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The Chapter in Review
Evolution in a Genetic Context

Microevolution involves several elements:

  • All the various genes of a population make up its gene pool.

  • Hardy-Weinberg equilibrium is present when gene pool allele frequencies remain the same from generation to generation. Certain conditions have to be met to achieve an equilibrium.

  • The conditions are (1) no mutations, (2) no gene flow, (3) random mating, (4) no genetic drift, and (5) no selection. Since these conditions are rarely met, a change in gene pool frequencies is likely.

  • When gene pool frequencies change, microevolution has occurred. Deviations from a Hardy-Weinberg equilibrium allow us to determine when evolution has taken place and to measure the extent of change.

Causes of Microevolution

Microevolution is caused by five conditions:

  • Mutations are the ultimate source for variation. Certain genotypic variations may be of evolutionary significance, only if the environment changes. Genetic diversity is promoted when there are several alleles for each loci and when single nucleotide polymorphisms (SNPs) exist.

  • Gene flow occurs when a breeding individual (in animals) migrates to another population or when gametes and seeds (in plants) are carried into another population. Constant gene flow between two populations causes their gene pools to become similar.

  • Nonrandom mating occurs when relatives mate (inbreeding) or when assortative mating takes place. Both of these cause an increase in homozygotes. Sexual selection, which occurs when a characteristic that increases the chances of mating is favored, favors random mating.

  • Genetic drift occurs when allele frequencies are altered by chance—that is, by random sampling errors. Genetic drift is particularly evident after a bottleneck, when severe inbreeding occurs, or when founders start a new population.

  • Natural selection (see Section 15.2).

Natural Selection

Adaptation occurs when the more fit individuals reproduce more than others. These individuals usually possess traits better suited for survival in the environment, and over generations, the frequency of this adaptive trait increases within the population.

Types of Selection

Most of the traits of evolutionary significance are polygenic, and a range of phenotypes in a population result in a bell-shaped curve. Three types of selection occur:

  • Directional selection: The curve shifts in one direction, as when dark-colored peppered moths become prevalent in polluted areas.

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  • Stabilizing selection: The peak of the curve increases, as when most human babies have the intermediate birth weight. Babies that are very small or very large are less fit than those of intermediate weight.

  • Disruptive selection: The curve has two peaks, as when British land snails vary because a wide geographic range causes selection to vary.

Adaptations Are Not Perfect

Adaptations may not be perfect because evolution builds on what variation exists. Only certain types of variations are available, and developmental procedures tend toward the same types of results. The result is often a compromise between a benefit and a cost. Sexual selection has a reproductive benefit that may not pertain to adaptation.

Maintenance of Variations

Despite constant natural selection, variation is maintained because:

  • Mutations and recombination still occur; gene flow among populations can introduce new alleles; and natural selection may not eliminate less favored phenotypes.

  • In sexually reproducing diploid organisms, the heterozygote acts as a repository for recessive alleles whose frequency is low. In sickle cell disease, the heterozygote is more fit in areas where malaria occurs, and therefore both homozygotes are maintained in the population.

Key Terms
Testing Yourself

Choose the best answer for each question.

  1. A population consists of 48 AA, 54 Aa, and 22 aa individuals. What is the frequency of the A allele?

    1. 0.60

    2. 0.40

    3. 0.62

    4. 0.42

    5. 0.58

  2. Which of the following is the binomial equation?

    1. 2p 2 + 2pq + 2q 2

    2. p 2 + pq + q 2

    3. 2p 2 + pq + 2q 2

    4. p 2 + 2 pq + q 2

For questions 3 and 4, consider that about 70% of white North Americans can taste the chemical phenylthiocarbamide. The ability to taste is due to the dominant allele T. Nontasters are tt. Assume this population is in Hardy-Weinberg equilibrium.

  1. What is the frequency of t?

    1. 0.30

    2. 0.70

    3. 0.55

    4. 0.09

    5. 0.60

  2. What is the frequency of heterozygous tasters?

    1. 0.495

    2. 0.21

    3. 0.42

    4. 0.2475

    5. 0.45

  3. Typically, mutations are immediately expressed and tested by the environment in

    1. prokaryotes.

    2. eukaryotes.

    3. prokaryotes and eukaryotes.

    4. neither prokaryotes nor eukaryotes.

  4. The offspring of better-adapted individuals are expected to make up a larger proportion of the next generation. The most likely explanation is

    1. mutations and nonrandom mating.

    2. gene flow and genetic drift.

    3. mutations and natural selection.

    4. mutations and genetic drift.

  5. The northern elephant seal went through a severe population decline as a result of hunting in the late 1800s. The population has rebounded but is now homozygous for nearly every gene studied. This is an example of

    1. negative assortative mating.

    2. migration.

    3. mutation.

    4. a bottleneck.

    5. disruptive selection.

  6. Which of the following generally results in a net gain in genetic variability?

    1. genetic drift

    2. mutation

    3. directional selection

    4. bottleneck

    5. stabilizing selection

For questions 9–15, indicate the effect of each of the conditions of the Hardy-Weinberg principle on genotype and allele frequencies. Each answer may be used more than once.


  1. alters genotype and allele frequencies

  2. alters genotype frequency only

  3. alters allele frequency only

  4. does not alter genotype or allele frequency

  1. mutation

  2. gene flow

  3. inbreeding

  4. assortative mating

  5. genetic drift

  6. bottleneck

  7. natural selection

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  9. A small, reproductively isolated religious sect called the Dunkers was established by 27 families that came to the United States from Germany 200 years ago. The frequencies for blood group alleles in this population differ significantly from those in the general U.S. population. This is an example of

    1. negative assortative mating.

    2. natural selection.

    3. founder effect.

    4. bottleneck effect.

    5. gene flow.

  10. Assuming a Hardy-Weinberg equilibrium, 21% of a population is homozygous dominant, 50% is heterozygous, and 29% is homozygous recessive. What percentage of the next generation is predicted to be homozygous recessive?

    1. 21%

    2. 50%

    3. 29%

    4. 42%

    5. 58%

  11. When a population is small, there is a greater chance of

    1. gene flow.

    2. genetic drift.

    3. natural selection.

    4. mutations occurring.

    5. sexual selection.

  12. Which of the following is not expected to help maintain genetic variability?

    1. gene flow

    2. mutation

    3. recombination

    4. disruptive selection

    5. genetic drift

  13. The sickle cell allele is maintained in regions where malaria is prevalent because

    1. the allele confers resistance to the parasite.

    2. gene flow is high in those regions.

    3. disruptive selection is occurring.

    4. genetic drift randomly selects for the allele.

  14. Complete the following by drawing a curve in the other two graphs to show the effect of directional selection.

  15. Complete the following by drawing a curve in the other two graphs to show the effect of stabilizing selection.

  16. Complete the following by drawing curves in the other two graphs to show the effect of disruptive selection.

Thinking Scientifically
  1. A behaviorist studying mountain bluebird reproductive behavior found that male aggression toward another male was highest during nest construction, and then it lessened after the first egg is laid, and again after hatching occurred. a. Knowing that natural selection favors behaviors that save energy, explain the reproductive behavior of the male mountain bluebirds. b. What might make you rule out genetic drift as an explanation?

Bioethical Issue
Who Should Reproduce?

The highly regarded population geneticist Sir Ronald Aylmer Fisher published a book in 1930 entitled The Genetical Theory of Natural Selection. In the book, he claims that civilizations fail because individuals with the highest level of fitness (those at the top of the societal ladder) do not reproduce as often as less affluent individuals. He suggests that high-income couples be paid to have children in order to improve the population. In other words, Fischer is suggesting that we carry out selection within the human population.

Can you envision any scenario in which our society should encourage reproduction by the most-fit and discourage it by the least-fit? If so, how should it be done? What characteristics would be appropriate to select for or against?

Essentials of Biology Website

The companion website for Essentials of Biology provides a wealth of information organized and integrated by chapter. You will find practice tests, animations, videos, and much more that will complement your learning and understanding of general biology.