Genetic Homeostasis

Genetic Homeostasis

Genetic Homeostasis

Genetic Homeostasis

Excerpt

DOBZHANSKY (1951 a ) has defined a Mendelian population as 'a reproductive community of sexual and cross-fertilized individuals which share a common gene pool'. One of the salient features of this spatio-temporal entity is an integrated structure which endows it with properties transcending those of its individual members. Older evolutionary theories have tended to consider Mendelian populations as relatively uniform, with reserves of variability carried as recessive mutants at individually low frequencies. The major process of evolutionary significance was presumed to consist of successive substitutions at each locus of alleles less advantageous to an individual by more advantageous ones. More recently justification has been obtained for the replacement of this idea by that of dynamic variability of the genetic composition of populations, lending emphasis to the concept of evolution as 'an irregularly shifting state of balance' (Wright, 1950). This change of outlook has caused the integrative properties of populations to assume paramount importance in evolutionary thought, not only for problems dealing with differentiation between populations, but with an even greater force for those relating to genetic phenomena within populations.

It seems obvious that properties of populations, as much as properties of individuals, have evolved under the action of natural selection. At the same time it is not immediately clear as to how natural selection operating on the individual level could lead to the development of integrating factors at the level of populations. Many students of evolution (e.g. Fisher, 1932; Simpson, 1941) have stressed the fact that natural selection cannot be concerned with properties of units higher than the individual (except in cases of inter-group selection, such as may be found in insects; see Haldane, 1932). The evolution of the integrative characteristics of Mendelian populations must have occurred under this view as a by-product of individual selection. The simplest example of this situation is found in the Hardy-Weinberg equilibrium. It is meaningless to attribute to individuals this property of large populations, whereby in the absence of selection, mutation or migration they tend to maintain their genetic composition constant. Yet it is the Mendelian behaviour of the units of inheritance linking individuals of successive generations . . .

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