In the late 1960's Kimura proposed the neutral theory of molecular evolution. In this theory he described how neutral (non-coding) alleles change, purely depending on genetic drift. It was not until the early 2000's (2001) until ecologist Stephen Hubbell proposed the neutral theory of biodiversity, describing how the assembly of a community changes due to stochasticity. When I refer to neutral theory in this text, I refer to the theory about biodiversity, not the molecular one.
In his book "The Unified Neutral Theory of Biodiversity and Biogeography" Stephen Hubbel explains his theory and provides examples showing fits of neutral theory with empirical data. Here I will not go into great (mathematical) depth on neutral theory, but provide a short, plain explanation on what neutral theory entails, and how it can be used. I advise the enthousiastic reader to further read in the links provided below.
Neutral theory tries to describe the assembly of a community. With the assembly of a community, we mean the number of species, and the number of individuals
per species within this community. It is important to note here that neutral theory does not explain the assembly of complete ecosystems, but
rather the assembly of subsets of these ecosystems, particularly groups of species that are trophically very similar (often called "guilds"). I will make clear why
this is important later.
Neutral theory is based upon two important assumptions:
The "neutrality" assumption
The "zero-sum" assumption
The "neutrality" assumption assumes that all species within the community behave similarly and are interexchangeable. This means that every species has equal birth and death rates, and is equally likely to go extinct. This is a rigorous assumption, and it is clear that "real" species do not behave this way. This is, however, a model, and to provide a workable model that is able to make predictions, often reality has to be simplified. By treating all species equally, complex interactions such as predator-prey dynamics, or parasite-host interactions are omitted. The neutrality assumption also prevents us from analysing highly dissimilar species. Examples of trophically similar communities would be for instance tree-communities or corals.
The "zero-sum" assumption assumes that the community is fully saturated. In effect this means that for every death, a birth occurs, hence keeping the population size constant. This assumption can be relaxed, still providing decent fits.
Obviously I can't go into great further detail of the complexity of this model, and I therefore encourage interested readers to read the Scholarpedia article as referenced at the end of this page.
Web Site (C) Thijs Janzen 2011-2014
Lake Tanganyika pictures courtesy Jen Reynolds