Evolution.

Society for Experimental Biology

Date:: 1953

Catalogue details

Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Evolution. Source: Wellcome Collection.

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xviii FOREWORD plants. The presence of anthocyanin in the corollas of P. juliae and P. amoena must certainly help to isolate them from the yellow-flowered species. When we can classify odours they will certainly prove to be biologically important taxonomic characters of plants, probably also of animals. Here modern biology is going back to Darwin. Sexual selection, though Darwin probably overrated it, has been underrated during this century, and it was Rendel in my laboratory who perhaps first showed decisively that both male 'releasers' and female response to them were genetically determined. Both positive and (as in Sheppard's case) negative assortative mating are forms of sexual selection, just as stabilizing, normalizing and disruptive selection (vide Waddington and Mather) are forms of natural selection, though none is very like any of the forms of selection of which Darwin wrote. The evolutionary possibilities of sexual selection in this wider sense have not, I think, been fully realized. If it be granted that in a lepidopteran species male preferences are in part genetically determined, and female releasers, whether visual patterns or correlated with them, are also geneti¬ cally determined, not generally by the same genes, we may have an explana¬ tion for the rapid divergence of Dowdeswell and Ford's island populations as the result of a mechanism much quicker than Wright's drift. For on Wright's hypothesis rare genotypes are not at a selective disadvantage, but only more likely to disappear by chance; but it may be selectively disadvantageous to possess preference for a rare type of releaser or con¬ versely. The biology of species possessing polyethism as well as poly¬ morphism, should be of the greatest interest, not only to students of evolution, but to ethologists, and Hovanitz's example shows that the genetical situation may often be simpler than one would have dared to hope. But Sheppard's work shows how hard it will be to estimate the selection pressures involved. We are coming back to Darwin in another respect. On the one hand he formulated the theories of natural and sexual selection. On the other he showed the superiority of outbred over inbred individuals. But he did not synthesize these two lines of thought. Mather, Thoday, Baker, Ponte- corvo and other contributors have begun to do so. If natural selection favours heterozygotes, this has important consequences for population structure and ultimately for evolution. I have outlined some of them elsewhere. To sum up, then, a number of workers are groping from their own different standpoints towards a new synthesis, while producing facts which do not fit too well into the currently accepted synthesis. The current instar of the evolution theory may be defined by such books as those of Huxley,