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.
Spatial Isolation
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
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.
Habitat Isolation
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
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.
Behavioral Isolation
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.
