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Daily Warm-up
February
th
7
What is genetic drift? What are the two types?
HW:
-Darwin video due tomorrow
-Read 21.4
Turn in:
-Nothing
5 Evolutionary Agents
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Natural selection
Sexual selection
Mutation
Gene flow
Small population size (genetic drift)
Genetic Drift
 Chance events that take out alleles at random
 Ex. Earthquake, flood
 More pronounced in small populations
 Two Types
 Founder effect
 Individuals become “founders” o f new population
 Bottleneck effect
 Population has experienced a “bottleneck” and certain
alleles may be over-represented
Founder effect example
Cheetahs as a bottleneck example
Genetic Drift
 Significant in small populations
 Causes allele frequencies to change at random
 Can lead to a loss of genetic diversity
 Can cause harmful alleles to be more prominent
Gene Flow
 Transfer of alleles into or out
of a population
Question
In what sense is natural selection more predictive than
genetic drift?
Natural Selection
 Only agent that consistently causes adaptive evolution
 Survival and reproductive advantage
 Can be measure by relative fitness
 Contribution to the gene pool by an individual relative to the other
members of the population
3 Types of Evolution
1. Directional selection= conditions favor individuals at
one extreme
2. Stabilizing Selection
 Intermediate phenotype versions are selected for by
conditions
3. Disruptive Selection
 Intermediate phenotype is selected against, both
extremes selected for by conditions.
Sexual Selection
 Traits that may or may not improve the likelihood of
obtaining a mate
 Sexual dimorphism- difference in
secondary sex characteristics
ex. Color, ornamentation, behavior
Intrasexual
Intersexual (mate choice)
*Birds of Paradise
Preservation of Genetic Variation
 Neutral variation- genes that do not confer any
advantage or disadvantage help maintain genetic
variation
 Also Diploidy- recessive allele is “hidden”
 Balancing selection- natural selection keeps two or more
forms in the population
 Heterozygote advantage
 Frequency-dependent
Heterozygote Advantage
 When heterozygotes have greater fitness than either
homozygous forms
 Ex. Sickle-cell disease and
malaria
Frequency-Dependent Selection
 Fitness of phenotype
depends on how common
in the population.
 Ex. Fish in lake
 Fungi in population of
insects (see video)
Why doesn’t natural selection=
perfect organism?
1. Selection can only act on existing variation
2. Evolution is limited by historical constraints
-Limited to ancestral structures
3. Adaptations are often compromises
4. Chance, natural selection, and environment interact