Disruptive Selection

Date: 2015-10-07; view: 456.

Directional Selection tends to favour phenotypes at one extreme of the range of variation. Insecticide resistance is an example. DDT was a widely used insecticide. After a few years of extensive use, DDT lost its effectiveness on insects. Resistance to DDT is a genetic trait that the presence of DDT in the environment made into a favoured trait. Only those insects resistant to DDT survived, leading over time to populations largely resistant to DDT (figure 32.2.).

Directional Selection

Stabilizing selection

Stabilizing selection favours the intermediate phenotype out of a range of phenotypes. The extremes in variation are selected against (figure 32.2).

Figure 32.2. Stabilizing selection, selection against the extremes in variation.

Figure 32.2. Directional selection.

Another example is the peppered moth (Biston betularia). Before the Industrial Revolution in the 18th and early 19th centuries, only light-colored moths were collected in light-colored woodlands in England. There was a rare, dark form. With the pollution caused by the buring of coal, the light-colored tree trunks became darker due to soot. The once rare dark-colored moths became more prevalent, while the once-common light-colored moths became increasingly rare. Reason: predation by birds. The color that had the greatest contrast with the background (tree trunk) was at a disadvantage. Cleanup of the forest during the 1950s caused the allele frequencies of light and dark moths to reverse to pre-Industrial Revolution levels, dark moths are now rare, light moths are now common.

The resistance of many bacterial species to antibiotics is another example of directional selection. Over 200 species show some degree of antibiotic resistance, necessitating the development and more prudent use of a new generation of antibiotic medicines.

Disruptive selection favours individuals at both extremes of variation: selection is against the middle of the curve. This causes a discontinuity of the variations, causing two or more morphs or distinct phenotypes. The African swallowtail butterfly (Papilo dardanus) produces two distinct morphs, both of which resemble brightly colored but distasteful butterflies of other species. Each morph gains protection from predation although it is in fact quite edible (figure 32.3.).

Figure 32.3. Disruptive selection.