The geographical distribution of animals is affected by both historical and present-day ecological factors. It is of great interest to distinguish between their effects. Unfortunately, both major classes of factors can yield similar biogeographic patterns, making it difficult to know which factor is more important. In addition, it is very important to examine all of the consequences of a particular hypothesis, as well as alternatives. Two examples are given: the Pleistocene forest refuge hypothesis and vicariance biogeography. The refuge hypothesis yields three predictions, but only one is upheld—concordance of centers of diversity; the distribution of positions and widths of contact zones is inconsistent with the hypothesis. The two alternative hypotheses, current ecology and current peripheral isolation, yield predictions which are upheld. The major prediction of vicariance biogeography, that concordant cladograms should indicate common vicariant sequences among the lineages, is rejected. Concordant cladograms can only result from common patterns of shared selection regimes and thus do not reflect vicariant patterns. More work needs to be done in distinguishing historical from ecological factors in species distributions.

In this paper, I examine the dynamics of species richness in a model system in which multiple species compete in a metacommunity (multiple patches linked by dispersal). Patches lie along an environmental gradient, and new species are introduced into the system by speciation of existing species. This model is used to explore how the ecological similarity of species influences the patterns in community structure that result and to determine whether patterns in fossil and systematics data may be signatures for different types of community structure. Making species more similar overall along the entire gradient or making new species that have more similar optimal positions along the gradient to their progenitor both increase the time required to drive species extinction. As a result, making species more similar ecologically to one another increases overall species richness because of an increased frequency of transient species in the system. Having more transient species in systems shifted the longevity distributions of species in the fossil record towards having a greater frequency of shorter duration species, and the age distribution of extant species that would be estimated from molecular phylogenies also had a higher frequency of younger aged species. Comparisons of these results with species longevity distributions extracted from two data sets and with species ages derived from species-level molecular phylogenies strongly suggest that transient species are an important component of real biological communities.