Hardy–Weinberg Equilibrium
- If Hardy–Weinberg Equilibrium is achieved then no evolution occurs.
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To achieve the equilibrium five conditions must be met:
- Population must be very large.
- Population must be isolated from other populations (no immigration or emigration).
- No mutations
- Random mating
- No natural selection (i.e. every individual has an equal chance of survival)
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If the five conditions are not met then evolution occurs:
- There is a change in allele frequency in the population.
- Hardy–Weinberg Equilibrium is not present.
Consequences of Violations of the Five Conditions
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A small population causes genetic drift
- Changes in allele frequency due to chance
- Example: a small number of coin tosses versus large number of coin tosses
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Special cases of genetic drift:
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Bottleneck Effect
- Occurs when some event reduces the population to a really small size.
- Example: Northern Elephant Seal population was almost hunted to extinction—only about a dozen individuals survived to repopulate the current large population.
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Founder Effect
- Occurs when a few individuals colonize a new area.
- Example #1: immigration of religious groups (in small numbers) to the United States that will not reproduce with other individuals. They maintain a very small isolated population.
- Example #2: Darwin's finches
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Non-isolated population causes gene flow
- Movement of individuals from one population to another, resulting in interbreeding of what were once isolated populations.
- Reduces genetic differences between populations.
- Mutations cause new alleles to be introduced into the population.
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Nonrandom mating causes more expression of recessive phenotypes
- Inbreeding: mating between closely related partners
- Assortive mating: individuals choose partners like themselves.
- In both cases, two organisms carrying a recessive gene are more likely to pair up, so a recessive phenotype (e.g. resulting from genotype aa) is more likely to be expressed.
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Natural selection causes the expression of successful alleles
- Certain alleles provide traits that create a better ability to survive and reproduce than the traits of other alleles do.