Testing the Stochasticity of Patterns of Organismal Diversity: An Improved Null Model

Evolutionary biologists, systematists, and paleontologists commonly have invoked deterministic explanations for certain patterns of diversity. However, few authors have considered the null model that the factors generating diversity may behave stochastically and that "pattern" therefore results from chance and does not require a deterministic explanation. We have developed a quantitative null model based on a randomly branching Markovian process to test the stochasticity of patterns of diversity as represented by phylogenies of contemporary organisms. The model allows one to consider real phylogenies containing large numbers of taxa and to determine whether the observed topology of those phylogenies might be explained by chance alone. We believe that our null model represents an improvement over earlier models. The simplicity of our null model makes feasible the testing of large numbers of phylogenies for stochasticity.