BIOL 107 Lecture Notes - Zygosity, Hybrid Zone, Assortative Mating

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Published on 25 Jan 2013
Mechanisms of Divergence
Dispersal, vicariance, and polyploidization only create the conditions for speciation
o For speciation to take place, genetic drift & natural selection have to act on mutations in
a way that creates divergence in the isolated populations
Genetic Drift
Effects of genetic drifts are random fixation of alleles and random loss of alleles
Drift can produce rapid genetic divergence in small, isolated populations
o Its effects are most pronounced in small populations
Normally, only tiny numbers of individuals are involved in colonization events; vicariance events
fragment large populations into two or more smaller ones; and polyploidization initially
produces only a handful of individuals
o Due to this matter, genetic drift has long been hypothesized as the key to speciation’s
second stage
A general model of speciation:
o Small populations that become isolated start out as a non-random sample of the
ancestral population. As drift continues to occur in the small, derived population, it
leads to a random loss of allele and the random fixation of existing and new alleles. As a
result, the isolated population should undergo rapid genetic divergence from the
ancestral population.
Bottleneck- a large-scale but short-term reduction in population size followed by an increase in
population size
o Has shown that when a population is reduced to a small size for a short period of time
only very rare alleles tend to be lost due to drift
For drift to change allele frequencies dramatically, the founding population has
to be extremely small and remain small for a significant period of time
Natural Selection
Marked genetic differences have to emerge between closely related populations for speciation
to occur
o Drift almost always plays a role in creating these genetic differences when at least one
of the populations is small
o But natural selection can also lead to divergence if one of the populations occupies a
novel environment or uses a novel resource
Natural selection can cause populations to diverge even when a small amount of gene flow
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
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