Scientists are studying the mating habits of a population of butterfly fish on a Caribbean coral reef. They discover that both the largest and smallest males get the most mates and pass on the largest number of genes to the population's offspring. The largest males get a large number of mates because they can guard the females, preventing most of the smaller males from mating. However, the very smallest males also get a large number of mates because they escape the notice of the larger males and are able to mate with the females quickly. Based on these observations, which selective force appears to be acting on male body size in this population?

Disruptive selection is a mode of natural selection that exhibits that two extreme values of phenotypic traits of the population are favored by this population than the population with their intermediate values. Disruptive selection occurs when there is a fitness advantage of extreme values of phenotypic traits over more intermediate phenotypes.

In this case, the largest males mates more, due to fact that they can guard the females, than smaller males. The smallest fishes also get a high number of mates as are able to mate with the females quickly without coming to the notice of larger males.