Question

A group of researchers studying the effect of severe thunderstorms on wing length in songbirds. The researchers hypothesize that in areas where severe storms are common, songbirds with short wings are more likely to survive the high gusting winds and thus are selected for, making them more frequent in the population.

The researchers notice that in the midwest, a certain species of songbird is found to have far more short-winged birds in the population than long or medium winged birds.
The type of natural selection operating on this bird population is

• A. Sexual selection
• B. Directional selection
• C. Stabilizing selection
• D. Disruptive selection
• E. Convergent evolution

Answer 1

The correct option is D Disruptive selection

A. Sexual selection describes a special case of natural selection which describes an organism's ability to copulate with its mate.

B. Directional selection occurs whenever the environment changes in a particular way and therefore, is a selective pressure for species to change in response to the environmental change. For example industrial melanism. Industrial melanism is a phenomenon shown by light coloured peppered moth (Biston betularia typica) and dark coloured moth (Biston betularia carbonaria). It is a evolutionary process in which darker individuals predominate over lighter individuals since the industrial revolution is a result of natural selection.

C. Stabilizing selection occurs when the environment does not change. Natural selection has to produce change and if an environment does not change there is no pressure for well adapted species to change. Fossil evidences suggest that many species remain unchanged for long periods of geological time.

D. Disruptive selection occurs where an environmental change may produce selection pressure that favours two extreme characteristics. For example, grass plant on welsh copper mines. Contaminated soil containing copper is lethal to normal grass plants but a chance mutation allowed one plant to grow. This plant prospered and reproduced but only on the contaminated soil. On normal soil, it grew more slowly than normal plants and was easily out competed.

E. Convergent evolution is a process during which organisms with different evolutionary history evolve similar phenotypic adaptations in response to common environmental conditions.

Hence, species of song birds having short wings and medium wings shows two extreme characteristics. So, they show Disruptive selection.

So, the correct answer is 'disruptive selection'.