The development of insecticide-resistant populations of insects or antibiotic resistance in bacteria is two example of directional selection. Directional selection can cause the evolution of a species with time, but does not increase bio-diversity . If a population is exposed to different patterns of selection in different parts of its range, the sub-populations may well diverge with time, each responding to the local selection pattern. Such disruptive selection can also be viewed as different patterns of directional selection occurring within the range of a species. If gene flow is restricted between the sub-populations, they may well evolve into different species, given time for the development of isolating mechanisms that would serve to keep the populations distinct, even if they later come into contact.
Disruptive selection can increase bio-diversity.
In the extreme, where one species evolves into several, the process is known as adaptive radiation . Disruptive selection can also operate within a species to produce distinctive morphotypes. The example of hooknose and jack phenotypes in male coho salmon is one such case. Where such selection generates pronounced differences between the sexes, as in the plumage and singing behavior of birds, the process is known as sexual selection . Sexual selection is probably best viewed as a special case of disruptive selection.