Rates of Evolution in Fossil Mammals
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Excerpt
Introduction
The study of distributions of evolutionary rates was initiated by Simpson (1944) and has later been pursued by the same author (1953) and others. The main interest has been directed towards taxonomic rates of evolution, which were studied on the basis of survival data for taxa on the genus or species level. The following conclusions may be cited:
Each group of animals has a typical or “normal” rate distribution, the horotelic rates of evolution.
Different groups of animals have different horotelic rates.
The horotelic distribution tends to be asymmetric, skewed to the left (modal rates higher than mean rates).
The existence of a distinct group of very slow rates, the bradytelic distribution, was indicated by survival data for some groups of animals.
Finally, theoretical considerations led to the conclusion that very fast rates, the so-called tachytelic distribution, have been operative at times, probably as brief episodes.
The present paper...
