Historical Background

Neutral theory gained its first major expression in ecology with the appearance in the 1960s of the theory of island biogeography by Robert MacArthur and Edward O. Wilson. This theory is neutral because its species are indistinguishable. The theory was erected in part to explain why islands or isolated habitat patches tend to have fewer species than similar-sized areas on the mainland or in continuous habitat. MacArthur and Wilson hypothesized that species richness on islands is a dynamic equilibrium between the rate of immigration of new species to the island and the rate of extinction of species once resident on the island. They argued that species richness on islands was lower because fewer species reach islands than are able to disperse similar distances on the mainland; and once species colonize the island, their extinction rates are higher because the smaller island populations tend to go extinct faster than larger mainland populations. Species are symmetric in the theory because they have identical immigration and extinction rates, and there is continual species turnover on the island as new species arrive and resident species go extinct. The theory predicts an equilibrium in species richness, but not an equilibrium of any particular assemblage of named species.

The theory of island biogeography is conceptually incomplete as a neutral theory of biodiversity because it does not include a process of speciation for the generation of new species. The mainland source area is treated as an unchanging pool of species that can potentially colonize any given island. Also, the theory only describes the dynamics of species richness, not the commonness and rarity of species, that is, relative species abundance. In the mid-1970s, the first neutral models of communities that took relative species abundance into account were developed by Watterson and Caswell. They adapted neutral theory from population genetics, and modeled species as undergoing a random walk in abundance, starting from an initial immigration event, analogous to genetic drift of a neutral allele originating from a mutation. Watterson noted that the distribution of relative species abundance in neutral communities was Fisher's logseries distribution. Caswell also noted that his simulations produced log-series-like distributions of relative species abundance.

At the time, the significance of this result and the connection of neutral theory to Fisher's logseries distribution was not generally appreciated. In the early 1940s, Ronald Fisher, the renowned statistician and evolutionary geneticist, and two entomologists, Corbet and Williams, published a seminal paper on the relative abundance of species, using data on moths collected at light traps in England and butterflies collected in Malaya. Fisher created a new two-parameter distribution, the logarithmic series, which fit the moth and butterfly data quite well. In the logseries, the expected number of species ^ having abundance n is given by

where a is a fitted diversity parameter, and x is a parameter whose value is close to but less than unity (if x >1 then the series does not converge). Since that time, Fisher's a, as parameter a is now known, has become one of the most widely used measures of species diversity because its value is almost invariant in the face of increasing sample sizes of individuals drawn from communities and sorted into species. Why Fisher's a should be so nearly constant, and the biological significance of both parameters, a and x, were not understood until the development of neutral theory.

In the 1970s, theoretical work on neutral models of phylogeny was done by Raup, Gould, Schopf, and Simberloff. However, the connection of this work to neutral theory in ecology was not realized until much later. Raup and his colleagues took a demographic approach to phylo-geny, in which they modeled monophyletic clades evolving by a stochastic birth-death branching process, based on the much earlier work of Yule. They were interested in whether random birth-death branching processes could duplicate observed patterns of phylogenetic diversification in the fossil record, and in particular the patterns of punctuated equilibria, that is, long periods of relative stability in diversity, punctuated by short bursts of rapid extinction and diversification. In these models, lineages were assigned birth (speciation) and death (extinction) rates. When the birth rate exceeded the death rate, the general outcome of these models was exponential growth in the number of descendant lineages, and somewhat slower growth if all extinct lineages were pruned out. However, in no case did the models yield the punctuational pattern postulated by Eldredge and Gould. Later, in the 1990s, work by Sean Nee and colleagues reached similar conclusions using analytical models. However, as the neutral theory in ecology would later show, these conclusions were somewhat premature.

In the early 1980s, the study of neutral theory in ecology languished for a time in which neutral models were heavily criticized for their lack of realism and biological content. Many of the early neutral models were simply sampling protocols for assembling species into random communities. A limitation was that they were not dynamical models built on fundamental processes in population biology (birth, death, dispersal). They simply assigned equal sampling probabilities to species without regard to their abundance or probability of dispersal. Moreover, some of these early models had serious statistical problems and other conceptual difficulties. These problems, along with the rise in the 1980s of deterministic theory for community assembly based on niche differences among species for exploiting limiting resources, led to a hiatus of nearly two decades before the subject of neutral theory was revisited in a serious manner.

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