ABSTRACT
Clustering of distribution areas is one of the most prominent biogeographic patterns. One mechanism that might cause this pattern is vicariance. According to the vicariance model, diversification is the result of a fragmentation of an ancestral biota by emerging barriers. Such a fragmentation will result in the formation of distinct biotic elements (i.e., groups of species with similar ranges) in areas of endemism. We use biotic elements as biogeographic units, because areas of endemism cannot be delimited if dispersal occurred. We propose a statistical test for clustering of distribution areas based on a Monte Carlo simulation with a null model that generates range data sets such that their range size distribution, the species richness distribution of the geographic cells, and the spatial autocorrelation of the occurrences of a taxon approach the parameters in the real data set. Biotic elements were delimited with model-based Gaussian clustering. We also tested another prediction of the vicariance model, namely that closely related species originate on different sides of an emerging barrier and, hence, belong to different biotic elements.
