I am working with Jon Payne
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.
- Heim, N.A., M.L. Knope, E.K. Schaal, S.C. Wang and J.L. Payne. 2015. Cope's Rule in the Evolution of Marine Animals.
Science, 347(6224):867-870. [URL].
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.
- Heim, N.A. 2008. A null biogeographic model for quantifying the role of migration in
shaping patterns of global taxonomic richness and differentiation diversity, with implications for Ordovician
biogeography. Paleobiology, 34: 195-209. [PDF]