recap

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20.2 recap

Evolutionary processes change the genetic structure of a population from one generation to the next. These processes include mutation, natural selection, gene flow, genetic drift, and nonrandom mating.

learning outcomes

You should be able to:

  • Distinguish between the everyday and scientific uses of the terms “adapt” and “evolve.”

  • Describe how each of the five processes of evolution can result in changes in gene frequencies across generations of a population.

  • Predict the effects of each of the five processes of evolution on a population.

Question 1

How is mutation a necessary component of the evolutionary processes?

Mutations provide the genetic variation on which all the other evolutionary processes act.

Question 2

What kind of traits would be more likely to evolve by genetic drift than by natural selection?

Neutral traits (traits that confer neither an advantage nor a disadvantage) are free to increase or decrease in a populations by drift alone. In small populations, chance effects have a large role. If the difference in fitness between individuals is relatively small, and there are few individuals in a population, then which individuals survive and reproduce is likely to be a result of factors that do not relate directly to the fitness of the organism. Under these conditions, even some mildly deleterious traits are expected to become fixed in the population over short periods of time.

Question 3

Describe conditions where nonrandom mating does and does not lead to evolution.

Self-fertilization reduces the frequency of heterozygotes but does not change the allele frequencies in a population. By contrast, sexual selection (nonrandom mating in preference for a particular phenotype) produces a directional change in the population. Hence, the population evolves.

Question 4

Can you think of reasons why gene flow might slow the evolution of a population?

A small population might be under strong selection for a particular trait that is favored locally. However, if there is extensive gene flow from neighboring populations where the particular trait is not favored, then selection for the trait in question will be overwhelmed by the gene flow from the surrounding populations.

The processes of mutation, natural selection, gene flow, genetic drift, and nonrandom mating can all result in evolutionary change. We will consider next how evolutionary change that results from these processes is measured.