TOWARD A GENERAL THEORY OF BIODIVERSITY1

Abstract

E. O. Wilson (2000) writes that ‘‘Community ecology in particular is about to emerge as one of the most significant intellectual frontiers of the 21st century.’’ If Wilson is correct, and I suspect that he is, Hubbell’s book will be viewed as the first milestone. It is a big, ambitious, creative, important book. Hubbell has set out to formulate a general synthetic theory of biological diversity. Even though his theory may ultimately be found to be partially incorrect or fatally flawed, it is a major achievement. Hubbell borrows liberally from the neutralist theory of evolutionary genetics developed by Kimura, Crow, Ewens, and others, and from the equilibrium theory of island biogeography developed by MacArthur and Wilson, to develop an unabashedly neutral theory of evolutionary ecology. Like MacArthur and Wilson (1967), he assumes that all species are, if not exactly identical in all respects, at least equivalent, so that any small differences among them do not influence the processes that determine species diversity. In Hubbell’s theory, diversity is the outcome of stochastic processes of both ecology and evolution operating on both local and regional scales. Ecological processes operate at the scale of a local community. Competition for shared, limited resources within and between species regulates the number of individuals, but deaths and births are random with respect to species. The result is a ‘‘zero sum game’’ in which the relative abundance of species is the outcome of stochastic events. Differences in diversity between local communities are attributed to random colonization and extinction processes. The local communities within a region are envisaged as forming a ‘‘metacommunity’’ whose diversity is generated and maintained by speciation. Random dispersal of propagules among the communities of a metacommunity leads to a balanced number of species. When a species goes extinct in a local community (because the last individual has died) it is on average replaced by a species that has colonized from the metacommunity. Hubbell develops mathematical models that specify the quantitative relationships among the above variables, processes, and assumptions. The theoretical development is very similar to the neutralist theory of evolutionary genetics. Indeed, Hubbell’s central model is mathematically identical to the infinite allele model of Ewens (1972), with species in a metacommunity being treated as analogous to alleles in a population. I have heard some colleagues mutter that all Hub-