Question

In a population of shrubs found at the perimeter of a deciduous forest, two shrub heights are present : short and tall. Over a period of several decades, biologists notice that the short shrubs become the predominant members of the shrub community.
Which of the following describes the type of selection occurring in the shrub population ?

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

Answer 1

Answer: (D)

Directional selection

This case shows natural selection. According to Darwin's concept of natural selection, as a struggle for existence, only those organisms could survive which have favourable variation to adapt with the environmental conditions. In other words, struggle for existence results in survival of the fittest which means that natural selection tends to favour those organisms which are most fit to survive with the reproductive age in a particular environment and to produce offspring.

Natural selection is of three types:

1. Directional selection: It 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.

2. Stabilizing selection: It 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.

3. Disruptive selection occurs where an environmental change may produce selection pressure that favours two extreme characteristics.

Thus, occurrence of short shrubs in the deciduous forest shows directional selection where the short shrubs become predominant over tall shrubs over a period of several decades.

So, the correct answer is Directional selection.