Evolution and Adaptation

by Thomas Hunt Morgan

It is, however, the process of sexual selection that brings out in the strongest contrast the difference between Darwin’s main idea of natural selection and the law of the survival of species. In sexual selection the competition is supposed to be always between the individuals of the same species and of the same sex. There can be no doubt in one’s mind, after reading “The Descent of Man,” that Darwin held firmly to the belief that the individual differences, or fluctuating variations, furnish the material for selection. In this way it could never happen that two competing species could exterminate each other, because in the one the males were better adorned, or killed each other off on a larger scale, owing to the presence of special weapons of warfare. It is clear that on the law of the survival of species, secondary sexual characters cannot be supposed to have evolved because of their value. Their origin is totally inexplicable on this view. In fact, the presence of the ornaments must be in some cases injurious to the existence of the species. The interpretation of this means, I think, that individual competition cannot be as severe as Darwin believed, and cannot lead to the results that he imagined it does. For this reason it seemed important to make as careful an examination of the claims of the theory of sexual selection as possible, and I hope that the outcome of the examination has shown quite definitely that the theory is incompetent to account for the facts that it claims to explain. It is certain in this case that we are dealing with a phenomenon that must be studied quite apart from any selective value that the secondary sexual organs may have. If this is granted, it will be seen that there is here a wide field for experimental investigation that is practically untouched.

It is evident that the first step that will clear the way to a fuller understanding of the problem of evolution must be a more thorough examination of the question of variation. Darwin himself fully appreciated this fact, yet until within the last fifteen years the study of variation has been largely neglected. With a fuller knowledge of the nature of fluctuating variation as the outcome of the studies of Galton, Pearson, De Vries, and others, and with a fuller knowledge of the possibilities of discontinuous variation as emphasized by Bateson and by De Vries, and, further, with a better knowledge of some of the laws of inheritance in these cases, we have begun to get a different conception of how evolution has come about. It may be well, therefore, to go once more over the main points in regard to the different kinds of variation.

While it has been found that no two individuals of a species are exactly alike, yet, taken as a group, the variations appear as though they followed the law of chance. The descendants of the group show the same differences. Thus the group as a whole appears constant, while the individuals fluctuate continually in all directions. This is what we understand by fluctuating variation. If the external conditions are changed, a new “mode” may appear, but the change is generally only a temporary one, and lasts only as long as the new conditions remain. Thus, while the direct influence of the environment may show for a time, the result is transient. Even if it were permanent, there is no evidence that the adaptation of organisms could be accounted for in this way unless the response were useful. It appears that this sometimes really occurs, especially in responses to temperature, to moisture, to the amount of salts in solution, to poisonous substances, etc. In this way, one kind of adaptation is brought about, but there is no evidence that the great number of structural adaptations have thus arisen.

The Lamarckian principle of the inheritance of acquired characters has also been supposed by many writers to be an important source of adaptive variation. An examination of this theory is not found to inspire confidence. We have, therefore, eliminated this hypothesis on the ground that it lacks evidence in its favor, and also because it appears improbable that in this way many of the adaptations in organisms could have been acquired.

Finally, there is the group of discontinuous variations. Of these there may be several kinds, and there is some evidence showing that there are such. For the present we may include all the different sorts under the term mutation, meaning that the new character or group of characters suddenly appears, and is inherited in its new form. From the results of De Vries it appears that mutations are sometimes scattering, at least in the case of the evening primrose. From such scattering mutations, the smaller species or varieties (in so far as these do not depend on local conditions) arise. There is here an important point of agreement with Darwin’s idea in regard to evolution, inasmuch as he supposed that varieties are incipient species. Our point of view is different, however, in that we do not suppose these varieties (mutations) to have been gradually formed out of fluctuating variations by a process of selection, but to have arisen at once by a single mutation. It also appears that in some cases a single new mutation may develop in a species. We may suppose that the new form might in such a case supplant the parent species by absorbing it, or both may go on living side by side, as will be more likely the case if they are adapted to somewhat different conditions.

A number of writers have supposed that evolution marches steadily forward toward its final goal, which may even lead in some cases to the final but inevitable destruction of the species. By certain writers this view has been called orthogenesis, although at other times the idea is not so much that there is advance in a straight line, as advance in all directions. This appears to be Nägeli’s view. It gives a splendid picture of the organic world, as irresistibly marching toward its goal,—a relentless process in some cases, leading to final annihilation, a beneficent process in other cases, leading to the fullest perfection of form of which the type is capable. Compared with the vacillating progress which is supposed to be the outcome of individual selection, this view of progression has a grandeur that appeals directly to the imagination. We must be guided, however, by evidence, rather than by sentiment. The case will, moreover, bear closer scrutiny. If evolution has indeed taken place by the survival of a series of mutations, whose origin has no connection with their value, does not this in the end amount to nearly the same thing as maintaining that evolution of organisms has been a steady progress forward,—a progress undirected by external forces, but the outcome of internal development? Admitting that innumerable creations have been lopped off, because they could find no foothold, yet, as Nägeli points out, the result is that, instead of a dense tangle of forms, there has been left relatively few that have been found capable of existing,—those that have found some place in which they can live and leave progeny. From this point of view it may appear, at first thought, that the idea of evolution through mutations involves a fundamentally different view from that of the Darwinian school of selection; but in so far as selection also depends on the spontaneous appearance of fluctuating variations, the same point of view is to some extent involved,—only the steps are supposed to be smaller. This point is usually ignored and passed over in silence by the Darwinians, but, as Wigand has pointed out, it makes very little difference whether the stages in the process of evolution are imagined to be very small or somewhat larger, so long as they are spontaneous. Selection does not do more than determine the survival of what is offered to it, and does not create anything new.

It is true that if the fluctuating variations that are selected be connected by very slight differences with an almost continuous series of other forms, and if little by little such a series be advanced in a given direction by selection, we get the idea of a continuity, whose advance is determined by selection. It is this conception that appears to give the theory of natural selection a creative power, which in reality it does not possess, and certainly not in the modified form in which the theory was finally left by Darwin. For Darwin found himself forced to admit that, unless a very considerable number of individuals varied at the same time and in the same direction, the formation of new species could not take place, and this idea of many individuals varying at the same time, and in the same direction, at once involves the conception that evolution moves forward by some force residing in the organism, driving it forwards or backwards. Instability comes, perhaps, nearer to expressing this idea than any other term, and yet to evolve from a protozoan to a man implies the idea of something more than simple unstableness.

The idea that Weismann has touched upon in this connection, namely, that the survival of a given form determines the future course of evolution for that form, is very plausible, and also fits in well with the results of our experience in the field of the inheritance of variations. We see new variations or mutations departing in some or in many characters from the original type, apparently by new combinations or perturbations of those already present. We never expect to see a bird emerge from the egg of an alligator. Thus it appears that by the survival of certain forms the future course of evolution is determined in so far as the new types of mutation are thereby limited. Weismann means, however, that in this way new plus or minus steps will be indefinitely determined amongst the new fluctuating variations, but this statement is contradicted by our experience of the results of artificial selection. The upper limit does not keep on pushing out indefinitely in the direction determined by the first selection, but is soon brought to a standstill. So that, as far as Weismann’s hypothesis is concerned, the idea appears to have no special value. On the other hand, this idea may be fruitful if applied to mutations, but here unfortunately we have not sufficient experience to guide us, and we do not know definitely whether a new character that appears as a mutation will be more likely, in subsequent mutations, to go on increasing in some of the descendants. Thus, while the mutation theory must assume that some new characters will go on heaping up, we lack the experimental evidence to show that this really occurs. It would be also equally important to determine, whether, if after several mutations have successively appeared in the same direction, there would be an established tendency to go on in the same direction in some of the future mutations. But here again we must wait until we have more data before we attempt to build up a theory on such a basis.

The attacks on the Darwinian school by the followers of the modern school of experimentalists are with few exceptions based on the assumption that the natural selectionists pretend that their principle is a sort of creative force,—a factor in evolution in the sense of being an active agent. This assumption of the selectionists has led many of them to ignore a fundamental weakness of their theory, namely, the origin of the variations themselves, although Darwin did not overlook or ignore this side of the problem, or fail to realize its importance, as have some of his more ardent, but less critical, followers. They have contented themselves, as a rule, with pointing out that certain structures are useful, and this has seemed to them sufficient proof that the structures must have been acquired because of their value. In contrast to this complacency of the selectionists, we find here and there naturalists who have, from time to time, insisted that the scientific problem of evolution is not to be found in the principle of selection, but in the origin of the variations themselves. It will be clear, from what has been said, that this is our position also, and for us adaptation itself does not appear to be any more a problem that can be examined by scientific methods, than the lack of adaptation. The causes of the change of whatever kind should be our immediate quest. The destruction of the unfit, because they can find no place where they can exist, does not explain the origin of the fit.

Over and beyond the primary question of the origin of the adaptive, or non-adaptive, structure is the fact that we find that the great majority of animals and plants show distinct evidence of being suited or adapted to live in a special environment, i.e. their structure and their responses are such that they can live and leave descendants behind them. I can see but two ways in which to account for this condition, either (1) teleologically, by assuming that only adaptive variations arise, or (2) by the survival of only those mutations that are sufficiently adapted to get a foothold. Against the former view is to be urged that the evidence shows quite clearly that variations (mutations) arise that are not adaptive. On the latter view the dual nature of the problem that we have to deal with becomes evident, for we assume that, while the origin of the adaptive structures must be due to purely physical principles in the widest sense, yet whether an organism that arises in this way shall persist depends on whether it can find a suitable environment. This latter is in one sense selection, although the word has come to have a different significance, and, therefore, I prefer to use the term survival of species.