AGE-DEPENDENT EXTINCTION AND THE NEUTRAL THEORY OF BIODIVERSITY

The apparent constancy of extinction risk over the lifetime of fossil taxa was central in the development of Van Valen’s (1973) Red Queen hypothesis, and the analysis of taxonomic survivorship curves has since motivated much paleobiological investigation. Yet contrary to the predictions of Red Queen, declining extinction risk with taxon age seems to be at least as common as Red Queen-type age-independent survivorship curves. Although much work has gone into characterizing empirical patterns in such curves, little progress has been made in developing theoretical alternatives to Red Queen. Here we develop and test a new approach based on the neutral theory of biodiversity, a population model in which the demographic properties of individuals are independent of species identity and which successfully predicts empirical patterns of species abundance distributions, species-area relationships, and phylogenetic tree shape.

We construct transition matrices for abundance dynamics of a single species in a community determined solely by speciation rate and community size (number of individuals), characterize the patterns of age-dependent extinction that result from these dynamics, and compare the results with paleontological data. We find theoretical survivorship curves have two phases: extinction risk is initially high as species expand from low initial abundance, but approaches a constant value after a number of lifespans roughly equal to the community size. Incompletely sampled survivorship curves look more like those predicted by Red Queen due to undersampling of young species. Neutral theory survivorship curves are strikingly similar to those observed in Ordovician-Silurian graptolites when parameterized with plausible estimates of speciation rate, community size, and sampling rate. Despite being an oversimplification of biological reality, neutral theory successfully predicts survivorship because it captures the population dynamics that determine extinction risk. Red Queen is not the only viable explanation for taxonomic survivorship curves: not only does the neutral theory account for a broader variety of empirical survivorship patterns, it also makes straightforward, testable predictions for abundance, community structure, and other aspects of ecology and evolution in the fossil record.