Question

Given below is a hypothetical situation
In a particular grassland, giraffes with long necks and short necks were present in almost equal numbers. As a result of drought and overgrazing by herbivores, surface vegetation became highly reduced and tall trees were the only plants that remained. It was observed that after a few generations, the percentage of tall-necked giraffes in the population increased to about 90%. This is a good example ___________

• A. stabilising selection
• B. directional selection
• C. disruptive selection
• D. diversifying selection

Answer 1

The correct option is B directional selection

As per the hypothetical situation given in the question, giraffes with long necks and short necks were present in almost equal numbers. As a result of drought and overgrazing by herbivores, surface vegetation became highly reduced and tall trees were the only plants that remained. Here, it can be assumed that there was a selection pressure against short necks, since individuals with short necks could not reach the branches of the trees to feed. The long-necked giraffes had a competitive advantage over the short-necked giraffes as they could reach and feed on the leaves of tall trees more easily.
As a result, the distribution of neck length shifted to favour individuals with long necks. This is an example of directional selection, a form of natural selection.
In directional selection, one of the extreme phenotypes is more fit than all the other phenotypes. Directional selection shifts the curve towards the favourable phenotype.

Stabilising selection favours individuals with intermediate phenotypes and diversifying or disruptive selection favours individuals with both the extreme phenotypes for a particular trait.