Panmictic
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A panmictic population is one where all individuals are potential partners. This assumes that there is no mating restrictions whether genetic or behavioural upon the population, and as such all recombination is possible. The Wahlund effect assumes that the overall population is panmictic.
[edit] Description
In simpler terms: It is the ability of individuals in a population to move about freely within their habitat, possibly over a range of hundreds to thousands of miles, and thus breed with other members of the population that defines panmixia (or panmicticism).
To signify the importance of this, imagine several different populations of the same species (for example: a grazing herbivore), isolated from each other by some physical characteristic of the environment (dense forest areas separating grazing lands). As time progresses, natural selection, assuming the populations are in differing environments, will slowly move the species' toward either separate speciation events or extirpation.
However, if the separating factor is removed before this happens, (a road is cut through the forest), and the individuals are allowed to move about freely, the individual populations will still be able to interbreed. As the species populations interbreed over time, they become more uniform, with a decrease in genetic diversity, and thus a decrease in total biodiversity.