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Evolution of Populations
Marie Černá
Microevolution
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Genetic drift in small population
Gene flow by migration
Mutation
Nonrandom mating
Natural selection
Genetic Drift
Genetic Drift - small populations
Genetic Drift - large populations
Genetic Drift - Bottle neck effect
Genetic Drift - Bottle neck effect
northern elephant seals of
the coast of Mexico
During the 1890’s,
their population was reduced
due to hunting
to about 20 individuals.
(It is also likely that
one male would have
fathered the offspring of the
entire group.)
Genetic Drift - Founder principle
Drosophila subobscura (fruit fly)
European
→
American
Genetic Drift - Founder principle
Gene flow - migration
Mutation
Nonrandom mating
• inbreeding
• assortative mating
Natural selection
• the differential ability
to survive and
produce viable, fertile
offspring
• is likely to adapt a
population to its
environment
(accumulates and maintains
favorable genotypes)
Genetic variation: the substrate
for natural selection
• within populations
(polymorphism)
• between populations
(geographical variation)
Polymorphism
Geographic variation
Geographic variation - Cline
Sources of Genetic variation
• Mutation (affection of function)
• Sexual recombination
Crossing over in meiosis
Independent assortment
Preservation of Genetic variation
• Diploidy (heterozygote protection)
• Balanced polymorphism
Heterozygote advantage
Frequency-dependent selection
Modes of natural selection
Directional selection
is most common
- during periods of environmental change
- when members of a population migrate
to some new habitat with different
environmental conditions
Directional selection
Desert cactus population
peccaries
Diversifying selection
Diversifying selection
a species of finch lives in Cameroon, West Africa
small-billed birds
(soft seeds)
large-billed birds
(hard seeds)
Stabilizing selection
This mode of selection reduces variation and
maintains the status quo
for a particular phenotypic character.
It keeps the majority of
human birth weights
In the 3 – 4 kg range.
For babies
much smaller or larger than this,
infant mortality is greater.
Stabilizing selection
Desert cactus population
Literature
Biology, eighth edition,
Campbell, Reece
Unit four: Mechanisms of Evolution
Chapter 23: The Evolution of Populations
Pages 468 – 486