We mentioned earlier that pre-mating mechanisms are concerned with the prevention of occurrence of interspecific crosses. In such cases,
a) the potential mates do not meet in which case the isolation is due to either geographical, seasonal or habitat reasons.
b) the potential mates do meet but fail to mate in which case the isolation is due to ethological reasons.
c) the potential mates meet and mate but no transfer of sperm takes place in which case the isolation is due to mechanical reasons.
Let us have each one of these mechanisms examined in detail.
Geographical Isolation
Many species of organisms are restricted in their distribution even as they may be found over a vast geographical regions. Studies using distribution maps of organisms have shown that although species may be continuously found over vast areas, a close scrutiny would reveal that the distribution is rather restricted to those regions with suitable ecological features. For instance, the American plane tree, Platanus occidentalis although distributed over more than half the United States, the natural groves of the plants are confined to bottom lands and stream banks. This is true of several species of organisms whose populations are separated by barriers of territory which they cannot use for ecological reasons. For instance, take the case of amphibians. A small body of salt water could be a barrier for their dispersal. Oceanic islands are not inhabited by amphibians except as in the case of Hawaii where they are introduced by man. Salt water is also a barrier for fresh water fishes. Pacific ocean receives many parallel streams of fresh water each of which had its own sub-species or species of fishes. Only the flooding during the rainy season join the streams and the fishes may be found together.