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Population Genetics and Causes of Microevolution

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BIOL 103
Peter T Boag

Population Genetics and Causes of Microevolution - Changes in allele frequencies of a population caused by microevolution: o Mutation (origin, but weak for evolution) o Nonrandom mating o Genetic drift o Gene flow o Natural selection o Table 22.1 - Hardy-Weinberg principle o Provides a ‘Null Hypothesis’ for what happens to gene frequencies between generations in absence of evolution o What we expect to happen in gene frequencies o Explains stability of allele frequencies over successive generations in populations at genetic equilibrium with random mating o Essential to understanding mechanisms of evolutionary change o For equilibrium to obtain, the following conditions must be true  Populations are very large  Individuals mate at random  Immigration and emigration are zero  Mutation is rare or absent - Hardy-Weinberg Equations o p = frequency of dominant allele o q = frequency of recessive allele o p + q = 1 2 2 o Genotype frequencies in population reflect the relationship p + 2pg +q = 1 o 2pq is frequency of heterozygous genotype o q is frequency of homozygous recessive genotype - In sexual reproduction, parental genotype frequencies determine allele frequencies in gamete pool - Alleles randomly unite into zygotes forming new genotypes, whose frequencies are the product of allele frequencies in the gamete pool – if parent frequencies change, take only 1 generation to reach new equilibrium - Cause of microevolution: o Nonrandom mating  Inbreeding; selfing or matings between close relatives  Can produce inbreeding depression, if population previously outbred  Expect every generation to produce some homozygous offspring, but homozygous offspring can only produce homozygous offspring  Does not change allele frequencies, but genotype frequencies do change as heterozygotes disappear (if return to random mating)  Assortative mating common in animals, e.g. tall humans prefer tall males  Both eventually decrease frequency of heterozygous genotypes  Inbreeding depression occurs in Lobelia cardinalis when plants self- fertilize – can outcross, or produce C (C produced by self-fertilization)  Self-fertilization has reproductive individuals with fitness that is low  Inbreeding depression is detectable in humans in first-cousins and closer unions o Genetic Drift  Random changes of allele frequencies in small populations, often due to ‘sampling’ = genetic drift  Decreases genetic variation within a population; may increase differences between populations  Changes are usually not adaptive, but can lead to population differentiation, even set the stage for speciation  Purely statistical p
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