A Method of Analysis for Historical Biogeography

Platnick, N. I., and G. Nelson (Departments of Entomology and Ichthyology, The American Museum of Natural History, New York, New York 10024) 1978. A method of analysis for historical biogeography. Syst. Zool. 27:1–16.—Historical explanations of biotic distribution fall into two classes, dispersal explanations and vicariance explanations. Dispersal models explain disjunctions by dispersal across pre-existing barriers, vicariance models by the appearance of barriers fragmenting the ranges of ancestral species. Distributional data seem insufficient to resolve decisively either dispersal or vicariance as the cause of particular allopatric distribution patterns. When faced with such a pattern our first question should therefore be directed not to its cause, but to whether or not it conforms to a general pattern of relationships shown by taxa endemic to the areas occupied. Two-taxon statements are always compatible with a general pattern; three-taxon statements are therefore the most basic possible units of biogeographic (as well as phylogenetic) analysis. Analysis of three-taxon statements involves converting a hypothesis about the interrelationships of taxa (a cladogram indicating relative recency of common ancestry) to one concerning the interrelationships of areas (a cladogram indicating relative recency of common ancestral biotas). The generality of the area hypothesis may be tested by comparison with other groups endemic to the relevant areas. If the area hypothesis is corroborated as general, a statement of the relative recency of interconnections among areas is obtained, and evidence from historical geology may allow us to specify the nature of those interconnections and thereby the cause of those distributions that conform to the general pattern. Analysis of four-taxon statements indicates that the availability of structurally different patterns and of groups that can serve as adequate tests of the generality of those patterns increases with the addition of taxa to the hypothesis, and that neither extinction nor the failure of some groups to respond (by speciating) to given dispersal or vicariance events interferes with the analysis.