EVOLUTIONARY MODE IN FOSSIL SEQUENCES: AN EMPIRICAL SURVEY

The most direct view of evolutionary patterns is afforded by repeated sampling of the same lineage over time. Paleontologists have long studied such examples of phyletic evolution in order to better understand the processes and patterns of evolutionary change. In recent years, a number of statistical tests have been developed to determine the mode of evolutionary change, with a focus on discriminating between directional evolution, (unbiased) random walks, and stasis.

Here, I describe a model-based approach for assessing the mode of evolutionary change in phyletic sequences. This approach is based in likelihood, and correctly accounts for sampling error as a source of variation among samples. It has the important advantage over previous methods that no model is granted privileged null status; all models are evaluated on an equal footing based on their compatibility with observed data, using standard model-selection tools such as the Akaike Information Criterion (AIC). I develop this approach to evaluate the trichotomy of evolutionary modes usually considered: directional change (modeled as a biased random walk), nondirectional change (modeled as an unbiased random walk), and stasis (as modeled by Sheets and Mitchell, 2001).

This statistical framework was applied to a large sample of over 100 evolutionary sequences compiled from the published literature. For each sequence, I fit three models corresponding to the standard modes of evolutionary change and evaluated the adequacy of the models using the AIC and related measures of model support. Only rarely, in less than 10% of cases, does the model of directional change best account for evolutionary patterns; even this modest percentage is almost certainly an overestimate due to investigator bias. Of the remaining sequences, approximately half are best fit by the unbiased random walk model, with the remaining best described by stasis. Analyses performed thus far do not find any systematic differences in evolutionary mode with respect to taxonomic groupings or trait type (size versus shape characters). These results support the view that periods of strongly directional evolution are rare and/or brief, and, over paleontological time-scales, most traits and lineages experience meandering or fluctuating evolutionary trajectories.