In order to keep a population from evolving certain characteristics must be present. Since many populations are not isolated, there is a large possibility that organisms will move into and out of a gene pool.
Populations vary in the types of individuals and their reproductive success.
Two species may live in the same area but in different habitats. Since there is little if any contact the possibility of successfully mating is drastically reduced. Since the breeding times of similar organisms are different there is no chance of reproductive contact. Birds, mammals, and insects have pre-mating rituals that attract the proper mate. Geographic isolation plays an important role in species development and maintenance.