BIOLOGICAL RACES IN INSECTS AND ALLIED GROUPS
Authors: THORPE W. H. 
Published: January 2008

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 theIthomiaeand 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, but in the meantime it is not very difficult to conceive how a biological race may be perpetuated. Even though the mating preference might not be very strong, yet under natural conditions in combination with slight differences in habitat and time of appearance, it might stop interbreeding almost completely. Moreover, as was pointed out above, if the oviposition reaction is in some degree determined by a “memory” of the larval food, as postulated by Walsh and others, then the early stages of such divergence are correspondingly easier to understand, for we are relieved of the necessity of imagining an immediate germinal change corresponding to the new habits. A new host plant race might thus be separated mnemonically for a very long time without any germinal change taking place.

Another point in favour of the view that such races are of some evolutionary significance has been brought out by Richards (1927) in a different connection. He says: “If the first barrier in the divergence of species were a physiological one of this nature it would be easy to see why the sterility of interspecific crosses should be so capricious in its incidence, for it might arise more or less accidentally, at any time after the species had ceased to interbreed.”

These considerations, then, indicate how a new race might be developed apparently irrespective of the action of natural selection, provided of course that the variation was not positively harmful. In the case, however, of a plant‐feeding insect the production of a new host form might be of very great value in that it might enable the insect to occupy an otherwise vacant ecological niche previously closed to it, and thus to colonise a large area. That the correlation of such important biological differences with minute morphological characteristics of no survival value may supply an explanation of the persistence of these latter has of course long been realised.

It also seems worth considering what bearing these facts may have on the rather academic question as to which comes first—physiological or morphological variation—academic because of the difficulty or impossibility of drawing any line of division between the two. Robson (1928, p. 183), discussing Darwin's view that it is immaterial which comes first or whether they both come together, says: “I do not think we can regard the matter as immaterial until we know more about the actual origin of habits. The doctrine of Natural Selection and Lamarckian argument do not require as an absolute condition that the acquisition of a new habit should precede a structural change; though, speaking broadly, both theories would be easier of acceptance if this condition were fulfilled.”

It will be obvious that the various cases described above do supply some very striking instances of significant physiological change, either without any visible morphological variation or with the latter relatively very slight. When morphological change is present it is, it is true, often extremely difficult or impossible to decide which came first, but we can at least say that, whereas the physiological changes are important and likely to be of survival value, the morphological ones are trivial in the extreme.

It seems then, that although much more evidence is desirable, there is no very great difficulty in visualising how a biological race having once begun, may persist and increase. The next question is—If this is a mode of evolution, how consistently does it operate in nature? Meyrick (1927), for instance, tends to restrict the application of the principle on the ground that many closely allied species have the same food.

In this connection let us consider first the phenomenon of large‐scale habit change which is such a feature of modern economic entomology, and which, as is well known, occurs in many groups of animals (see Ritchie, 1920, pp. 400–416). There are many cases on record of a large scale and relatively sudden change of habit (e.g.food, oviposition preference), as a result of which a species is able to colonise new areas or new environmental niches. Such new niches may then be occupied apparently in preference to, if not to the exclusion of, the old. Robson (1928, pp. 110 and 156) lists several interesting cases of this in groups other than insects. Among insects the theory of Anopheline zootrophy and the cases ofRhagoletis pomonellahave already been mentioned, and such cases as the Colorado Beetle,Leptinotarsa, and the Parsley Stalk‐Weevil (Listronotus), Boyce (1927), come to mind. A particularly interesting case, in that the whole process has been observed, is that of certain Capsid bugs in the fruit‐growing areas of East Anglia.Lygus pabulinushas long been known to Hemipterists as one of the commonest Capsids to be obtained in summer by sweeping herbaceous plants; it was not known previously to 1928 that it was bi‐voltine, the winter eggs being laid in the stems of woody plants. From the work of Petherbridge and Husain (1917) on the Capsid pests of apple in East Anglia, it is certain thatL. pabulinusdid not at that time occur on apple, and was rare on currants or other bush fruit. By 1926 it was found frequently on apple and had become a serious pest in currant and gooseberry plantations, owing to its newly acquired habit of using these plants for a winter host. In this area it appears to have deserted the original winter host, apparently blackberry, almost entirely (Petherbridge and Thorpe, 1928).

Although it is not proved in any one case, with the doubtful exception ofRhagoletis, that such habit change is correlated with the development of a separate biological race; yet in view of the rapidity with which, in certain instances, such races appear to have been produced experimentally, it seems more than likely that this is the case.

So much for the evidence of the plasticity of food habits; but viewing the matter from another standpoint it at once becomes obvious that such rapid development of new races cannot be going on uniformly in nature, or there would be no specific stability at all. In reality there are many instances in which it seems certain that the feeding habits of a species or genus of insects must have remained constant over a very long period of time. Such cases are postulated by Harrison's theories (1920, 1924) as to the origin of the different races and species ofAriciaandOporabia, and Brues (1920) cites the instances of the long period during which the holarctic and nearctic Vanessids or the world‐wideAristolochia‐feeding Papilios must have remained constant in food habits. Constancy of food habits might, of course, be explained on the assumption that great mortality is always caused at the commencement of a host plant change, and that under natural conditions the few survivors would have an extremely slender chance of perpetuating the newly formed habit. Nevertheless, were the food habits of all insects as easily modified as, say, those ofPontaniaorRhagoletisappear to be, even such initial difficulties at establishment would not suffice to account for the case of the Papilios and Vanessids cited above, nor for many others.

It will have been realised from what has been said already that there are certain difficulties in the way of considering the host selection principle as of wide application in the production of new forms, especially in that the stronger the tendency of a species to develop a host preference the more difficult it becomes for that species to adopt new hosts. If, however, as seems possible in many cases, the host preference is mainly upon the psychological plane, without anyphysiologicalinability to change to a new host, we may have an explanation of certain of these difficulties. Thus, under normal conditions, the insect would remain constant, but during periods of exceptional stress, due perhaps to climatic or other environmental changes, the psychological preference might be overcome, with the result that new host plants would be adopted and a new race thus developed.

We can only suppose in our ignorance that while fixity of habits is the rule, species and genera pass through periods of plasticity, perhaps coinciding with particular climatic or environmental changes, during which habits are easily changed and during which the origin of new races and species proceeds apace, to be succeeded perhaps by another long period of staid conservatism.

Finally, since certain workers in this field have regarded their results as evidence of the inheritance of acquired characters, the question presents itself: Can the origin of biological races be explained on any non‐Lamarckian theory? And this in turn depends on the question: Is the evidence really satisfactory that selection of preexisting strains will not explain the phenomena?

In the majority of experiments quoted above, there is certainly noproofthat the results could not be explained on the basis of selection from a mixed population; this is especially the case in experiments such as those of Pictet (1911) onLasiocampa quercusand Schroder (1903) onPhratora vitellinae, in which there was a very great initial mortality. It must be said, however, that such an explanation seems highly inadequate for most of the results described, and in one or two cases it appears to be ruled out almost certainly. Thus, whilst a number of the Nematode experiments could be accounted for in this way, the theory seems quite inadequate to account for those instances in which the Nematode population becomes so specialised on a new host as to be unable to return to the original food plant. Another difficulty is found in the work of Tischler (quoted above), in which it is stated that, if left to choose, theentirepopulation goes toCircaea lutetianaandnonetoC. intermedia, but that if the former is absent infestation of the latter will take place. It seems that such a result, if adequately substantiated, cannot be explained on any theory of selection. Unfortunately the subsequent behaviour of a strain forced on tointermediadoes not appear to have been studied. In view, however, of the long period during which some Nematodes are capable of living without nutriment and of our uncertainty as to the factors which govern ability to infest fresh hosts, it is perhaps unwise as yet to lay too much stress on these points. It should also be mentioned that, because of the resistant eggs and long life of die adult resulting in the probability at any given time, that all the various stages of the Nematode will be present, it is far more difficult to eliminate the possibility of a “mnemonic” theory than it is in experiments with insects. Such a theory, however, seems quite as incapable as that of selection of accounting for the extreme cases just cited.

There are also serious difficulties in the interpretation of some of the insect experiments on these grounds. Thus Harrison (1927) states that of the experimentally producedSalix rubrarace ofPontania, nonereturned toS. andersonianain the course of three years. Since, as was remarked above, the possibility of “memory” was not eliminated this case cannot be regarded as conclusive evidence; a possible explanation of the results might be as follows. Suppose the Sawflies on the initial food plantXto be composed of two strains:A, capable of development on both plantsXandY, andB, capable of development onXonly. Suppose also that both these have the “mnemonic” preference for oviposition onX.They can thus be represented asAxandBx.Now force this population on to hostY.By selectionBis then eliminated, but strainAestablishes itself, and in the second generation its mnemonic preference becomes changed so that we can represent it asAy.If this populationAyis now given a choice ofXandY, it will remain onYbut could easily beforcedback on toX.Only if there wasdifficultyin doing this, or if a continued preference forYoverXwas shown in the second generation or after—only then should we be entitled to put forward a Lamarckian explanation.

It must be said that this explanation is perhaps unlikely. Nevertheless its possibility should be considered.

In the work of Nuttall and others on the transformation ofPediculus capitisintoP. corporiswe come up against the difficulty that the change did not appear suddenly in the first generation after the change in feeding methods, but developed gradually over at least four generations. Keilin and Nuttall (1919, p. 325) expressly state that Lice,P. capitis, which had been reared by Bacot for two years in the laboratory, wereintermediatein structural characters between typicalcapitisandcorporis.Here again it is very difficult to find an orthodox explanation.

In conclusion, then, we may say that many of these experiments are easily explained on some form of Lamarckian theory, but extremely difficult to account for on any other lines. If the hypothesis were not such a debatable one the evidence might well be regarded as almost conclusive for, to quote Thompson and Parker (1928), “it is probably the only intelligible theory of a natural evolutionary process ever advanced.” It seems equally certain, however, that none of the experiments recorded in these pages has been on a sufficiently extensive scale to carry complete conviction on the point. They do, however, suggest most profitable fields for further work of this nature and, taken together, they provide a quite considerable amount of the ever‐growing body of circumstantial evidence for the theory.

1This apparently is also the point of view taken by Hering (1926a) with regard to the Lepidoptera when he states that the polyphagous forms are phylogenetically the oldest and that monophagy is a more recent development. For a detailed discussion of the same subject, with special reference to leaf‐mining larvae, see Hering,Die Ökologie der blattminierenden Insektenlarven, 1926, pp. 120–44. A similar view is taken by Howard (1924b, p. 37) with regard to the parasitic Hymenoptera.