One problem that has always bedeviled biologists trying to clarify relationships between organisms is the matter of time. How long has it been since the ancestor of two modern species split into the two separate lines leading to the present kinds? The fossil record helps, but is spotty. The field of molecular genetics now is beginning to supply answers.
Not all genes are subjected to natural selection—that is, they are
neither constrained to remain unchanged by mutations nor are they weeded out of the
genetic material. These neutral genes thus tend to change through time because of
mutations. When a species splits into two species, the neutral genes of each new
species build up differences independently. Taken over long spans of time, the
differences tend to accumulate at constant rates. If we can calculate this rate, all we
have to do is follow the rate backwards to find the time of the splitting of the
ancestor. Finally, we should be able to trace back the time of origination from their
common ancestor of such desert creatures as kangaroo rats.
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.