PARAPATRIC SPECIATION

Another form of speciation, called parapatric (Gr. para, beside)
speciation, occurs in small, local populations, called demes. For
example, all of the frogs in a particular pond or all of the sea
urchins in a particular tidepool make up a deme. Individuals of a
deme are more likely to breed with one another than with other
individuals in the larger population, and because they experience
the same environment, they are subject to similar selection pressures.
Demes are not completely isolated from each other because
individuals, developmental stages, or gametes can move among
demes of a population. On the other hand, the relative isolation of
a deme may mean that its members experience different selection
pressures than other members of the population. If so, speciation
can occur. Although most evolutionists theoretically agree that
parapatric speciation is possible, no certain cases are known. Parapatric
speciation is therefore considered of less importance in the
evolution of animal groups than allopatric speciation.