Variability and Evolutionary Rate in the Oreodonts

Bader, Robert

This paper gives the results of a comparison of the variability and evolutionary rates of 23 characters in two subfamilies of oreodonts. Measurements were taken on approximately 325 individuals representing 10 different species, 5 in each subfamily. The time is Middle to Late Miocene encompassing approximately 12 million-years and the locale is the Great Plains (with the exception of one California species). This study is concerned with the evolutionary rate and the variability in two subfamilies of oreodonts, an extinct group of Tertiary mammals. The two subfamilies, the Merycochoerinae and the Merychyinae, evolved through approximately the same Middle to Late Miocene sequence, a span of about 12 million-years. The Merycochoerinae are considerably larger, with rather pig-like proportions and a reduction of the nasal bones indicative of a tapir-like proboscis. The habitat, at least in the later members of the line, was probably semiaquatic. The smaller group, the Merychyinae, were extremely conservative, lightly built forms with bullet-like heads. They remained plainsdwelling, cursorial forms throughout the sequence. The geographic setting is The Great Plains area of western Nebraska, eastern Wyoming, southwestern South Dakota, and northeastern Colorado. Twenty-three measurements were made on the 5 species of each subfamily. Variability was measured by the Pearsonian coefficient of variation. Evolutionary rate was given as a mean percent change in each unit character per million-years. In neither subfamilial lineage is there a marked secular trend in the magnitude of the variation. An analysis of the variation in 3 species of the Merychyinae demonstrates an important interrelationship between variability and speciation. A large, widespread, highly variable species, M. minimus, gave rise to M. arenarum by the process of speciation. M. elegans, the direct descendant of M. minimus through phyletic progression, and M. arenarum both show significantly less variation than the parental population, M. minimus. Part of this reduced variability is later restored in the elegans line. In comparing the two subfamilies, the Merychyinae have the higher mean variation in 20 of the 23 characters studied. The evolutionary rates are considerably below comparable figures for metric characters in the evolution of the horse. The Merycochoerinae have a greater rate than the Merychyinae in 19 of the 23 characters examined. This difference is statistically significant. In 18 of the 23 characters the Merycochoerinae evolved at the more rapid rate while the Merychyinae exhibit the greater variability in the same traits. Evolutionary factors which may have played a role in this relationship are discussed. These include relative position in the history of the group, mutation rates, length of generation, population size, and selection. It is concluded that the greater intensity of selection on the Merycochoerinae is the principal factor accounting for their greater evolutionary rate and lower variability.

Variability and Evolutionary Rate in the Oreodonts

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