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October 12, 2004

Fisher on common ancestors

I recently quoted from a 1929 letter of R. A. Fisher on the subject of common ancestry.

I now find that there is a much more accessible discussion in Fisher's Genetical Theory of Natural Selection (1930):

The intimate manner in which the whole body of individuals of a single species are bound together by sexual reproduction has been lost sight of by some writers. Apart from the intervention of geographical barriers so recently that the races separated are not yet regarded as specifically distinct, the ancestry of each single individual, if carried back only for a hundred generations, must embrace practically all of the earlier period who have contributed appreciably to the ancestry of the present generation.... It is only the geographical and other barriers to sexual intercourse between different races, factors admittedly similar to those which condition the development of incipient species as geographical races, which prevent the whole of mankind from having had, apart from the last thousand years, a practically identical ancestry. The ancestry of members of the same nation can differ little beyond the last 500 years; at 2,000 years the only differences that would seem to remain would be those between distinct ethnographical races; these, or at least some of the elements of these, may indeed be extremely ancient; but this could only be the case if for long ages the diffusion of blood between the separated groups was almost non-existent. (Dover edn., pp.138-9)

This last remark seems to overlook the possibility of the evolution and maintenance of geographical differences as a result of differing selective pressures in different parts of a continuous 'cline'. In fact, however, the next section of the book goes on to consider this possibility, and the balance of factors that may lead either to an equilibrium or to a fission of the population into different species. Fisher does not use the term 'cline', which was introduced by Julian Huxley some years later, but the concept is esentially the same.

Posted by David B at 03:55 AM