Neutral theory and community ecology

I review the mathematical and biological aspects of Hubbell's (2001) neutral theory of
species abundance for ecological communities, and clarify its historical connections with
closely related approaches in population genetics. A selective overview of the empirical
evidence for and against this theory is provided, with a special emphasis on tropical plant
communities. The neutral theory predicts many of the basic patterns of biodiversity,
confirming its heuristic power. The strict assumption of equivalence that defines neutrality,
equivalence among individuals, finds little empirical support in general. However, a weaker
assumption holds for stable communities, the equivalence of average fitness among
species. One reason for the surprising success of the neutral theory is that all the theories of
species coexistence satisfying the fitness equivalence assumption, including many theories
of niche differentiation, generate exactly the same patterns as the neutral theory. Hubbell's
neutral theory represents an important synthesis and a much needed demonstration of the
pivotal role of intraspecific variability in ecology. Further improvements should lead to an
explicit linking to niche-based processes. This research programme will depend crucially on
forthcoming theoretical and empirical achievements.