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

Hint: The type of selection occurring in the shrub population is a part of a natural selection. Charles Darwin gave the concept of natural selection. In this type of selection occurring in the shrub population, the changes occur in one path, and extreme phenotype is preferred over other phenotypes.

Complete answer:
• Natural selection: It brings a change in the frequency of genes and promotes adoption as a result of evolution. It is of three types: Stabilizing selection, Directional selection, and Disruptive selection.
• Directional selection: This type of selection happens due to the condition of the environment which changes in one direction. Here extreme phenotype is preferred over other phenotypes and elimination of medium phenotype and one extreme phenotype occurs. It plays a major role in speciation.
• Disruptive selection: This type of selection can be influenced by many factors. It favors both extreme phenotypes and eliminates medium phenotype.
• Stabilizing selection: In this type of selection, the condition of the environment remains almost unchanged. It favors medium phenotype and eliminates both extreme phenotypes.

So, in the above question directional selection happens when the short shrub becomes predominant over the tall shrub in the shrub community.
Hence, the correct answer is option (D).

Note: In directional selection, the evolutionary change occurs progressively as it causes rapid changes in allele frequency. In stabilizing selection, the evolutionary changes do not occur as there are no drastic changes in a trait. In disruptive selection, evolutionary changes occur but it is very rare.