Book - Evolution and Genetics 10
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Chapter 10 Natural Selection and Evolution
Darwin's Theory of Natural Selection still holds today first place in every discussion of evolution, and for this reason the theory calls for careful scrutiny ; for it is not difficult to show that the expression "natural selection" is to many men a metaphor that carries many meanings, and sometimes different meanings to different men. While I heartily agree with my fellow biologists in ascribing to Darwin himself, and to his work, the first place in evolutionary philosophy, yet recognition of this claim should not deter us from a careful analysis of the situation in the light of all that has been done since Darwin's time.
The Theory of Natural Selection
In his famous book on the Origin of Species, Darwin tried to do two things: first, to show that the theory of evolution furnishes an adequate explanation of the facts. No such great body of evidence had ever been brought together before, and it convinced most thinking men that the theory of evolution of living things furnished a rational explanation of what is known about their relationships and past history.
Darwin also proposed several theories as to how evolution has taken place. He pointed to the influence of the environment, to the effects of use and disuse, and to natural selection. It is to the last theory that his name is especially attached. He appealed to a fact familiar to everyone, that no two individuals are identical and that some of the differences that they show are inherited. He argued that those individuals that are best suited to their environment are the most probable ones to survive and to leave offspring. As a consequence, their descendants should in time replace through competition the less well-adapted individuals of the species. This is the process Darwin called natural selection, and Spencer called the survival of the fittest.
Stated in these general terms there is nothing in the theory to which anyone is likely to take exception ; for, it may appear little more than a truism to state that the individuals that are the best adapted to survive have a better chance of surviving than those not so well adapted to survive. But Darwin did much more than appeal to any such generality. He pointed out that variations occur in all directions; that at least some of these variations are transmitted; and that on an average more offspring are produced by each pair than survive. He appealed directly to a large amount of biological evidence in support of his theory.
Since 1859 a great deal of work has been done that bears on the interpretation that Darwin placed on the facts to which he appealed. Before deciding on the merits of natural selection this evidence must he examined.
The Measurement of Variation
If we measure, or weigh, or classify any character shown by the individuals of a population, we find much variabilitv some of which we ascribe to the varied experiences that the individuals have encountered in the course of their lives, i.e., to their environment, hut we also recognize that some of the differences may he due to individuals having different inheritances. A few familiar examples will help to bring out this contrast.
Fig. 53. Series of leaves of a tree arranged according to size. (After DeVries.)
If the leaves of a tree are arranged according to size (fig. 53), we find a continuous series, but there are more leaves of medium size than extremes. If a lot of beans be sorted out according to their weights, and those between certain weights put into cylinders, the cylinders, when arranged according to the size of the beans, will appear as shown in figure 54. An imaginary line running over the tops of the piles will give a curve (fig, 55) that corresponds to the curve of probability (fig. 56).
Fig. 54. Beans put into cylinders according to size of beans. The cylinders are arranged according to the size of the contained beans. (After DeVries.)
If we stand men in lines according to their height we get a similar arrangement.
The differences in size shown by the individual beans or by the individual men are due in part to heredity, in part to the environment in which they have developed. This is a familiar fact of almost every-day observation. It is well shown in the following example. In figure 57 the two boys and the two varieties of corn, which they are holding, differ in height. The pedigrees of the boys (fig, 58) make it probable that their height is partly inherited, and the two races of corn are known to belong to a tall and a short race respectively. Here, then, the chief effect or difference is due to heredity. On the other hand, if individuals of the same race develop in a favorable environment the result is different from what their development would be in an unfavorable environment (fig. .59) . Here to the right the corn is crowded and in consequence dwarfed, while to the left the same kind of corn has had more room to develop and is taller.
Fig. 55. A curve resulting from arrangement of beans according to size. (After DeVries.)
Fig. 56. Curve of probability. (After Johannsen.)
Fig. 57. A short and a tall boy, each holding a stalk of corn. The short boy holds a stalk of a race of short corn, and the tall boy one of tall corn. (After Blakeslee.)
Fig, 58. Pedigrees of boys shown in fig. 57.
Fig. 59. Corn raised under diiferent conditions. That to the left is spaced, that to the right is crowded. (After Blakeslee.)
Darwin knew that if selection of particular kinds of individuals of a population takes place the next generation is affected. If the taller men of a community are selected, the average of their offspring will be taller than the average of the former population. If selection for tallness again takes place, still taller men will on the average arise. If selection again makes a choice, the process may continue (jig. 60) .
Fig. 60. Curves showing (hypothetically) how selection might be supposed to bring about progress in the direction of selection. (After Goldschmidt.)
Now while we recognize that this statement contains an imj^ortant truth, it has been found that it contains only a part of the truth. Any one who repeats for himself this kind of selection experiment will find that while the average class will often at first change in the direction of selection, the process slows down as a rule rather suddenly (fig. 61) . He finds, moreover, that the limits of variability are not necessarily transcended as the process continues even although the average may for a while be increased. More tall men may be produced by selection of this kind, but the tallest men are not necessarily any taller than the tallest in the original population.
Selection, then, has not produced anything new, but only more of certain kinds of individuals. Evolution, however, means producing new things, not more of what already exists.
Darwin's interpretation as to the effect of continued selection in the same direction may seem to imply that the range of variation shown by the offspring of a given individual about that type of individual would be as wide as the range shown by the original population, but Galton's work first made clear that this is not the case in a general or mixed population. If the offspring of individuals did continue to show as wide a range of variability about the new average as did the original population, then it would follow that selection could slide successive generations along in the direction of selection.
Fig. 61. Diagram ilustrating the results of selecting for extra bristles in Drosophila melanogaster. Selection at first produces rapid effects, which soon slow down and then cease. (After MacDowell.)
Darwin himself was extraordinarily careful, however, in the statements he made in this connection, and it is rather by implication than by actual reference that one can ascribe this meaning to his views. Some of his contemporaries and many of his followers, however, appear to have accepted this sliding scale interpretation as the cardinal doctrine of evolution. And in this connection we should not forget that just this sort of process was supposed to take place in the inheritance of use and disuse. What is gained in one generation forms the basis for further gains in the next generation. Now, Darwin not only believed that acquired characters are inherited but turned more and more to this explanation in his later writings. Let us, however, not make too much of the matter; for it is not so important to find out whether Darwin's ideas were as definite on this point as our own as it is to make sure that our own ideas are clear in the light of the more recent and extensive studies of variation that have been made since Darwin's time.
Selection and Variation
If, then, all that selection can do is to produce more individuals of a given type, it may appear that this part of Darwin's evidence fails to support his assumption that the observed variability of animals and plants suffices to furnish selection with its necessary material. It is here that the mutation they has a contribution to make. Since 1900 much evidence has been obtained showing that new variations may appear that transcend the extremes of variation of the original ty2)e. These, if they are inherited, are called mutants. In some cases the new type so far transcends the original type that the extreme fluctuations of the two do not overlap ; but in other cases the new type may be nearer to the original one and the fluctuations of each may overlap.
Darwin knew^ of cases of sudden mutation and called them sports or monstrosities. He thought that they could seldom supply materials for evolution because they changed a part so greatly as to throw the organism as a whole out of harmony with its environment. This argument for rejecting extreme or monstrous forms seems to us today as valid as it did to Darwin ; but we now recognize that sports are only extreme types of mutation, and that even the smallest changes that add to or subtract from a part in the smallest measurable degree may also arise by mutation. We identify these smaller mutational changes as the most probable variants that make a theory of evolution possible both because they do transcend the original types, and because they are inherited. If there are other kinds of heritable variations than mutants, it seems scarcely possible that they should have been overlooked; for, many thoroughgoing studies of variation have now been made.
The work of the Danish botanist, Johannsen, published in 1909, furnishes the most critical evidence relating to the inheritance of variations that has as vet been obtained. There are, moreover, special reasons why the material that he used is better suited to give definite information than any other so far studied.
Johannsen worked with a garden bean (Phaseolus vulgaris nana ) , weighing the seeds or else measuring them. The plant multiplies by self-fertilization. Taking advantage of this fact Johannsen kept the seeds of each plant separate from the others, and raised from them a new generation. When curves were made of these new groups it was found that some of them had different modes from that of the original general population (fig. 62, A-E, bottom group) . They are shown in the upper groups (A,B, C, Dj E) . The general population is a composite of all the groups.
That his conclusion is correct is shown by rearing a new generation from one plant or indeed from several plants of any one of these lines. Each line repeats the same modal class. There is no further breaking up into groups. Within the line it does not matter at all whether one chooses a big bean or a little one — they will give the same result. In a word, the germ-material in each of these lines is pure, or homozygous, as we say. The differences that are found between the weights (or sizes) of the individual beans are due to their location in the pod or in the plant on which they have developed.
Fig. 62. Pure lines of beans. The lower figure (A-E) gives the general population, the figures above give the pure lines within the population. (After Johannsen.)
Johannsen's work shows that the frequency distribution of a pure line is due to factors that are extrinsic to the germ-jjlasm. It does not matter then which individuals in a pure line are used to breed from, for they all carry the same germ-material.
We can now understand more clearly how selection acting on a general population brings about, at first, changes in the direction of selection.
An individual is picked out from the population in order to test its particular kind of germ-material. Although the different classes of individuals may overlap, so that one can not always judge an individual from its appearance, nevertheless, on the whole, chance favors the picking out of the kind of genn-material sought. In species with separate sexes there is the further difficulty that two individuals must be chosen for each mating, and superficial examination of them does not insure that they belong to the same group — their germ plasm cannot be inspected. Hence selection of bi2)arental forms is a precarious process, now going forward, now backwards, now standing still. In time, however, the process forward is almost certain to take place provided the selection is from a heterogeneous population. Johannsen's work was simplified because he started with pure lines. In fact were this not the case his work would not have been essentially different from that of any other selection experiment.
It has since been pointed out by Jennings and by Pearl that a race that reproduces by self-fertilization, as does this bean, automatically becomes pure in all of the factors that make up its germ-material. Since self-fertilization is the normal process in this bean the purity of the germ-plasm of each line already existed when Johannsen began to experiment.
In addition to the variability due to external factors acting on the individual during its development there are also differences in the germ-materials — genetic factors — that are known to produce slight differences in the extent to which some particular part or character develops. Inasmuch as some of these minor or modifying genetic factors produce their results only in the presence of the chief character, they may be concealed and only manifest their presence when the chief character develops. The discovery of the occurrence of such modifying genetic factors has gone a long way in making clear some of the effects of selection — effects that have at times led some of the neo-Darwinians to assume that the selection process could bring about a change that causes the organisms to transcend its original type. Castle stated in 1916: "JNIany students of genetics at present regard unit characters as unchangeable. . . . For several years I have been investigating this question, and the general conclusion at which I have arrived is this, that unit characters are modifiable as well as recombinable. JNIany Mendelians think otherwise but this is, I believe, because they have not studied the question closely enough. The fact is unmistakable that imit characters are subject to quantitative variation." That Castle was not carelessly playing fast and loose with the term factor (gene) and character is shown by the whole context of the entire chapter in which tliis sentence occurs. It is intended to be understood to mean that unit characters may not only be altered by the recombination of modifying characters, for, Castle has always recognized this possibility, but also that the gene (factor) varies quantitatively and that selection not only produces its results by selecting larger or smaller genes but in doing so it brings about progressive and further advances in the direction of selection. This interpretation is attributed by Castle to Darwin himself, as another quotation from the same chapter shows: "Selection as an agency in evolution must then be restored to the important place which it held in Darwin's estimation, an agency capable of producing continuous and progressive racial changes."
Now the only really critical piece of work done on this subject, that of Johannsen, had already led to the opposite result. It can hardly be said, therefore, that the subject had not been studied closely enough. The evidence from Castle's own experiments with hooded rats when studied more criticallv has shown that it still remains to be proven that genes are subject to quantitative variations and are amenable to selection.
The evidence discussed in this chapter is consistent with the view that the individual gene is not affected by selection, and that the initial changes commonly observed when selection is practised on a mixed population are due to recombinations of the different kinds of genes affecting the same character that are present in most populations. Since these modifying genes behave in inheritance strictly in accordance with ^Mendel's laws there are no grounds for assuming that they are different from other genes. Selection ceases to produce any further effects after these genes have been sorted out and the material has become homozygous for them.
It follows, that if new characters transcending the extremes of the original population arise, this must come about through a change in one or more of the genes themselves. At present we have discovered only one way in which such a change takes place — by a mutation in a gene.
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