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Panmixia (or panmixis) means uniform random fertilization, which means individuals do not select a mate based on physical traits.King C and Stanfield W.D. (1997). Dictionary of genetics. Oxford University Press. . p. 262: "Panmixia (panmixis): random mating as contrasted with assortative mating."Merriam-Webster Medical Dictionary. "Panmixia: Random mating within a breeding population." A panmictic population is one where all potential parents may contribute equally to the gamete pool, and that these gametes are uniformly distributed within the gamete population (gamodeme). This assumes that there are no hybridising restrictions within the parental population: neither genetics, cytogenetics nor behavioural; and neither spatial nor temporal (see also Quantitative genetics for further discussion). True panmixia is rarely, if ever, observed in natural populations. It is a theoretical model used as a null hypothesis in population genetics. It serves as a point of comparison to understand how deviations from random mating affect allele and genotype frequencies.[2] Therefore, all gamete recombination (fertilization) is uniformly possible. Both the Wahlund effect and the Hardy Weinberg equilibrium assume that the overall population is panmictic.
In genetics and heredity, random mating usually implies the hybridising (mating) of individuals regardless of any spatial, physical, genetical, temporal or social preference. That is, the mating between two organisms is not influenced by any environmental, nor hereditary interaction. There is no tendency for similar individuals (positive assortative mating) or dissimilar individuals (negative assortative mating) to mate.[3] Hence, potential mates have an equal chance of being contributors to the fertilizing gamete pool. If there is no random sub-sampling of gametes involved in the fertilization cohort, panmixia has occurred. This scenario is considered rare as it is very idealized. In real life, there are many different factors that can influence mate choice. Such uniform random mating is distinct from lack of natural selection: in viability selection for instance, selection occurs before mating.
To signify the importance of this, imagine several different finite 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 and genetic drift will slowly move each population toward genetic differentiation that would make each population genetically unique (that could eventually lead to speciation events or Local extinction).
However, if the separating factor is removed before this happens (e.g. 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's populations interbreed over time, they become more genetically uniform, functioning again as a single panmictic population.
In attempting to describe the mathematical properties of structured populations, Sewall Wright proposed a "factor of Panmixia" (P) to include in the equations describing the gene frequencies in a population, and accounting for a population's tendency towards panmixia, while a "factor of Fixation" (F) would account for a population's departure from the Hardy–Weinberg expectation, due to less than panmictic mating. This equation describes how the allelic and genotypic frequencies remain constant in a non-evolving population.[4] In this formulation, the two quantities are complementary, i.e. P = 1 − F. From this factor of fixation, he later developed the F statistics.
Panmixia allows for species to reach genetic diversity through gene flow more efficiently than monandrous species. However, outside population factors, like drought and limited food sources, can affect the way any species will mate. When scientists examine species mating to understand their mating style, they look at factors like genetic markers, genetic differentiation, and gene pool.
Knoxdaviesia proteae, a fungus that lives on flowers of Protea repens, shows extensive genetic variation and weak genetic differentiation. These genetic factors mean the fungus population is well-mixed and maintains panmixia across the population by spreading widely via beetles. This fungus uses mites to travel short distances, but it has been found that instead, Knoxdaviesia proteae rides on beetles to pollinate Protea repens. This allows for frequent genetic exchange due to the fungus's interaction with other colonies instead of cloning itself. [6]
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