Connecting Concepts Across Chapters 9

CONCEPTS SUMMARY]_

• Population genetics examines the genetic composition of groups of individuals and how this composition changes with time.

• A Mendelian population is a group of interbreeding, sexually reproducing individuals, whose set of genes constitutes the population's gene pool. Evolution occurs through changes in this gene pool.

• Genetic variation and the forces that shape it are important in population genetics. A population's genetic composition can be described by its genotypic and allelic frequencies.

• The Hardy-Weinberg law describes the effect of reproduction and Mendel's laws on the allelic and genotypic frequencies of a population. It assumes that a population is large, randomly mating, and free from the effects of mutation, migration, and natural selection. When these conditions are met, the allelic frequencies do not change and the genotypic frequencies stabilize after one generation in the Hardy-Weinberg equilibrium proportions p2, 2pq, and q3, where p and q equal the frequencies of the alleles.

• Nonrandom mating affects the frequencies of genotypes but not alleles. Positive assortative mating is preferential mating between like individuals; negative assortative mating is preferential mating between unlike individuals.

• Inbreeding, a type of positive assortative mating, increases the frequency of homozygotes while decreasing the frequency of heterozygotes. Inbreeding is frequently detrimental because it increases the appearance of lethal and deleterious recessive traits.

• Mutation, migration, genetic drift, and natural selection can change allelic frequencies.

• Recurrent mutation eventually leads to an equilibrium, with the allelic frequencies being determined by the relative rates of forward and reverse mutation. Change due to mutation in a single generation is usually very small because mutation rates are low.

• Migration, the movement of genes between populations, increases the amount of genetic variation within populations and decreases differences between populations. The magnitude of change depends both on the differences in allelic frequencies between the populations and on the magnitude of migration.

• Genetic drift, the change in allelic frequencies due to chance factors, is important when the effective population size is small. Genetic drift occurs when a population consists of a small number of individuals, is established by a small number of founders, or undergoes a major reduction in size. Genetic drift changes allelic frequencies, reduces genetic variation within populations, and causes genetic divergence among populations.

• Natural selection is the differential reproduction of genotypes; it is measured by the relative reproductive successes of genotypes (fitnesses). The effects of natural selection on allelic frequency can be determined by applying the general selection model. Directional selection leads to the fixation of one allele. The rate of change in allelic frequency due to selection depends on the intensity of selection, the dominance relations, and the initial frequencies of the alleles.

• Mutation and natural selection can produce an equilibrium, in which the number of new alleles introduced by mutation is balanced by the elimination of alleles through natural selection.

• Molecular methods offer a number of advantages for the study of evolution. The use of protein electrophoresis to study genetic variation in natural populations showed that most natural populations have large amounts of genetic variation in their proteins. Two hypotheses arose to explain this variation. The neutral-mutation hypothesis proposed that molecular variation is selectively neutral and is shaped largely by mutation and genetic drift. The balance model proposed that molecular variation is maintained largely by balancing selection.

• Different parts of the genome show different amounts of genetic variation. In general, those that have the least effect on function evolve at the highest rates.

• The molecular-clock hypothesis proposes a constant rate of nucleotide substitution, providing a means of dating evolutionary events by looking at nucleotide differences between organisms.

• Molecular data are often used for constructing phylogenies.

[important terms_

Mendelian population (p. 000) gene pool (p. 000) genotypic frequency (p. 000) allelic frequency (p. 000) Hardy-Weinberg law (p. 000) Hardy-Weinberg equilibrium (p. 000) positive assortative mating (p. 000)

negative assortative mating (p. 000) inbreeding (p. 000) outcrossing (p. 000) inbreeding coefficient (p. 000) inbreeding depression (p. 000) equilibrium (p. 000) migration (gene flow) (p. 000) sampling error (p. 000)

genetic drift (p. 000) effective population size (p. 000) founder effect (p. 000) genetic bottleneck (p. 000) fixation (p. 000) fitness (p. 000) selection coefficient (p. 000) directional selection (p. 000) overdominance (p. 000)

underdominance (p. 000) phylogeny (p. 000) proportion of polymorphic loci (p. 000) expected heterozygosity (p. 000) neutral-mutation hypothesis (p. 000) balance hypothesis (p. 000) molecular clock (p. 000)

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