Ii polimorfismo

Abstract

Polymorphism, according to Ford, is the occurrence in the same locality of two or more discontinuous forms (called morphs) of a species, in such propor*lions that the rarest of them cannot he maintained by recurrent imitation. According to this definition, polymorphism includes both genetic and non-genctic discontinuous variation, provided that the various forms arc not exceedingly rare. This paper does not deal with non-genetic polymorphism and the occurrence of sexes, although they arc cases of polymorphism. Polymorphism is a widely spread phenomenon. It regards a series of characters, of which a review is given.Polymorphism can he distinguished into balanced and transient polymorphism. In the first ease, the various forms tend to he maintained at a given proportion which can he modified by the environmental conditions; in the second one, a form tends to increase in number up to reach the amount of 100% of the population.Various mechanisms are involved in the maintenance of the balanced polymorphism, among which there arc: (i) the advantage of the heterozygotc; (ii) the advantage depending on the rarity of the form; (iii) the advantage of various forms on heterogeneous substrates (sub-strate polymorphism).Ill various instances, the balanced polymorphism can he detected hy the departures from the genotype frequencies which are calculated hy means of the Hnrdy-Wcinhcrg law.Then the A. refers on many important and well studied examples of polymorphism. Most of them can he well explained as eases of balanced polymorphism maintained hy natural selection, often through the hctcrozygote advantage mechanism, sometimes enhanced hy other mechanisms.In many Lcpidoplcra and Odonala polymorphism is confined to the females; Colins exhibits hourly variation, due to physiological characters which explain its polymorphism hy means of natural selection.The polymorphism of many lady beetles exhibits geographic, ecological, season, and long time variation, all probably due to natural selection. The polymorphism of many lady beetles exhibits geographic, ecological, season, and long time variation, all probably due to natural selection.The polymorphism of the snail Ccpcoa ncmoraUs, whose ecological variation and selective predation are object of dispute, lias given the opportunity for a discussion between the supporters of natural selection and those of genetic drift.In the Crustacean Tishc retirulnta polymorphism is also maintained hy natural selection, since there is hetcrozygotc advantage, physiological characters and mating preference. In Sphaeroma senalum, on the other hand, polymorphism just appears to he a neutral one, hut ecological variation and physiological characters arc all elements which point out the importance of selective forces in determining it, in many fishes polymorphism, often confined to the males, has been recently interpreted as due to selective forces, often hy means of reproductive advantage. In man, many instances of polymorphism are associated with physiological characters or pathological conditions, so revealing with a clear evidence that they arc maintained hy natural selection, the frequencies of the forms being eventually modified locally by genetic drift.A particular ease of polymorphism is the chromosomic one, well known in Drosophila, in which it is concerned with the presence of various paracentric inversions. It is generally balanced, since there is hetcrozygotc advantage, seasonal variations, long time variations, geographic variations and allitudinal elincs.In a number of cases the balanced polymorphism is maintained by advantage of the rarest forms. In the Batcsian and aggressive mimicry the advantage of a mimetic form depends 011 its rarity in respect to its own model; the Mullerian mimetic species, whose advantage is not dependent on their rarity, are not polymorphic. In ease of a mating preference and of the occurrence of sexes, too, the advantage of the forms depends on their rarity.The best known ease of transient polymorphism is (lie spreading of mclanic forms of several moths all over industrial districts; this industrial melanism is due to the selective value of the mclanic forms in the changed environmental conditions under severe predation by birds.Neutral polymorphism is not known with certainty. The genetic drift seems to be responsible of geographic variation of balanced selective polymorphism only.The balanced polymorphism is likely to he originated from plciotrop'ie mutations through selection, when the genes arc dominant for favourable effects and recessive for unfavourable ones; or through the accumulation in the same supergenos hy means of natural selection of dominant favourable genes ami recessive unfavourable ones. Evolution of dominance can also occur. Selection will cause the accumulating and sheltering of lethal recessive genes in supcrgcncs, and consequently the hetcrozygotc advantage will be enhanced. © 1965 Taylor & Francis Group, LLC.