We have a wealth of information on current evolution. Such things as increasing resistance to antibiotics and pesticides or rapid changes in populations of vertebrates as environments change are now old news. Fossil evidence, of course, shows vast, higher-level change through geologic time. But there are patterns within the last 50,000 years where it is difficult to differentiate between migration and evolution.
Many vertebrates who survive today in our Chihuahuan Desert Region show
differences from regional fossils of the same species that date from the last ice age.
Commonly, these differences are in size and size-related features. The modern species
often differ in size geographically, individuals in the north, for example, being
larger or smaller than those farther south. The problem arises when we see fossils
different from current inhabitants but similar to those far away. Did the fossil
populations come to resemble today's distant populations by adapting to similar
conditions or did conditions similar to those enjoyed by today's distant
populations allow migration into the Southwest? As scientists so often end up their
reports, "More data are needed".
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.
Findley, J. S. 1965. Shrews from Hermit Cave, Guadalupe Mountains, New Mexico. Journal of Mammalogy 46:206-210.
Harris, A. H. 1988. Late Pleistocene and Holocene Microtus (Pitymys) (Rodentia: Cricetidae) in New Mexico. Journal of Vertebrate Paleontology 8:307-313.