Most species of complex organisms are clearly separated from one another. Those relatively few cases where hybridization occurs are notable largely for the insight they give us into how splitting of species comes about. When viewing a species line through time, though, we have a problem: species change more or less gradually, and immediately after a split, are still very similar.
The extinct Western Vulture, common in Southwestern fossil sites,
demonstrates the problem. It is obviously related to the modern Black Vulture of the
southern United States and points farther south. The primary difference is in the
larger size of the fossil form. So, are we dealing with two separate species that split
apart with one of them later becoming extinct or are we dealing with a single species
whose members became smaller in size as Pleistocene ice sheets melted away?
Unfortunately, questions such as these can be answered authoritatively only if we can
find an occurrence of both at the same time and place or, on the other hand, we find
fossils of increasingly smaller birds through time.
Listen to the Audio (mp3 format) as recorded by KTEP, Public Radio for the Southwest.
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.