This section describes mechanisms by which species may become reproductively isolated before a zygote can be formed. In some cases mating between species cannot occur, while in other cases mating occurs, but fails to produce a zygote.
The most obvious form of reproductive isolation is spatial isolation. If members of two populations never encounter each other, they will never mate and no gene flow will occur. Spatial isolation occurs between populations that are separated by great distances, but it can also take place between populations that inhabit different parts of the same area.
Geographical isolation occurs when a physical barrier separates populations. This can happen in two ways: 1) A previously continuous population can be divided by the appearance of a barrier, an event known as vicariance; 2) A population may also be divided when some members cross an existing physical barrier, known as dispersal. In some cases there is no barrier other than great distance. For example, some species of frogs inhabit areas from the Northeastern US to Mexico. Within this distribution, neighboring populations are not isolated from each other and may interbreed. However, populations from the two ends of the distribution never meet and cannot mate.
For habitat isolation to occur, populations do not need to be separated by great distance. Instead, they must occupy different habitats, even within the same area. For example, if two populations of flies exist in the same geographical area, but one group lives in the soil and another lives on the surface of the water, members of the two populations are very unlikely to meet and reproduce.
Temporal isolation represents another way in which populations living in the same area can be prevented from mating. Different populations may be ready to mate at different times of the year. For example, two populations of plants may produce flowers in different seasons, making mating between the populations impossible.
In many animals, courtship displays and rituals are vital to reproduction. Such behaviors can be very specific, varying between closely related species. Male behaviors such as courtship calls, songs, and dances will only be recognized by females of the same species. Some species of crickets are morphologically identical, but can be distinguished by the fact that females will only respond to the mating songs of males of their own species. Males of other species are ignored.
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.
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.