Question

Complete the analogy
Stabilizing selection: sickle cell anaemia: Directional selection: ......................
A. Elongation of neck of giraffes
B. Australian marsupials

Answer 1

Natural selection is the process due to which the organisms adapt and change. The change is heritable and is expressed over generations. Environmental change is the main driving force for adaptation.

Complete Answer:
To solve this question, we must know about the types of natural selection.
- The stabilizing selection and the directional selection are the types of natural selection. Let us discuss them.
1. Stabilizing selection - In this type the population stabilizes on a single non-extreme trait. It is the most common type of natural selection. It results in the decrease of population’s genetic variation. This type balances the two traits as in sickle cell anaemia which has two alleles. One allele is lethal which is eliminated by the selection of the other which protects against malaria.
2. Directional selection - It is the selection in which only one trait is favoured and the favoured trait moves in one direction and reaches the extreme. Example is the long neck of a giraffe. The long neck evolved in the single direction flow of sequence from short neck to long neck with respect to the availability of food.
Australian marsupial is an example of convergent evolution.

Thus the correct answer is (A). Stabilizing selection: sickle cell anaemia: Directional selection: Elongation of neck of giraffes.

Note: The organisms undergoing directional selection have greater reproducing capabilities than the other. It tends to the phenotype which is more fit. It can lead to more quick and vast change in the population.