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SHAPING EVOLUTIONARY THEORYSECTION 15.3
Mrs. Pachuta
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Hardy-Weinberg Principle
Genetic equilibrium (no evolution) will occur with the following conditions:1. No genetic drift
2 Types: 1. Founder Effect 2. Bottleneck
2. No gene flow (due to immigration/emigration)
3. Random mating4. No mutations5. No natural selection
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Hardy-Weinberg Principle
Genetic Drift – any change in allele frequencies in a population that is due to chance In larger populations, enough alleles ‘drift’
so that allele frequency stays fairly constant from one generation to another
In smaller populations, the effects of ‘drift’ are more pronounced
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Hardy-Weinberg Principle
Founder Effect – An extreme example of genetic drift; occurs when a small sample of a population settles in a location separated from the rest of the population. Result: Uncommon genes might become
more common in this group Example: Polydactyly in Amish population
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Polydactyly in Amish Population
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Hardy-Weinberg Principle
Bottleneck – another extreme example of genetic drift; occurs when population declines to very low number, then rebounds. Result: The gene pool of rebound
population often genetically similar to population at its lowest level – which means decreased diversity. Example: Cheetah population in Africa – almost
died out 10,000 yrs ago and again 100 yrs ago
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Bottleneck
Cheetahs are now so genetically identical, that they appear to be inbred. This decreases fertility and increases the
chance for extinction.
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Hardy-Weinberg Equation
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Directional selection – an extreme version of the trait is favored.
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Types of Natural Selection
Stabilizing selection – the average phenotype is favored
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Disruptional selection – favors both extreme versions of the trait; divides population into 2 groups
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Sexual selection – males and females differ significantly in appearance Darwin was intrigued that qualities of
attractiveness to the opposite sex appeared to decrease chance of survival
Example: peacocks
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PATTERNS OF EVOLUTION
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Divergent Evolution(diverge = moving apart)
Also called “adaptive radiation” One species gives rise to many species
as result of different environmental conditions Habitat Food source
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Convergent Evolution(converge = “coming together”)
Occurs when unrelated species evolve similar traits because they live in a similar environment
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Coevolution
Species in a close relationship evolve in response to one another
Example: plants and pollinators
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RATE OF SPECIATION
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Gradualism
Theory that says evolution proceeds in small, gradual steps.
(left side)
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Punctuated Equilibrium
Theory that says evolution occurs through abrupt changes
(right side)