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Chapter Summary

  1. The field of population genetics provides a quantitative way of describing, modeling, and predicting how allele and genotype frequencies in populations change over time.
  2. The Hardy–Weinberg model serves as a null model in population genetics, telling us what happens to allele frequencies and genotype frequencies when no evolutionary processes—natural selection, mutation, nonrandom mating, migration, and genetic drift—are operating.
  3. When none of these five evolutionary processes are operating, the Hardy–Weinberg model makes three predictions: (a) allele frequencies will not change over time, (b) genotype frequencies will be the so-called Hardy–Weinberg equilibrium frequencies, and (c) a population with genotype frequencies away from Hardy–Weinberg equilibrium frequencies will return to these frequencies in a single generation.
  4. For a locus with two alleles, A1 and A2, at frequencies p and q respectively, the Hardy–Weinberg genotype frequencies are as follows: f[A1A1] = p2, f[A1A2] = 2pq, f[A2A2] = q2.
  5. Natural selection, mutation, nonrandom mating, and migration can each drive changes in genotype frequencies in a population.
  6. Natural selection can take on various forms. Directional selection, overdominance, and underdominance are types of frequency-independent selection in which the fitness of a genotype is independent of its frequency in the population; these contrast with positive and negative frequency-dependent selection, in which the fitness of a genotype depends on the genotype frequencies in the population.
  7. Mutation–selection balance can maintain deleterious alleles at low frequency in a population.
  8. Assortative mating, in which individuals tend to mate with similar individuals, increases the frequency of homozygotes in a population; disassortative mating, in which individuals mate with dissimilar individuals, increases the frequency of heterozygotes.
  9. Migration between populations brings their allele frequencies closer to one another.
  10. The evolutionary processes considered in this chapter have diverse but predictable effects on variation within and between populations.