Evolution Lecture 25
Chapter 18
Topics for today:
Four modes of geographic speciation
1. Genetic factors that lead to reproductive isolation
2. Evidence from:
• natural populations
• experimentation
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. 18.1 new 16.1 old
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
o complete isolation
o hybridization
Evidence for allopatric speciation
 Incipient prezygotic and/or postzygotic mating barriers among geographic
populations
o Experimental pairing of individuals within and between populations
o Do they mate?
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Molecular differentiation among allopatric populations
o Mitochondria DNA of six freshwater fish species
o Distinct eastern and western clades in all species
Fig. 18.3 new 16.3 old
Molecular differentiation among allopatric populations
o Mitochondria DNA of six freshwater fish species
o Distinct eastern and western clades in all species
o Gene flow reduced by Appalachian moutains
o Isolation occurred 3-4 myo
Filg 18.3 new 16.3 old
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. 18.5 new 16.5 old
Causes of reproductive isolation in allopatry?
 Genetic divergence via ecological selection
o Reproductive isolation evolves as a by-product of selection on other traits
(pleiotropic effects)
o Example 1
o Test mating compatibility of herbivorous insects that specialize on
different hosts
Fig. 18.8 new 16.7 old
o Example 2
 Evidence is stronger if the pattern of isolation is repeated (parallel
divergence)
 Three-spined stickleback
Fig. 18.8 new 16.7 old
 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. 18.12 new 16.10 old
o Example 2
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Sexual selection on males is strongest when there is high variance
in reproductive success
Fig. 18.12 new 16.9 old
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. 18.7 new 16.6 old