Allopatric speciation: the great divide
speciation is just a fancy name for speciation by geographic
isolation, discussed earlier. In this mode of speciation, something
extrinsic to the
organisms prevents two
or more groups from mating with each other regularly, eventually
causing that lineage to speciate. Isolation might occur because of
great distance or a physical barrier, such as a desert or river,
as shown below.
Allopatric speciation can occur even if the barrier is a little "porous," that is, even if a few
individuals can cross the barrier to mate with members of the other group. In order for a speciation
even to be considered "allopatric," gene flow between the soon-to-be species must be greatly reduced but
it doesn't have to be reduced completely to zero.