Biological Races in Insects and Allied Groups

Abstract

Discussion and Summary. Turning now to a consideration of the general significance of these numerous examples, the question which first presents itself is: Do they really represent the commencement of evolutionary change? In other words: Are they species in the making? A large number of workers (e.g. Schroder, Pictet, Cuénot, Nuttall, Harrison, Adkin and Sandground, to mention only a few) have answered in the affirmative with regard to the particular case with which each was concerned1. It seems worth while now to consider the question as a whole. If these races are indeed nascent species, then we must assume that their characteristics are sooner or later germinally fixed and that there is some degree of isolation, physiological or habitudinal, which ensures their continuance. How much evidence is there for these assumptions? It seems to the writer that perhaps the most important consideration in this connection is that of isolation. As has long been realised, the great difficulty in regarding mutations or any other inheritable variations produced in small numbers, as the starting point of new species, is that of accounting for the spread of such throughout a population. If the differences which serve to distinguish one species from another were of a kind likely to be of immediate survival value, then the problem would be simpler, but this does not appear to be the case with the general run of specific characters, still less of varietal ones. The importance of geographical isolation in fostering such variations is undoubted, but in many cases it is unlikely that this factor can come into play until a comparatively late stage in the process; the first steps in the spread of a new variant are little assisted by geographical barriers. If, however, there is some form of physiological isolation such as is provided by various small biological differences and by a repugnance for cross‐mating, then the process seems much easier to comprehend. Charles Darwin saw this clearly, as is evinced by his great interest in Bates' statement (Trans. Linn. Soc.23, 501): “The process of the creation of a new species I believe to be accelerated in the Ithomiae and allied genera by the strong tendency of insects, when pairing, to select none but their exact counterparts.” As Carpenter (1913, Proc. Ent. Soc. p. lxxxviii) points out, Darwin (Life and Letters, 2, 392) wrote to Bates regarding this generalisation: “I look upon this fact as very important”; but later rather severely criticised him for making the statement in general terms without sufficient evidence. That the evidence even now is far from being as extensive as could be wished is made clear by Richards (1927, pp. 345–347). As has already been seen in this review, in the few instances among naturally occurring biological races in which the matter of selective mating has been put to the test, the results have been positive, although not always, it must be said, on a scale large enough to be very convincing. There is here a big field for further work of a very great importance, ...

Oviposition Behaviour as an Ecological Factor in Woodland Canopy Leafhoppers

“…It thus supposes a conditioned response in the adult by previous larval experience. The early literature on this theory and its importance in evolution was fully reviewed by Thorpe (1930). Many authors have since advocated the theory in evolutionary discussions, but there seems to be very little evidence for it.…”

The evolutionary process in talitrid amphipods and salamanders in changing environments, with a discussion of "genetic assimilation" and some other evolutionary concepts

“…The Baldwin effect, or organic selection, was primarily concerned with behavioral changes in evolution. Huxley (1948), citing examples mainly from the works by Thorpe (1930Thorpe ( , 1940 on food plant preference in insects, emphasized the conditioning effect to particular host plants as a nonhereditary barrier which may serve as the first stage in evolutionary divergence by the split of a population. He pointed out that the change is not impressed upon the germ cells, but holds the strain in an environment where mutations tending in the same direction will be selected and incorporated into the constitution.…”

Group selection and the development of the biological species concept

“…Dobzhansky argued these 'races' were actually good biological species, even though he encountered strong opposition from Alfred Sturtevant and taxonomists in so doing. In the same passage Dobzhansky also cited a well-known review of 'biological races' in insects (Thorpe 1930). Thorpe showed that insects may specialize on different hosts or ecological niches, and can separate into cryptic 'biological races' that become physiologically isolated.…”