Microevolution
• Microevolution is the Change in allele frequency
due to one of the following processes:
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Natural Selection
Mutations
Genetic Drift
Gene flow
Types of Natural selection
• Direction Selection
– Selection favors one of
the extreme types in
the population.
– Often the result in
directional changes in
the environment
– Examples: Pesticide
and antibiotic
resistance
Types of Natural Selection cont.
• Stabilizing Selection
– Selection favors the
intermediate forms
while selecting against
the extremes.
– Occurs in stable
environments in which
organisms are well
adapted
Types of Natural Selection cont.
• Disruptive Selection
– Selection favors both
extremes while the
intermediate forms are
selected against.
– Over long periods of
time disruptive
selection may lead to
speciation.
Sexual Selection
• Individuals will select potential mates
according to specific characteristics.
• These traits may or may not be healthy for
the species.
• Example: Irish Elk
Mutations
• Mutations are the raw material for all new
alleles.
• New mutations will change gene frequency.
• Natural selection will either amplify or
diminish mutations in a population.
Genetic Drift
• Genetic drift is a random change in allele frequencies over
the generations, brought about by chance events.
• Genetic drift is most powerfully felt in small populations.
• The founder effect and the bottleneck effect are two
examples where genetic drift changes allele frequency in a
population.
Founder Effect
• The founder effect occurs when a small group
becomes isolated from its larger population.
• The new group will likely have a different allele
frequency than the parent group it originated from.
• Example: Mutiny on the Bounty
Bottleneck
• A bottleneck is a severe reduction in a population
brought about by intense selection pressure or
some natural calamity.
• The population is regenerated by means of the few
individuals left.
• The genetic diversity is greatly diminished
compared to the original population.
Gene Flow
• The movement of alleles between different
populations of the same species.
• Alleles are lost from a population when
individuals leave (emigrate). Alleles enter a
population when new individuals move in
(immigrate)