In the decades since its introduction, the neutral theory of evolution has become central to the study of evolution at molecular level, in part because it provides a way to make predictions that can be tested against actual data. The neutral theory holds that most variation at the molecular level does not affect fitness and, so,
evolutionary fate of genetic variation is best explained by stochastic processes. This theory presents a framework for ongoing exploration of two areas of research: biased gene conversion and impact of effective population size on effective neutarlity of genetic variants.
Niche complementarity, in which co-existing species use different forms of a resource, has been invoked to explain most debated patterns in ecology, including maintenance of diversity and relationships between diversity and ecosystem function.
Niche differences, or the unique ways which each species 'makes its living' in nature, are the classical explanations that ecologists have used since Darwin to explain the diversity of life on Earth. Precisely, a species' niche encompasses all of the factors it requires for growth and reproduction and how a species impacts its environment. For example, plants require water and nutrients in some amounts and by growing, plants decrease the availability of those resources, which negatively affects growth of competitors. Because many factors limit organisms and since no organism is best adapted for all conditions, species have tradeoffs, which allow them to perform better in some environments but worse in others. Niche differences are a mechanism that can maintain biodiversity by allowing species to co-exist.
Building on foundational ideas of island biogeography and the neutral theory of molecular evolution, the neutral theory of species diversity makes the assumption that all individuals are ecologically identical and that niche differences are not needed to explain biodiversity patterns. Individuals of particular species may all share characteristics that make them look or function differently from other species but those differences do not influence diversity. An individual in a community interacts with and experiences its neighbours as though they were exactly the same, regardless of species. This assumption of equivalence is the necessary feature of neutarlity, which differs from niche-based assumptions that an individual's fitness depends on who its neighbours are: are they stronger or weaker competitors or do they belong to different species with different niche requirements? Neutral theory predicts that species have perfectly overlapping niches - at other extreme would be species with unique, non-overlapping niches. Other communities likely represent neither of these extremes but are somewhere in the middle.