TESTING THE EVOLUTIONARY EFFECTS OF ABUNDANCE IN THE FOSSIL RECORD

Abundance (number of individuals within a species) is a basic ecological observation and aspect of a species’ biology, yet there is no consensus about its effects on evolutionary processes. On one hand, abundant widespread species may have high potential to give rise to new descendants (as Darwin suggested). On the other, the abundance of species in the tropics (where diversity is highest) is usually low; some evolutionary theory (e.g., speciation by founder effect or peripatry) suggests that smaller populations evolve more rapidly; and some recent paleontological studies have found a strong negative relation between geographic range (often a proxy for abundance) of parental species and the rate of speciation of daughters. Various authors have suggested that decreases in abundance may lead to an increase in speciation, due to fragmentation of species ranges, or to a decrease in speciation, due to extinction or hybridization of isolated populations.

Examining these issues in the fossil record is hindered by the difficulty of estimating original abundance in living populations. Yet if this can be overcome, it should, in principle, be possible to examine the role of abundance in particular evolutionary events. I have attempted to do so via three approaches: 1) comparing peak abundances in species to the timing of evolutionary events in multiple lineages of Cenozoic turritelline gastropods; 2) comparing geographic ranges of Paleogene mollusk species from the U.S. Gulf and Atlantic coastal plains to clade diversity; and 3) plotting changes in sample-standardized abundance data from the Paleobiology Database through multiple phylogenies of a variety of benthic marine macroinvertebrate clades from the literature.

Results suggest that the effects of abundance on evolution are complex, and depend on both evolutionary mode (anagenesis vs. cladogenesis) and the temporal stage of the evolutionary process under consideration.