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Published byRhoda Fowler Modified over 9 years ago
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A change in allele frequency
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Q: How do scientists know when this occurs? A: They compare it to a non-changing population = Ideal population (like a “perfect” car… it only exists in a showroom)
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5 Characteristics of an “Ideal Population” Population is very large Large depends on the population, but thousands or hundreds are assumed Mating is random No phenotype has a greater chance of mating
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No mutation of alleles No immigration or emigration No movement into or out of population No selection occurs No survival advantage goes to one phenotype over another
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If these 5 conditions are met… There is no change in allele/genotypic frequencies… and no evolution When these rules are broken, microevolution is said to be occurring.
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For Hardy-Weinberg Equilibrium: p stands for the frequency of the dominant allele Ex. “A” q stands for the frequency of the recessive allele. Ex. “a” In H-W, or 100% of the alleles p + q = 1
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p 2 = frequency of homozygous dominant genotype Ex. “AA” q 2 = frequency of homozygous recessive genotype Ex. “aa” 2pq = frequency of heterozygous genotype Ex. “Aa” p 2 + 2pq + q 2 = 1, or 100%
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Example 1: Approximately 9% of Americans of African descent suffer from sickle cell anemia, which is inherited as a recessive trait. What is the frequency of the sickle cell allele? Approximately what percentage of this sub- population carries the sickle cell allele?
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Example 2: Cystic fibrosis is known to occur as a recessive trait in human populations. In a genetic study, the frequency of the recessive allele for a population was found to be 2.0%. What percentage of the population would be expected to exhibit Cystic Fibrosis? What percentage of the population would be normal, but carry a CF allele?
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What Hardy-Weinberg taught us… 1. We can track changes in genes 2. If genes change, a population is evolving
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How does this happen? Natural Selection Types: A. Stabilizing Selection B. Directional Selection
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Disruptive selection
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Not all changes in genes are due to natural selection, though…
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Other ways to change gene frequencies…
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Mutation a) Point b) Duplication
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Sexual recombination
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Genetic Drift Change in allele frequencies that is due to chance More common in small populations Several types of genetic drift…
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A. Founder effect Small sample of a population settles in a new location (migrates)
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B. Bottleneck Effect Population declines to a very low number due to “event” then rebounds… New “composition” to the population
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2. Gene Flow Genes “move” into or out of a population…
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