Mechanical Isolation

Mechanical isolation occurs when mating is physically impossible. Often this occurs because the genitalia of different species are incompatible. Bush babies, a group of small arboreal primates, are divided into several species based on mechanical isolation. Each species has distinctly shaped genitalia that, like locks and keys, only fit with the genitalia of its own species.

Gametic Isolation

In some cases, there are no barriers to mating, but the gametes are unable to fuse and fertilization does not take place. Often this occurs because the female immune system recognizes sperm as foreign and attacks it. In other cases, the sperm or pollen may not be adapted to the environment of the female reproductive track.