Diversity patterns in the fossil record are often interpreted as functions of only origination and extinction, while taxonomic dispersal among regions is rarely considered. We present a null biogeographic model of global diversity, that calculates taxonomic richness (S) and similarity (J), as measured by the Jaccard Coefficient. The model contains three regions, each initially containing a unique set of taxa. At each time step, each taxon is randomly allowed to go extinct, create a new taxon, disperse or remain unchanged. At the end of each time step, global S and mean global J are calculated. The probabilities for origination and extinction are set to be equal and the model was run with origination/extinction and dispersal probabilities that spanned over four orders of magnitude.The model has two significant results. (1) If the probability of dispersal is sufficiently large relative to origination/extinction, regional and global richness increase exponentially. For non-negligible probabilities of origination, the combined effect of dispersal and origination exceeds the effect of extinction, causing the exponential increase in diversity. (2) If richness increases, global mean J will reach equilibrium (Jeq). When plotted against the ratio of dispersal probability to origination/extinction probability, Jeq follows a sigmoidal curve.This null biogeographic model is useful for predicting dispersal probabilities in the fossil record when origination and extinction probabilities are known. Taxonomic dispersal among regions has the potential to play a significant role in shaping diversity trends, and should be considered when analysing Phanerozoic diversity trends.