VARIABILITY AND EVOLUTIONARY RATE IN THE OREODONTS

An increasing awareness of the importance of the role of the population as opposed to that of the individual has been one of the major trends of the past several years in evolutionary research. This trend can be discerned in both the botanical and zoological aspects of such disciplines as genetics, ecology, taxonomy, and paleontology. The shift in interest from individual to population has neces-, sitated a renewed study of variation in natural groups since individuals in nature are never phenotypically, and almost never genotypically, identical. The study of variation and its relation to the mechanics of evolutionary change in fossil materials has been hampered in the past by the paucity of species for which an adequate sample was available as well as the reluctance of many paleontological practitioners to stray outside the confines of the typology concept. Happily, continued success in the field has alleviated (at least in part) the first obstacle and an increasing understanding of evolutionary dynamics, the second. Simpson (1944, 1953) has given the most complete expression of recent paleontological thought on problems in which characteristics of populations, including variation, are of prime concern. Most types of variation, to be studied adequately, must be approached not only fronm the biological point of view but also with the statistical tools of the biometrician. In the above-mentioned work, Simpson analyzed statistically the relationship between variability and speciation in a phyletic series of Jurassic ammonites. He interpreted the reduction of variation at points of speciation as due to an unequal segregation of variability in the parental population. This point has received practically no attention in studies of vertebrate evolution. It will formu one of the main topics of interest in the present paper. The term, variability, will be used throughout as the potentiality or the capacity of organisms to differ from each other as distinguished from its actual realization or expression, variation. In genetic terms variability would represent genotypic differences, variation phenotypic differences. The degree of correlation between the two is obviously very high. As the gaps in many of the well-known phylogenetic series have become progressively reduced by new field discoveries, paleontologists have concerned themselves not only with readjustments and greater refinements of the phylogenies involved but also with the rates at which such changes took place with an eye to the causal factors involved. Although most fossil lineages are too imperfectly known to lend themselves to such quantitative treatment, the past decade has seen a great increase in the number of vertebrate groups whose evolutionary rates have been studied quantitatively. The problem of the relationship between variation and rates of change in fossil populations has received little direct study up to the present time. Simpson (1944) made a preliminary analysis of pairs of allied living species, one of which has evolved rapidly, the other at more moderate rates (e.g. horses and tapirs). He found that the more slowly evolving member of each pair showed a variability at least as great as in the allied, more rapidly evolving line and adds "there is