[2] The theory has sparked controversy,[3][4][5] and some authors consider it a more complex version of other null models that fit the data better.

Wilson's theory of island biogeography[1] and Stephen Jay Gould's concepts of symmetry and null models.

Asymmetric phenomena such as parasitism and predation are ruled out by the terms of reference; but cooperative strategies such as swarming, and negative interaction such as competing for limited food or light are allowed (so long as all individuals behave alike).

The theory predicts the existence of a fundamental biodiversity constant, conventionally written θ, that appears to govern species richness on a wide variety of spatial and temporal scales.

Although not strictly necessary for a neutral theory, many stochastic models of biodiversity assume a fixed, finite community size (total number of individual organisms).

There are unavoidable physical constraints on the total number of individuals that can be packed into a given space (although space per se isn't necessarily a resource, it is often a useful surrogate variable for a limiting resource that is distributed over the landscape; examples would include sunlight or hosts, in the case of parasites).

Because the Unified Theory refers only to communities of trophically similar, competing species, it is unlikely that population density will vary too widely from one place to another.

Hubbell considers the fact that community sizes are constant and interprets it as a general principle: large landscapes are always biotically saturated with individuals.

Exceptions to the saturation principle include disturbed ecosystems such as the Serengeti, where saplings are trampled by elephants and Blue wildebeests; or gardens, where certain species are systematically removed.

The UNTB suggests that it is not necessary to invoke adaptation or niche differences because neutral dynamics alone can generate such patterns.

This equation shows that the UNTB implies a nontrivial dominance-diversity equilibrium between speciation and extinction.

If m < 1, dispersal is limited and the local community is a dispersal-limited sample from the metacommunity for which different formulas apply.

are coefficients fully determined by the data, being defined as This seemingly complicated formula involves Stirling numbers and Pochhammer symbols, but can be very easily calculated.

Both community dynamics are modelled by appropriate urn processes, where each individual is represented by a ball with a color corresponding to its species.

Since one basic assumption is saturation, this reproduction has to happen at the cost of another random individual from the urn which is removed.

Hubbell calls this simplified model for speciation a point mutation, using the terminology of the Neutral theory of molecular evolution.

At each time step take one of the two actions : The metacommunity is changing on a much larger timescale and is assumed to be fixed during the evolution of the local community.

) and are derived by Etienne and Alonso (2005),[9] including several simplifying limit cases like the one presented in the previous section (there called

The topic is of great interest to conservation biologists in the design of reserves, as it is often desired to harbour as many species as possible.

The most commonly encountered relationship is the power law given by where S is the number of species found, A is the area sampled, and c and z are constants.

for some constant k, and if this relationship were exactly true, the species area line would be straight on log scales.

Such species are thus considered to be evenly split between the two adjacent classes (apart from singletons which are classified into the rarest category).

Preston called this the veil line and noted that the cutoff point would move as more individuals are sampled.

For biodiversity evolution and species preservation, it is crucial to compare the dynamics of ecosystems with models (Leigh, 2007).

[12] Within this framework the population of any species is represented by a continuous (random) variable x, whose evolution is governed by the following Langevin equation: where b is the immigration rate from a large regional community,

The model can also be derived as a continuous approximation of a master equation, where birth and death rates are independent of species, and predicts that at steady-state the RSA is simply a gamma distribution.

This suggests that the neutral assumption could correspond to a scenario in which species originate and become extinct on the same timescales of fluctuations of the whole ecosystem.

[13] The theory has been criticized as it requires an equilibrium, yet climatic and geographical conditions are thought to change too frequently for this to be attained.

[13] Tests on bird and tree abundance data demonstrate that the theory is usually a poorer match to the data than alternative null hypotheses that use fewer parameters (a log-normal model with two tunable parameters, compared to the neutral theory's three[6]), and are thus more parsimonious.

[15] It also fails to explain why families of tropical trees have statistically highly correlated numbers of species in phylogenetically unrelated and geographically distant forest plots in Central and South America, Africa, and South East Asia.