Our biological classification systems are based on hypotheses of relationship. For example, members of the genus Ammospermophilus—antelope squirrels, that is—are more closely related to each other than to members of any other genus. If we traced their genealogy, we would find a single species that was the ancestor of all the living species of Ammospermophilus. This implies, of course, that species split, forming new species, a process we call speciation.
Most speciation events occur when a species becomes divided into
separate geographic ranges by ecological barriers. There are no real arguments until we
get to considering what is the most common way in which a species becomes fragmented.
Some argue that small groups manage to bypass a preexisting barrier; others that a
barrier arises within the original range, a phenomenon called vicariance. A probable
example of this occurred when the Rio Grande finally became established as a
through-flowing stream, separating the ancestor of our antelope squirrels into eastern
and western populations; populations now the separate species that we know as the
White-tailed and the Texas antelope squirrels.
Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.
Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.