What Types of Barriers Isolate Evolution?

Timing

• Different timing of breeding cycles can prevent closely related species from mating. Two species of field crickets -- Gryllus pennsylvanicus and Gryllus veletis -- provide an example of this form of prezygotic barrier. While the former species reaches reproductive age in fall, the latter is not ready until spring. If a species seeks mates at a particular time of day, this can also reduce its chance of breeding with species that are on different schedules.

Mechanical and Structural Barriers

• Sometimes, two species cannot breed due to molecular or structural blockage of the formation of the zygote. In some cases, male gametes may be unable to reach the female egg due to anatomical incompatibility. In others, the gametes may reach the egg, but be unable to fertilize it.

Geography

• When species are isolated by geographic barriers, they evolve independently. For example, freshwater fish in an isolated alpine lake will evolve independently of similar freshwater fish in a Scandinavian lake. This was most famously demonstrated by avian evolution in the remote Galapagos Island chain, where Charles Darwin discovered a wide variety of finch species that had evolved in isolation to exploit very different ecological niches.

Habitat

• Many species exist only in certain habitats. The boundaries of these habitats serve as barriers that reduce the likelihood of a particular species meeting and breeding with a mate from a different habitat. Habitats can be large (e.g., a rainforest), or tiny. As an example, two species of Japanese ladybirds will only mate on their respective host plants: Epilachna nipponica mates on thistles, and Epilachna yasutomii mates on blue cohosh.

Behavior

• Prezygotic isolation can also occur when courtship displays, vocalizations and other mating signals differ. Among birds, one species' courtship display or song will typically fail to attract a mate of another species. Similarly, moths secrete species-specific pheromones, and fireflies produce light displays that appeal solely to insects of their own species.

Hybrids

• Postzygotic isolation occurs when a hybrid is born, but fails to breed. Hybrids may die before they reach reproductive age, or simply be unable to reproduce. This is the case with mules, which are sterile offspring of donkeys and horses. In some cases, a hybrid may successfully reproduce, but its offspring are normally unable to reproduce.