ALLOPATRIC SPECIATION

Allopatric (Gr. allos, other patria, fatherland) speciation occurs
when subpopulations become geographically isolated from one another.
For example, a mountain range or river may permanently separate
members of a population. Adaptations to different environments
or neutral selection in these separate populations may result in
members not being able to mate successfully with each other, even if
experimentally reunited. Many biologists believe that allopatric speciation
is the most common kind of speciation.
The finches that Darwin saw on the Galápagos Islands are a
classic example of allopatric speciation, as well as adaptive radiation
Adaptive radiation occurs when a number of
new forms diverge from an ancestral form, usually in response to
the opening of major new habitats.
Fourteen species of finches evolved from the original finches
that colonized the Galápagos Islands. Ancestral finches, having emigrated
from the mainland, probably were distributed among a few
of the islands of the Galápagos. Populations became isolated on various
islands over time, and though the original population probably
displayed some genetic variation, even greater variation arose. The
original finches were seed eaters, and after their arrival, they probably
filled their preferred habitats rapidly. Variations within the original
finch population may have allowed some birds to exploit new islands
and habitats where no finches had been. Mutations changed the
genetic composition of the isolated finch populations, introducing
further variations. Natural selection favored the retention of the
variations that promoted successful reproduction.
The combined forces of isolation, mutation, and natural selection
allowed the finches to diverge into a number of species
with specialized feeding habits. Of the 14 species of
finches, six have beaks specialized for crushing seeds of different
sizes. Others feed on flowers of the prickly pear cactus or in the
forests on insects and fruit.