A DIFFERENTIATION INDEX, AND A NEW MEASURE OF EVOLUTIONARY RATES

Abstract

Paleontological studies of relative growth have shown that allometric growth pattern may often remain stable during long evolutionary sequences; but also that clear-cut shifts from one growth pattern to another may occur. As examples may be noted the results of Robb (1935), later developed in more detail by Reeve and Murray (1942); Phleger and Putnam (1942); Kurten (1954 a, b). Most of these studies concerned only one or a few variate pairs. However, it may be useful to extend the method to a greater number of variate pairs. It is then possible to study the relative frequency of identical and differentiated growth patterns in the populations compared; and this may be expressed as an of giving the percentage of significantly differing allometry trends. This index may be expected to run more or less parallel with actual taxonomic and genetic differentiation, because the growth patterns have a genetic basis, and changes in them arise from genetic change. It is suggested that this index may be used as a taxonomic character, to help us infer whether the populations we are comparing belong to one species or two, to two subspecies or only two slightly differentiated demes not meriting formal recognition. I do not mean that it should be the sole basis for such conclusions, but it may contribute in a valuable way; it should give highly objective characters, which may be useful as a guide in decisions on border-line cases in taxonomy. In order to make the method serviceable, the first step is to compare populations in which the taxonomic relationships are already well known. Indices obtained by such studies will indicate the scale of differentiation which we may expect on the deme, subspecies, and species level, and so on. Being quantitative, the method will also yield a measure of evolutionary rates. If the lapse of time between ancestral and descendant population is known, we have an absolute measure; but it may also be used as a relative measure, when two collaterally evolving phyla are studied at an earlier and a later point in time. In view of these interesting potentialities, some examples are given in the present paper. The main object of study has been the Pleistocene to Recent evolutionary series of the spotted hyena; but I have also drawn on hitherto unpublished data assembled by myself and Dr. Robert Rausch (Anchorage, Alaska) in joint studies of Recent circumpolar species; and on selected published data.