Book - Evolution and Genetics 11
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Chapter 11 The Origin of Species by Natural Selection
The question still remains whether under natural selection the mutational changes that appear sporadically will suffice to supply the materials for new species. Genetics has shown that in all probability only one gene of a pair mutates at a time. If the mutation occurs late in the history of the germ-cells, the mutated gene might be retained in an egg or in two sperm-cells. If the mutation occurred earlier in the germ track the mutated gene might, if in a female, remain in several eggs after the polar bodies are formed, or if in a male, remain in many sperm cells. If a germ-cell carrying the new gene happens to combine with a germ-cell of another normal individual, the cells of the embryo so produced will contain the gene in only one of its chromosomes, and such an individual will not show the character if the gene is recessive. In this individual, half of the mature germ-cells will now contain the gene and half its normal partner. If such an individual mates with a normal individual, half of its offspring will carry the gene in only one chromosome of each cell. Here, for the first time, the new gene is present in many individuals — in half as many as are produced by the mating. This process may by chance be repeated over and over again, but sooner or later two individuals each carrying the gene may mate. One fourth of their offspring will then show the new character, which now appears for the first time. Several or many mutant individuals w^ill then suddenly emerge, and since they are the output of the same female, their proximity will increase the chance that two at least mate with each other and produce progeny with the new character.
These considerations are significant for the selection theory. They show that there is no danger of a new mutant being lost at its inception, except in so far as chance works against the survival of the offspring of any one individual. They show also that a new gene may, by chance, become distributed in the race before its character comes to the surface. If, when it appears, the new character is one that better fits the individuals to some environment at hand, such individuals have a better chance of survival and, other things being equal, of leaving offspring. If one of them mates with an individual of the original race the same process takes place all over again, but as often as this happens, the new gene may spread in the race at the expense of the old, and may replace it if the character it stands for is one better suited to the old environment ; or, if better fitted to a new environment within reach, it will then give rise to a new type leaving the original type in possession of the old station.
The integrity of a new gene protects it from being lost through crossing with the old type, because there is no blending of the gene with the original one each time the two are brought together. Darwin confused here the characters that may blend in the hybrid with the genes that do not blend. The maintenance of the gene's integrity overcomes a serious difficulty in Darwin's theory of natural selection as he first formulated it.
Soon after the appearance of the Origin, Fleming Jenkin — a Scotch engineer — pointed out that single variants would disappear by "swamping" even although slightly beneficial, if, as Darwin supposed, blended inheritance is the rule after crossing; for, the new character will lose some of its advantage each time it combines with the original type. The chance is always greater, at first, of a mating with an individual of the original type owing to the larger numbers of such individuals. In later editions of the Origin Darwin acknowledged the force of this objection and tried to meet it by postulating that the new character must be already present in a large number of individuals if it is going to have anything like a chance of sur^dval. But one may ask if a new adaptive character can appear in so large a number of individuals that it swamps virtually the original type, what becomes of the theory of natural selection? If the adaptive change has already taken place in so many individuals it is simpler to assume that it might soon appear in all as the result of whatever change induced its appearance in so many.
Now this dilemma is to some extent at least met by the modern theory of the stability of the gene. A gene is not lost by residing in the same cell with a gene of another kind. It may, if mere chance favors its perpetuation, spread, and once inoculated in the race it may produce in time enough individuals to start a new type. Our modern knowledge of the behavior of the gene meets to some extent the difficulty raised by Fleming Jenkin and reestablishes the strength of Darwin's theory, but, on the other hand, it should be clearly understood that the chance of a recessive gene becoming widely disseminated is extremely small even though the character it represents may be a beneficial one.
The stability of the gene also enables us to understand how a gene, if it is recessive, representing a character that is even injurious to the race, may become spread, locality at least, in a group without detriment to the individuals carrying one gene. This explains the frequent occurrence, in certain restricted populations, of the appearance at times of certain abnormal types, as seen, for instance, in night blindness, and "bleeding" in man.
There is another result, clearly established by the genetic work on Drosophila, that is favorable to the final establishment of a new type or character if it is beneficial.Most, perhaps all, of the mutations appear more than once. This improves their chances of becoming incorporated in the species, and if the mutation produces a character that favors survival the chance of its becoming established is still further increased. But it is also not to be overlooked that since most of these mutational changes are not beneficial, their recurrence acts as a drag on the race, because in so far as their genes become disseminated they give rise to defective individuals whenever two such genes are brought together. The early death of defective individuals in the wild state may make their appearance less noticeable than under the more favorable conditions for survival under domestication.
It is sometimes implied that a mutational change that is dominant has a better chance than a recessive. This is not the case, however, if the dominant character is neither advantageous nor injurious, but neutral. It, then, has the same chance as a recessive. But if the dominant is beneficial, it has a somewhat better chance than a recessive, because, since it comes to expression from the beginning in the hybrid type, it improves the chances of that type in comparison with the original type. If the dominant is injurious it will be more quickly eliminated than a recessive character that is injurious.
These theoretical considerations do no more than suggest certain possibilities concerning the theory of natural selection. Before we can judge as to its actual efficiency we must be able to state how much of a given advantage each change must add to give it a chance to become established in a population of a given number. Since only relatively few of the individuals produced in each generation become the parents of future generations, numbers count heavily against any one individual establishing itself. This is a most difficult problem for which we have practically no data, and as yet only the beginning of a theoretical analysis has been made of this side of the selection problem. Haldane has developed a partial analysis of the problem for a few Mendelian situations. He points out that the problem is extremely complex and that there is at present not much quantitative information to furnish material for such a study of natural selection by means of gene mutations.
The Diagnostic Characteristics of Species and the Origin of Species by Natural Selection
It has often been pointed out that the characters used by systematists to separate species have as a ride nothing whatsoever to do with the adaptive features of species. The latter are largely physiological. Yet if species have originated through adaptive modifications, it might be expected that physiological characters would be the most distinctive ones that distinguish species from one another.
The solution of this paradox is, I think, to be found in the many-sided effects produced by each gene. It has been shown, particularly clearly in the mutant types of Drosophila, that visible, superficial changes are nearly always accompanied by other changes that have a more physiological aspect, such as the vigor, or length of life, or productivity of the individual. From the point of view of evolution these physiological effects are those of most significance, while the superficial changes are trivial in comparison.
Now it is highly probable, if definite structural changes have definite accompanying physiological changes, that other mutations that bring about physiological changes produce, at the same time, superficial structural effects. If so, we mav find here an explanation of the constancy of the latter when they are by-products of important physiological characters. Hence their constancy and their value as diagnostic characters of species.
The study of the mutation process has to a large extent also concerned itself with superficial characters, while the concomitant physiological modifications are referred to only in passing; but from the evolutionist's point of view it is the internal physiological accompaniments of the superficial effects that are of much greater significance. It is not surprising, therefore, that a good deal of the discussion of the bearing of mutants on the theory of evolution may seem rather far afield. If the mutation process were studied as contributor to the theory of evolution rather than in its genetic bearings we would reverse our present attitude and study minutely the effects of each new gene on the changes that it brings about in the life of the individual and on its productivity. We would then regard the superficial characters as by-products of the invisible effects, unimportant in themselves and at best only indices of internal modifications.
Chance and Evolution
When we consider the innumerable physiological adjustments of any organism, and the many structural adjustments of the parts of the body to each other and to the environment, an appeal to evolution through chance variation may seem preposterous. Stated in this general way the theory of evolution by chance variations seems repellent to the traditional thinking of many persons. It is this supposed difficulty, I think, that has driven some biologists and laymen, either to the acceptance of some sort of external guiding principle responsible for evolution, or to the assumption of an internal mystical property (entelechy) of living things, or to the cruder appeal to the inheritance of acquired characters. There is, however, a well known property of living organisms that puts the theory of chance, as the sufficient agent in evolution, on a very different footing from chance as generally understood. This is the property of living things to multiply their kind indefinitely i.e., to reproduce an indefinitely large number of individuals with the stamp of a lucky throw. For example, no one would maintain that so complex a mechanism as that of a living organism could suddenly appear by the accidental coming together of the materials of which it is at present composed. This is as inconceivable as that an automobile could develop through the chance meeting of wood, iron, rubber, oil, and gasoline ; or to use Paley's old image, that a watch could be produced by the accidental accumulation of pieces of iron. The parts of the automobile and of the watch have been brought together under the direction of a human agent, but what has brought the parts of the organism together? The implication in this question is that there must have been a directing agent of some sort, since by chance such a fortuitous combination is inconceivable. The statement ignores certain properties of living materials that put the two problems in a different light. These are the property of growth by which living matter can increase indefinitely in volume; the property of multiplication by which a given sam^^le may duplicate itself without limit; and the possibility of changes in the material that furnish new stable conditions. We may not be able at present to explain fully how growth takes place, but there is nothing in growth, as far as known, that is inconsistent with chemical processes. We may not be able to state in detail how cells divide, but the purely physical character of the process can scarcely be doubted. We may not be able to give the cause of a new variation, but we find nothing in the occurrence of a change that produces a variation that is inconsistent with chemical or structural alterations in the germ material. If this much be conceded, the problem of self-construction of even a complicated piece of mechanism is not beyond our comprehension. At any rate the problem is obviously different in kind from that of constructing a mechanism whose materials do not possess these properties. So long as the processes of division and growth take place faster than the process of accidental destruction or death, the living material can maintain itself indefinitely. The stability of such an organism is no greater, of course, than that of the chemical material of which it is composed. If this changes in those parts that have the property of division and growth (without affecting these properties) something new will result, a new type, and if this is able to maintain itself we can imagine at least something new may be established. It is not necessary to suppose that all changes will have a survival value, but only that some of them may. The alteration may bring the organism into a new relation with its environment, or through competition with the old type replace it, or it may make it possible for the new type to move into an environment different from that of the original type and hence escape competition/ Such a process may or may not lead to greater complexity, but would be an evolutionary change in any case. It should not ])e overlooked that only a limited number of living things are relatively complicated structures. An immense world of apparently simple organisms exists on the earth at the present time. Evolution has not meant the substitution of the simpler by the more complex; both exist side by side today, each standing in a different relation to the environment, but neither more capable of remaining in existence than the other.
The phrase natural selection, or its equivalent, the survival of the fittest, is generally understood to mean that a new type that appears, being better adapted to the same environment, displaces the old type by competition. One new "species" replaces its parent species. Something new has evolved, and by implication something "better," i.e., something with better chances of survival than the original species. While such replacement of an old type by a new one through competition may be one of the ways that new types evolve, it would be erroneous to suppose that Darwin limited the term in this wav. It would be unfortunate to identify selection with such an interpretation. Darwin by no means restricted the application of the term natural selection to the substitution for the parent type of a better adapted new type. Perhaps it is owing to the various ways in which he used natural selection — often as a metaphor — that it has come to have so many different meanings and is often confusedly used as synonymous with evolution.
1 The situation is essentially the same if the new type is fitted to establish a new relation with a different part of the same original environment — as when birds developed wings to take advantage of the air. Such a change may, it is true, lead through competition to a substitution of the new for the old type, but at other times it may also remove the new from competition with the old type. Birds for example have not replaced lizards.
It has been pointed out that the power to reproduce itself puts the problem of the construction of a living organism on a different footing from the construction of a complex machine out of inorganic (not living) material. This question is so important for the theory of evolution that its significance must be further indicated.
Whenever a variation in a new direction becomes established the chance of further advance in the same direction is increased. An increase in the number of individuals possessing a particular character has an influence on the f utiu'e course of evolution, — not because the new type is more likely to mutate again in the same direction, but because a mutation in the same direction has a better chance of producing a further advance since all individuals are now on a higher level than before. When, for example, elephants had trunks less than a foot long (fig. 63) the chance of getting trunks more than one foot long would be in proportion to the length of the trunks already present and to the number of individuals in which such a character might appear. In other words, evolution once begun in a given direction is in a favorable position to go on in the same direction rather than in another (fig. 64) , so long as the advance does not overstep the limit where further change is advantageous.
Fig. 63. Evolution of elephant's trunk; above Maeritherium, in the middle Tetrabelodon (After Lancaster) ; below African elephant (After Gambier Bolton).
Fig. 64. Evolution of elephant's trunk. (After Lull.)
The duality of the evolution process from the point of view of natural selection has not always been sufficiently emphasized. A series of events that can be given a strictly causal interpretation leads to the occurrence of a new individual, which, through other properties inherent in living matter, can reproduce a group of individuals like itself. Another and entirely unconnected series of events in the outer world has produced another situation as when the land was lifted above the water. If the new type happens to come into relation with the new world it may perpetuate itself there. This is adaptation — the fortuitous coming together of the results of two processes that have developed independently of each other. The fitness of the animal or plant to an environment that it finds existing, gives the false impression that its relation to the environment, its adaptation, has come about through a response to the environment. The central idea of natural selection, as generally understood at the present time, is that the relation is purely fortuitous. The organism has been produced by one series of events, the environment by another; the relation of the two is secondary.
The Dominance of the Wild Type Genes
The genes that arise by mutation have been found to be largely recessive to the genes already present in the original type which are said, therefore, to be dominant to the new genes. If the original genes also arose by mutation there is no obvious reason why new genes are not as often dominant as recessive to the original ones. It may be frankly admitted that at present we cannot give a satisfactory explanation of this relation if we assume that evolution has come about by the same kind of processes that we observe today when new mutants arise. There are, however, certain considerations that put the situation in a somewhat different light.
In the first place there is no such sharp contrast as implied in the statement just made between dominant and recessive genes. Many genes classified as recessive produce some effect in hybrid combination on the character most affected.
In the second place if recessive mutant genes may sometimes revert to the original type (for which there is some evidence at present but not enough perhaps to be entirely convincing) it follows that there may be no essential difference between the kinds of genes in question.
In the third place it is possible that some or even many of the commonly observed mutant genes represent degradation products of the old genes (that is, similer chemical bodies) that are more frequently produced than more com^ilex bodies. Even if this is true it does not follow that more complex genes may not also arise by mutation and some of these might be dominants to the old gene. At present, however, this is purely speculative.
In the fourth place it is known that new dominant genes do arise. There need be no necessary relation between the dominance of a gene and an increase in the character affected. In fact, while some dominant mutants add something to the original character (size or complexity), others diminish the same character.
In the fifth place it is possible that under natural conditions dominant advantageous characters have a far better chance to become established than recessive advantageous characters, because, by definition, they produce a greater or less effect on the hybrid and give it an advantage from the start. Tempting as is such a suggestion, it would be hazardous at present to use it to exj^lain the observed dominance of many of the characters of the wild types as compared with the recessiveness of many of the new mutant types that aj)pear or are preserved under cultivation.
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