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BIOL 1602 Lecture Notes - Reproductive Isolation, Mate Choice, Genetic Drift

Course Code
BIOL 1602
Rebecca Jubis

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Natural selection is a potent force for divergence
Sexual Selection
Sexual selection acts on characters involved in mate choice. Changes in sexual selection can
cause reproductive isolation and trigger rapid divergence
Sexual selection is a form of natural selection that results from differences among individuals in
their ability to obtain mates
It has been shown that changes in the way that a population of sexual organisms chooses or
acquires mates can lead to rapid differentiation from the ancestral populations
The key point is that sexual selection promotes divergence efficiently because it affects gene
flow directly
16.4- Secondary Contact
Suppose a particular speciation event begins with the geographic isolation of two populations
and a corresponding reduction in gene flow, and then continues as selection, mutation, and drift
cause genetic divergence between the two groups
If populations have diverged during a period when the groups lived in different geographic
areas, then any hybrid offspring that are produced should have markedly reduced fitness
relative to individuals in each of the parental populations
o Basically if natural selection, genetic drift and adaptation worked against heterozygous
offsprings, then hybrid offsprings would have decreased fitness
Due to this, there should be strong natural selection in favour of assortative mating- meaning
that selection should favour individuals that choose mates only from the same population
Selection that reduces the frequency of hybrids in this way is called Reinforcement
Reinforcement occurs when hybrid offspring have low fitness, and natural selection leads to
assortative mating and the prezygotic isolation of populations
The reinforcement hypothesis predicts that when closely related species come into contact
andhybridize, a mechanism that prevents hybridization will evolve
o For ex. Selection might favour mutations that alter aspects of mate choice or life history
such as the timing of breeding
o Divergence in these traits prevents fertilization from occurring and results in prezygotic
isolation of the two species
o Prezygotic Isolation- reproductive isolation between populations caused by differences
in mate choice or timing of breeding, so that no hybrid zygotes are formed
But populations can also remain genetically isolated in the absence of the
reinforcement if hybrid offspring are sterile or infertile- known as postzygotic
Postzygotic Isolation- reproductive isolation between populations caused by
dysfunctional development or sterility in hybrid forms
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