In a long series of papers dating from 1913, Sewall Wright has developed and propounded the theory that evolutionary progress will be most favoured by a population structure of many small, partially Isolated, sub-units or demes. In such a situation, with demes continually becoming extinct, and being restored by migrants from more successful demes, simultaneous random drift in all sufficiently small local populations strongly heterallelic with respect to nearly neutral alleles, gives a broad basis for the local appearance of novel favourable interaction systems (Wright, 1977).

This theory was based partly on observations made by Wright on the development of some breeds of livestock, in particular Shorthorn cattle. The breed records indicated the appearance of high excellence in a few  herds at different times, and subsequent diffusion of this excellence, principally by use of bulls from superior herds, rather than by mass selection throughout the breed as a whole (Wright, 1977).

R. G Banks, J. SF Barker

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 7. Symposia (1), , 29-32, 1982
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