Monday, 15 April 2013

J. Huxley anticipated H.J. Muller by misinterpreting R.A. Fisher

R.A. Fisher (1932, The bearing of genetics on theories of evolution. Science Progress 27:273-287) published an address delivered before the Royal Society of Dublin, 31 January 1932. In it, he explained the inbreeding depression as due to the accumulation of recessive deleterious mutations in out-crossing species, which inbreeding will expose, and the lack of such a depression in naturally inbreeding species as due natural selection preventing such an accumulation. The context is the evolution of dominance and recessiveness and the purpose is explaining why some organisms experience inbreeding depression and others not. Here's the relevant passage:
"It is the habitual cross-pollination of the maize plant, when grown commercially for seed, that has permitted the accumulation of the great swarm of defects which are revealed by self-fertilisation. In species commonly self-fertilised such recessives would be quickly eliminated, and the habitual procedure in scientific maize improvement now lies in the selection of those self-fertilised lines which are most free from serious defects, and which, on crossing, do, in the first or second generation, outyield every commercial variety of maize obtainable.        The injury observed on close inbreeding is thus exposed in an entirely new light. It is not perpetual self-fertilisation, but the first few generations, and especially the first generation, that is dangerous. The inbred lines show no perceptible further deterioration after eight or ten generations. Moreover, it is not the racial potentialities that are injured, but only the individual expression of them. It is not the species, but the individual, which suffers. The various devices which exist in nature to ensure exogamy are not for the benefit of the species, for which, so to speak, Natural Selection cares nothing, but to ensure the well-being of the immediate progeny ; to guard them against the recessive defects, which may lie latent in their parents." Fisher (1932, p. 283)
Julian Huxley (1942, Evolution. The Modern Synthesis. Harper & Brothers Publishers) construed this as a mutation clearance argument for the maintenance of sex similar to Muller's Ratchet:
"In addition, as Fisher (1932) stresses, it [sexual reproduction] has a function to perform in relation to the deleterious nature of most mutations. For, by allowing recombination, it permits mutations to appear in homozygous form, and thus facilitates the elimination of the more deleterious. Elimination will be greater when the frequency of homozygosis is increased by inbreeding or self-fertilization." (Huxley 1942, p. 84)
I cannot help but feel that this is a misinterpretation of Fisher (1932), who wanted to explain why most organisms experience an inbreeding depression, while some did not, and not deploy an argument for the evolutionary maintenance of sex (not here anyway). Nevertheless, the misinterpretation was luckily in line with later reasoning by Muller (1964, The Relation of Recombination to Mutational Advance. Mutation Research 1:2-9).

P.S.: Muller does not cite Huxley, so we do not know whether Huxley's idea/misinterpretation had consequences. However, they knew each other personally. In 1915, Huxley hired Muller, then a student at Thomas Hunt Morgan's fly lab, as his assistant at the Rice Institute in Huston.

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