13. Evolution and Natural Selection


13.7. Patterns of Selection


Understanding the nature of the environment is key to determining how natural selection will affect how a species will change. In general, three forms of selection have been identified: stabilizing selection, directional selection, and disruptive selection (figure 13.10).


FIGURE 13.10. Patterns of Selection

In stabilizing selection, both extremes for a characteristic are selected against. Thus, gene frequencies do not change, and the original range of phenotypes is maintained. In directional selection, one extreme is selected for and the other extreme is selected against. This results in a shift in gene frequency and range of phenotype in a direction toward one extreme and away from the other. In disruptive selection, both extremes are selected for and the intermediate condition is selected against. This leads to the development of two distinct phenotypes with different gene frequencies.


Stabilizing Selection

Stabilizing selection occurs when individuals at the extremes of the range of a characteristic are consistently selected against. This kind of selection is very common. If the environment is stable, most of the individuals show characteristics that are consistent with the demands of the environment. For example, for many kinds of animals, there is a range of color possibilities. Suppose a population of mice has mostly brown individuals and a few white or black ones. If the white or black individuals are more conspicuous and are consistently more likely to be discovered and killed by predators, the elimination of the extreme forms will result in a continued high frequency of the brown form. Many kinds of marine animals, such as horseshoe crabs and sharks, have remained unchanged for thousands of years. The marine environment is relatively constant and probably favors stabilizing selection.


Directional Selection

Directional selection occurs when individuals at one extreme of the range of a characteristic are consistently selected for. This kind of selection often occurs when there is a consistent change in the environment in which the organism exists. For example, when a particular insecticide is introduced to control a certain species of pest insect, there is consistent selection for individuals that have alleles for resistance to the insecticide. Because of this, there is a shift in the original allele frequency, from one in which the alleles for resistance to the insecticide were rare to one in which most of the population has the alleles for resistance. Similarly, changes in climate, such as long periods of drought, can consistently select for individuals that have characteristics that allow them to survive in the dryer environment, and a change in allele frequency can result. Recent evidence also shows changes resulting from periods of rapid climate warming. The Yukon red squirrel, pitcher-plant mosquito, and European blackcap warbler show genetically based shifts in the timing of their seasonal reproduction, dormancy, or migration.


Disruptive Selection

Disruptive selection occurs when both extremes of a range for a characteristic are selected for and the intermediate condition is selected against. This kind of selection is likely to happen when there are sharp differences in the nature of the environment where the organisms live. For example, there are many kinds of insects that feed on the leaves of trees. Many of these insects have colors that match the leaves they feed on. Suppose the species of insect ranges in color from light green to dark green, and medium green is the most common. If a particular species of insect had some individuals that fed on plants with dark green leaves, whereas other individuals fed on plants with light green leaves, medium green insects could be selected against and the two extremes selected for, depending on the kind of plant they were feeding on.



14. Distinguish among stabilizing, directional, and disruptive selection.