Panmixia

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Panmixia

 

the free crossbreeding of individuals within a population or some other intraspecific group.

Complete panmixia exists only when each individual has the possibility to mate with any individual of the opposite sex. However, in nature pairs do not form for crossbreeding by chance. The choice of partners is conditioned chiefly by similarity in behavior and physiology; the partner chosen also inhabits the same part of the area of distribution. In the latter case, divergences from complete panmixia may be the result of crossbreeding between close relatives (inbreeding). Therefore, when one refers to panmixia in natural groups of individuals, and first and foremost in populations, what is usually meant is the extent to which panmixia occurs: it must be more common within a given group than among individuals of neighboring groups.

The extent to which panmixia occurs varies among species and depends on differences in the manner of reproduction. Some species form long-term, sometimes lifelong, pairs; other species form pairs only for the reproductive season. Some species, such as many galliforms, do not form lasting pairs; in other species, for example, many insects and arachnids, the females are fertilized only once in their lives. Species in which fertilization takes place externally, such as fishes and amphibians, do not form pairs. Here, a cluster of eggs from a single female may be fertilized by spermatozoa from different males. To whatever extent it occurs, panmixia safeguards the genetic and evolutionary unity of intraspecific groupings and of the species as a whole. The term “panmixia” was introduced in 1885 by A. Weismann.

REFERENCE

Weismann, A. Die Continuität des Keimplasmas als Grundlage einer Theorie der Vererbung, 2nd ed. Jena, 1892.

A. V. IABLOKOV

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