobzhansky-Muller)

3. Cytoplasmic incompatibility

4. Chromosome divergence (polyploidy, cytological change)

5. Recombination in hybrids

Four geographical modes of speciation

Fig. 16.1

A. Allopatric speciation: physical barrier

• Most common mechanism of speciation?
• Defined by reduction in movement of individuals and gametes
• Distance not necessarily important
• Diverge due to drift and selection
• Reproductive isolation is a by-product of selection
• Renewed contact may result in
• complete isolation
• hybridization

Evidence for allopatric speciation

• Incipient prezygotic and/or postzygotic mating barriers among geographic populations
• Experimental pairing of individuals within and between populations
• Do they mate?
• Molecular differentiation among allopatric populations
• Mitochondria DNA of six freshwater fish species
• Distinct eastern and western clades in all species

Fig. 16.3

• Molecular differentiation among allopatric populations
• Mitochondria DNA of six freshwater fish species
• Distinct eastern and western clades in all species
• Gene flow reduced by Appalachian moutains
• Isolation occurred 3-4 myo

Fig. 16.3

• Increasing geographic overlap of species

· Allopatric species originally separated

· How should the geographic overlap of closely related species pairs change over time?

· Can only increase or stay at zero

· Genetic divergence used as an index for time since divergence

Fig. 16.5

Causes of reproductive isolation in allopatry?

• Genetic divergence via ecological selection

• Reproductive isolation evolves as a by-product of selection on other traits (pleiotropic effects)
• Example 1

o Test mating compatibility of herbivorous insects that specialize on different hosts

Fig. 16.7

• Example 2

• Evidence is stronger if the pattern of isolation is repeated (parallel divergence)
• Three-spined stickleback

Fig. 16.7

• Genetic divergence – sexual selection

o Different male traits may evolve by female choice in different populations ultimately causing reproductive isolation

o Two populations of bush crickets

o Example 1 Ephippiger ephippiger

§ Different male traits may evolve by female choice in different populations ultimately causing reproductive isolation

§ Two populations of bush crickets

§ Differences in♀ choice drive differences in ♂ song

Fig. 16.10

o Example 2

§ Sexual selection on males is strongest when there is high variance in reproductive success

Fig. 16.9

• Dobzhansky-Muller incompatibility

o Populations have same allele frequencies initially

o Mutations occur that have positive epistatic effects with alleles at other loci & are fixed by selection

o Hybridization breaks up positive epistasic relationships that differ among populations

Fig. 16.6