There is a certain type of fish in a lake. Fisherman can catch the largest of these fish using nets. They don't like the little fish and most of the smaller ones can get through the holes of the nets. After a few decades of this type of fishing, how do you think the fish population may change? Use the ideas explored in this article to give an explanation for your answer.

After a few decades, there will be only small fishes, as directional selection will be acting in this population.

Explanation:

Directional selection increases in the proportion of individuals with an extreme phenotypic trait. Individuals expressing the other phenotypic extreme might be receiving different pressures that drive them to decrease their fitness. These pressures, in general, might be related to food access, niches availability, reproduction, predation, among others.

In this example, there are two extreme phenotypic traits, large and small fishes. Large fishes are being predated by fishermen, who prefer large individuals rather than small ones. Large fishes fitness start to decline, as they are being hunted. Only small fishes get to survive, they get to reproduce and hence, to have better fitness. Natural selection is acting on the size of these animals, favoring the small-sized ones. Over a few decades, the directional selection will increase the proportion of small fishes.