Modeling Cope's Rule in Marine Animals

I am working with Jon Payne, Matt Knope, and Steve Wang to develop an evolutionary branching model to test whether or not observed trends in the mean, minimum and maximum size of marine animals is due to natural selection or neutral drift. The null hypothesis, proposed by Stanley (1973), is that increases in body size over time could be due to neutral (i.e., non-selective) drift away from an initially small size. Comparisons of observed trends with our models robustly rejects the hypothesis of neutral drift from an initial small size, suggesting that has been a persistent selection for larger body size in marine animals over the last 550 million years.

Modeling the influence of immigration on paleodiversity

Migration is an important evolutionary process that is often neglected in paleontological studies conducted at the regional or continental scale. Migration has a profound impact on delta diversity, which in recent years has become a feature of fossil assemblages that is of increasing interest to paleontologists. Delta diversity is similar to beta diversity, but rather than measuring the diversity gained when environmentally homogenous, local samples are pooled, delta diversity measures the diversity gained when samples from environmentally heterogeneous landscapes or continents are pooled. A null model is presented that evaluates the role of migration, in concert with origination and extinction, in shaping patterns of delta diversity. Delta diversity is computed as one minus the Jaccard coefficient of similarity. The model results show that, over time, delta diversity reaches a dynamic equilibrium and that migration is a process that reduces delta diversity while origination and extinction are processes that increase delta diversity.

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