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Chapter Summary

  1. In finite populations, allele frequencies will fluctuate as a result of random sampling effects. This process is known as genetic drift.
  2. Genetic drift operates more strongly in smaller populations than in large populations.
  3. Genetic drift reduces the heterozygosity—the fraction of individuals who are heterozygous at a given locus—within a population by causing alleles to be fixed or lost even in the absence of natural selection.
  4. Because genetic drift is a random process, it causes divergence between populations over evolutionary time.
  5. We can trace the genealogy of individual gene copies through a population. For any sample of gene copies at a single locus, somewhere in the past there is an ancestral gene copy from which all copies in our sample are descended.
  6. Tracing this genealogy of gene copies back in time, we derive the coalescent tree. In a sexual population, every locus has a different coalescent tree.
  7. Population bottlenecks, in which populations are temporarily reduced to a small number of individuals, accelerate genetic drift and can cause substantial changes in allele frequencies.
  8. Allele frequencies in peripheral and island populations can differ greatly from allele frequencies in the populations from which they were derived because of the founder effect.
  9. Drift reduces heterozygosity in a population, but mutation creates new variation. The mutation–drift balance represents a steady state between these two processes. Drift also interacts with natural selection and can reduce the ability of selection to fix favorable alleles.
  10. The neutral theory of molecular evolution proposes that most variation in a population is neutral and most substitutions that occur over evolutionary time are neutral substitutions. If so, it follows that genetic drift plays a major role in the evolutionary process.
  11. Under the neutral model, the fixation rate in a population is equal to the mutation rate in an individual in that population.
  12. At many loci, molecular changes occur at an approximately constant rate over time. The behavior of this molecular clock makes it possible to assign dates to the branch points on a phylogeny using DNA sequences.