Natural selection can both change and stabilize phenotypes within populations. Directional selection can result in continuous change to phenotypes; stabilizing selection slows change from a favored phenotype; and disruptive selection can result in one population that exhibits multiple distinct phenotypes.
learning outcomes
You should be able to:
Describe the effect of each type of selection (stabilizing, directional, and disruptive) on phenotype distribution.
Distinguish conditions in which selection is likely to be directional, stabilizing, or disruptive.
Describe the different expected outcomes of stabilizing, directional, and disruptive selection.
Stabilizing selection results in a reduction in variation within the population and an increase in frequency of the modal phenotype. Directional selection results in a change in the modal phenotype of the population in one direction. Disruptive selection results in a population with a bimodal distribution of phenotypes.
Why would you expect selection on human birth weight to be stabilizing rather than directional?
As shown in Figure 20.13, mortality increases with both smaller and larger birth weight compared to the optimal birth weight of about 7.5 pounds. Significantly smaller babies are more likely to be premature or undernourished, and thus less likely to survive. Significantly larger babies may cause difficulties in delivery, putting both the baby and mother at greater risk and stress during childbirth. Thus, babies close to the modal birth weight are most likely to survive and grow into adults, thereby increasing the relative frequency of alleles for intermediate birth size in the population.
Can you think of examples of extreme phenotypes in animal or plant populations that could be explained by directional selection?
Almost any phenotype of an organism that differs markedly from its close relatives is likely the result of directional selection. Among the many thousands of possible examples, consider the long necks of giraffes, the long trucks of elephants, the great body size of whales, the large brain of humans, the great height of giant sequoias, and the large floating leaves of water lilies. Directional selection obviously can result in reduction in size as well, so also consider the tiny body size of hummingbirds compared to other birds, or the tiny leaf size of duck weed compared to other flowering plants. Undoubtedly, you can think of many other examples.
Genetic drift, stabilizing selection, and directional selection all tend to reduce genetic variation within populations. Nevertheless, as we have seen, most populations harbor considerable genetic variation. What processes produce and maintain genetic variation within populations?