Chapter 19: Evolutionary Change in Populations

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Genotype, Phenotype, and Allele Frequencies

• All the individuals that live in a particular place at the same time make up a population.
  • Each population has a gene pool, which includes all the alleles for all the loci present in the population.
• Population genetics is the study of genetic variability within a population and of the forces that act on it.
• A genotype frequency is the proportion of a particular genotype in the population.
  • A phenotype frequency is the proportion of a particular phenotype in the population.
  • An allele frequency is the proportion of a specific allele of a given genetic locus in the population.

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The Hardy–Weinberg Principle

• The Hardy–Weinberg principle states that allele and genotype frequencies do not change from generation to generation (no evolution is occurring) in a population at genetic equilibrium.
• The Hardy–Weinberg principle applies only if mating is random in the population, there are no net mutations that change the allele frequencies, the population is large, individuals do not migrate between populations, and natural selection does not occur.
• In the Hardy–Weinberg equation, p = the frequency of the dominant allele, q = the frequency of the recessive allele, and p + q = 1.
• The genotype frequencies of a population are described by the relationship p2 + 2pq + q2 = 1, where p2 is the frequency of the homozygous dominant genotype, 2pq is the frequency of the heterozygous genotype, and q2 is the frequency of the homozygous recessive genotype.

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Microevolution

• Microevolution is a change in allele or genotype frequencies within a population over successive generations.
• In nonrandom mating individuals select mates on the basis of phenotype, indirectly selecting the corresponding genotype(s).
  • Inbreeding is the mating of genetically similar individuals that are more closely related than if they had been chosen at random from the entire population.
  • Inbreeding in some populations causes inbreeding depression, in which inbred individuals have lower fitness (relative ability to make a genetic contribution to the next generation) than non-inbred individuals.
  • In assortative mating individuals select mates by their phenotypes.
  • Both inbreeding and assortative mating increase the frequency of homozygous genotypes.
• Mutations, unpredictable changes in DNa, are the source of new alleles.
  • Mutations increase the genetic variability acted on by natural selection.
• Genetic drift is a random change in the allele frequencies of a small population.
  • Genetic drift decreases genetic variation within a population, and the changes caused by genetic drift are usually not adaptive.
  • A sudden decrease in population size caused by adverse environmental factors is known as a bottleneck.
  • The founder effect is genetic drift that occurs when a small population colonizes a new area.
• Gene flow, a movement of alleles caused by the migration of individuals between populations, causes changes in allele frequencies.
• Natural selection causes changes in allele frequencies that lead to adaptation.
• Natural selection operates on an organism’s phenotype, but it changes the genetic composition of a population in a favorable direction for a particular environment.
• Stabilizing selection favors the mean at the expense of phenotypic extremes.
  • Directional selection favors one phenotypic extreme over another, causing a shift in the phenotypic mean.
  • Disruptive selection favors two or more phenotypic extremes.

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Genetic Variation in Populations

• Genetic polymorphism is the presence of genetic variation within a population.
• Balanced polymorphism is a special type of genetic polymorphism in which two or more alleles persist in a population over many generations as a result of natural selection.
• Heterozygote advantage occurs when the heterozygote exhibits greater fitness than either homozygote.
  • In frequency- dependent selection, a genotype’s selective value varies with its frequency of occurrence.
• Neutral variation is genetic variation that confers no detectable selective advantage.
• Geographic variation is genetic variation that exists among different populations within the same species.
  • A cline is a gradual change in a species’ phenotype and genotype frequencies through a series of geographically separate populations.

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