Area effects and the structure of peripheral populations of Cepaea nemoralis

Abstract

Neither of the peripheral populations of Ceraea nemoralis studied showed any signs of unique races such as those found in Cleithronomys, and in this respect they do not differ from central populations. In spite of this negative result, the study of populations of C. nemoralis at the limits of their distribution has been more than justified by the indirect evidence resulting on the causes of area effects.Considered simply as area- effects, neither the Montrose nor the Yugoslav Cepaea populations support the theory that area- effects are determined by local climatic selection, as in neither case do the distributions of gene-frequency show any apparent relationship to local topographical factors might cause, for example, accumulation of cold air. This is particularly .noticeable in the transects, both of which traverse remarkably uniform regions (sand-dune and polje respectively), and both of which nevertheless show considerable variations in gene-frequency.The mere existence of colour and banding area effects at the Northern limit of C. nemoralis' distribution is in itself significant, as it if suggests that if area effects are maintained in the face of intense selection by cold ( which is presumably acting to prevent the species from extending its range), then environmental selective forces which are so small as to virtually undetectable in most cases arc unlikely to be the sole agents maintaining area-effects elsewhere in C. nemoralis' range.The situation in the South is particularly interesting. Climatic selection seems to 1)6. Pausing great alterations in the frequencies of the colour genes, but to have relatively little effect on the relative frequencies of the genes for shell banding. This suggests strongly that these banding genes are in some way more deeply integrated into the genecomplex than are those for shell colour. The colour genes are this susceptible to intense environmental selection by high temperatures, while the frequency of the banding genes in any particular area is primarily a function of the genetic environment rather than the ambient temperature (or whatever else is limiting C. nemoralis' range). It is probably significant in this connection that the Yellow morph has been shown to be more resistant to extreme conditions of various kinds than are the other colours.Both the coadaptive and the selective theories of area effects may therefore be possessed of an element of truth, and Clarke's compromise between them may provide a satisfactory unitary hypothesis.It appears that the genetic basis of Cepaea area-effects may he a complex phenomenon, with different genes showing different degrees of integration with the gene complex. The degree of coadaptation may itself be a fairly labile attribute of a population - King for example, has shown that stocks of Drossphila subjected to intense selection by DDT developed differently-integrated gene-complexes within twelve generations of selection, so that crosses between F₁₂ lines showed significantly lower DDT resistance than did either parental line. Lewontin's experiments on Drosophila populations held in constant environments showed even more rapid evolution of different coadapted genecomplexes favourable to high or to low frequencies of a particular gene. Coadaptetion may therefore develop rapidly and, as we have suggested in Cepaea, different genes may show different amount s of interaction with a coadapted complex of genes. In Cepaea, this may be based on an inversion system, whereby the abnding genes are held on an inversion bearing other genes which interact strongly with the animal's genetic environment, whereas the colour gens are not so strongly associated with such powerful elements of the genecomplex.Area effects in other species are also likely to be based on a balance between genetic and environmental selection. Cavalli-Sforza's finding that small isolated human populations show an increased variance in blood-group frequencies, and that classification of the human races according to such frequencies agrees well with the classifications based on other characters is best interpreted as a response of blood-group frequencies to different genetic backgrounds in small isolated populations while, on a larger scale, related rates have generally similar genetic backgrounds, and therefore have similar gross proportions of the various blood groups. Allison's work on "area -effects" in various haemoglobin variants in man, however, is equally likely to be a response of a particular gene (or group of genes) to environmental selection, in this case by disease.In any event, area effects in any species deserve further study, not only because of the light which they may throw on human population genetics, but also because of their intrinsic evolutionary interest.