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Although species are the basic units of many paleontological and evolutionary studies, the term “species” applied to the fossil record does not convey the same concept to all workers. Simpson’s “evolutionary species” incorporated time into the species concept, but considered each non-branching lineage as a separate species; longer lineages with more continuous fossil records may require subdivision into successional species. One’s perception of paleontological species affects, and is affected by, evolutionary philosophy and models of how new species form and evolve. For example, if species actually arise abruptly and persist for much longer periods essentially unchanged (punctuated equilibria), discrimination of paleontological species should be a relatively simple matter. Alternatively, if there is continuous change within and between successive species (gradualism), species boundaries would be nebulous, and would have to be imposed arbitrarily. We summarize our study of omomyid primates and cite other supportive evidence which suggests that, where the record is sufficiently dense, gradual evolution (requiring arbitrary boundaries) is common between species and even genera.

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