Problems in distinguishing historical from ecological factors in biogeography

SYNOPSIS. 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.