Many things complicate trying to figure out evolutionary rates for groups of animals over geologic time. One problem is that we can't just use geologic periods as units of time because their lengths vary greatly. Luckily, we now have the time frame down to the point where we can look at the amount of change per million years or other time unit.
Another problem has been, how can you compare the rate of change
between two very different sized animals? An evolutionary increase of 1 mm in the leg
bone of a mouse in a million years may be major, but the same increase in the leg bone
of an elephant utterly insignificant. The solution has been to use the natural
logarithm to decrease the influence of overall size, and a formula has been devised to
compare rates of evolution per 1 million years regardless of the sizes of the animals.
The height of horse teeth during one long interval, for example, evolved at the rate of
0.113 units. Units of what? Why darwins, of course!
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.