Book - Evolution and Genetics 4
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Chapter 4 The Materials of Evolution
The apparent permanence of the types of animals and plants living at the present time is a common fact of observation. If this were the whole story it would appear that evolution had come to an end. If living things at the present time were really stable, we could give no good reason why it has not always been so in the past. On the other hand, if this stability is deceptive, we might expect to find evolution still taking place at the present time as in the past, and if this is true we might hope by a careful study of what is happening in living things about us to be able to get some information as to the way in which the process has taken place in the past. It is, of course, also conceivable that, even if evolution went on in the past, it has actually come to an end at the present time, or at least, having reached its climax, a declining process may be the order of the day — a process the reverse of that by which the upward trend of evolution went on in the past. It is also conceivable that the process of evolution is so slow that we may not be able to detect or measure it with means at our disposal. It may be true, furthermore, that certain species at least have become so far adjusted to the present conditions of the earth, that they are no longer advancing, and that only a few species are producing new or better types.
While it is well to keep these possibilities in mind, an appeal to the actual evidence furnishes no grounds for the belief that the process of evolution has come to an end. The conditions of land, water, and atmosphere have in all probability changed slowly since life first appeared, as they are changing today, and if in the past, evolution has progressed while the external world has been so slowly changing, it is a fair presumption that, to some extent at least, we may expect to find evolution taking place at the present time. It should also not be forgotten that the readjustment of animals and plants to each other may be as important a condition of evolution as their adjustments to the external world. From the latter point of view, the extraordinary complexity of the relation of living things to each other may even seem to furnish ample opportunities for further adjustments. It is certain, at least, that with the advent of man's interference with the natural conditions essential to other animals and plants, widespread changes may be expected to take place in part destructive but possibly also constructive. His domestication of several kinds of j)lants has produced changes in them that are very striking, but whether by the same kind of processes that take place in nature is a matter of dispute.
There is then, on the whole, a fair a priori expectation that evolution may still be taking place, and that it may not be so slow as to be beyond our powers of observation and even experimentation. The study of living things has, I think, confirmed this expectation despite the fact that there exist at present rather wide differences of opinion as to the legitimate conclusions from the evidence.
A great deal of the discussion about evolution has centered about the Origin of Species. Historically, the question as to what constitutes a species goes back at least as far as Linne (1707-1778) who classified all plants and animals known to him into such groups. The systematic arrangement of living things into species, genera, families, etc., still attracts the attention of a large number of naturalists.
Many difficulties arise Avhen an attempt is made to arrange animals into groups. It is generally recognized that, in some groups, species have a different value from that in others. While some taxonomists prefer to arrange individuals into large species, other systematists split these large species into several or many smaller ones, still calling them species. It is generally admitted that the classification into species is often an arbitrary procedure, one that is useful, of course, in order to indicate the resemblances and differences that a study of animals and plants reveals. A few students of the subject still attempt to arrange their species in such a way as to indicate their relationship by descent, but the attempt, while desirable from the standpoint of evolution, has proven so difficult that it has been tacitly abandoned or ignored by many taxonomists.
It is unfortunate, in my opinion, that ever since the time of Darwin the question of the origin of species has occupied so much of the forefront of speculation concerning the evolution of living things ; for if, as I have stated, "species" are to a large extent arbitrary arrangements of animals and plants — arrangements that may be essential for the purpose for which they are made, but entirely unsuited for evolutionary study — many unnecessary difficulties may arise in an attempt to explain their origin by natural processes, if some species are only groups of individuals arranged by taxonomists for convenience.
Nevertheless, there are certain important considerations connected with attempts to classify living things into species that cannot be dismissed by the foregoing treatment of the subject.
In the first place, experience has shown that most animals and plants do fall into groups that are more like each other than like other groups; and in the second place, that within each group the individuals freely cross and leave fertile offspring, while between most species crossing is rare or impossible, and when it occurs the offspring are generally sterile. When two types are infertile with each other they are generally admitted to be "good species" and even when they produce offspring, if the latter are sterile, the two types are recognized as separate species. It becomes necessary, therefore, to examine further into the significance of these relationships.
There are found in nature many interbreeding individuals that are sharply defined from other groups. It is convenient to have a name for all of the individuals of such a group, even when, as in the human species, the individuals living in the different parts of the world may present striking differences in structure, color, temperament, and social behavior. There are other types that are different from all others within a circumscribed region, but in other regions are represented by individuals that show slight but constant differences from the former. Here it becomes a more difficult matter to decide whether to call them all one species or whether to make two species. In practice it is generally agreed amongst taxonomists to give one specific name to such groups, if intermediate forms between the groups are found, but, if intermediate types are absent, the two separated groups are called different species. It is obvious, therefore, that the distinction between such species is largely arbitrary and artificial, especially since in the great majority of cases no tests of cross-breeding are made to determine whether the extreme types will cross and leave fertile offspring, or whether they are infertile or if fertile leave sterile offspring.
That the visible structural differences between groups of individuals is not a safe guide on which to construct a real distinction, is exemplified by numerous cases in which groups may exist side by side, that are so closely similar that only an expert can separate them, yet are either infertile when crossed or else leave offspring that are sterile. For example, two species of Drosophila, often found together, are so similar that complete separation is extraordinarily difficult. They can be made to cross only after many matings, but the offspring are completely sterile. With this information at hand no one would hesitate to call them different species, while without it the two would be placed in the same species.
It is true, in general, that the more "different" in structure two groups are, the smaller the chance that they will be found to cross, and still smaller the chance that they will have fertile offspring. This brings us back to the second question concerning the relation of infertility between species and the sterility of species hybrids. The infertility may appear at first sight to be due to the observed differences, but there mav be some other less obvious relation that is responsible for their failure to produce offspring. Darwin was familiar with this problem and has written about it extensively. He pointed out that there are no sharp lines with respect to infertility between species, and gave many examples, especially in plants, in which cross fertilization between individuals belonging to groups, unmistakably species according to ordinary standards, takes place. Darwin was also impressed by the fact that even self-sterility often occurs in hermaphroditic species, not only because self-fertilization is often made difficult in one way or another, but also because it does not actually occur even when the sperm has access to the egg, as has been shown by suitable tests.
Darwin was also familiar with the sterility of hybrids from species-crosses, and here again he emphasized the lack of any sharp distinction; for in some cases the sterility is complete, in others partial, and in still others it is not present. All this is in harmony with his conception of the gradual breaking vip of a species into new groups or varieties which as they become more and more different may show, when crossed, various degrees of fertility, and of sterility in their offspring.
Since Darwin, the subject has not advanced much further, although genetics has contributed a little more information and holds out promise of furnishing more. It has been shown in one or two hermaphroditic species that genetic factors are present that are concerned with self-sterilitv, and in a few other cases it has been shown that similar conditions are explicable on the assumption of more than one such factor. It has been found both hi plants (tobacco) and in one animal (Ciona) that the failure to self fertilize is not due to incompatibility of any sort between the egg and the sperm, but to a physiological block to the penetration of the sperm into the egg. It has also been shown in the case of several marine animals (sea urchins and fishes) that the eggs may be entered by spermatozoa of widely separated species — belonging even to different families — and start development. The failure to produce a normal embryo is due in some cases to the failure of the sperm to develop normally in the foreign egg; in other cases to the failure of the chromosomes derived from the two sources to become normally distributed in the cleavages of the egg, and in still other cases to the inability of the introduced chromosomes to function in the cytoplasm of the foreign egg.
All this is satisfactory and carries us a step further in an understanding of the problem of the infertility between species. We may add a further consideration in line with what genetics and embryology lead us to expect, namely, that the genetic factors present in the chromosomes of the fertilized egg derived in part from the egg, in part from the sperm, are acting on the cytoplasm throughout the process of development. So long as these pairs of factors are alike or identical (one maternally derived, one paternally) the course of development runs smoothly, but if one member of the pair acts in a different way from the other member it is easy to see why sooner or later the result is disastrous owing to a conflict of competing factors.
The egg's cytoplasm that has been formed under the dual influence of the maternal set of chromosomes appears to determine the early stages of development so that even if the sperm introduces factors that would act disastrously on these stages their influence does not at first show itself, but as development proceeds the influence of the paternal chromosomes comes more and more into play and further progress is arrested. This is, in fact, what is seen, in a way, when widely different species of echinoderms or of fish are crossed. The early stages of cleavage run smoothly and follow the maternal type, but as the embryo develops further, irregularities and delays occur that bring progress to an end.
Thus while some advance has been made in the study of infertility between species, the other problem, namely the sterility of the hybrids, when such are produced between species not too different, is not so well understood, but in some cases at least the immediate cause of the sterility has been found. It has been shown, in recent years, to result from changes that take place in the ripening of the germ cells. There comes a time when the pairs of chromosomes unite throughout their length and subsequently separate to go into sister cells. This is the so-called conjugation process. Now in certain hybrids, the mule for example, it has been shown that the maternal and paternal chromosomes fail to pass successfully through this ordeal with the result that later they are separated very irregularly. In consequence the germ-cells contain all kinds of assortments of the chromosomes and become abnormal. The result is that the individual is sterile.
While these observations do not explain why the chromosomes fail to unite, they do account for the sterility of the hybrid. Until we learn more concerning the conditions that bring about the union of the chromosomes, it may be unsafe to offer any explanation of the process ; nevertheless, for the present at least, it is not irrational to ascribe the failure to the differences in the hereditary factors carried by the chromosomes of the two species.
Now while all this will, I think, be conceded as theoretically possible, the fact remains that in no case has a mutant type been seen to arise that has produced individuals that are fertile inter se, but sterile with the parent species.^ Bateson has recently emphasized this point and has insisted that until it can be met we are not justified in assuming that new species are formed by mutation.
He says: "The production of an indubitably sterile hybrid from completely fertile parents which have arisen under critical observation from a common origin," — this "is the event for which we wait." Bateson has worded his requirements in such a way as to render the demonstration well-nigh impossible, but a somewhat different view, of the origin of species through mutation may put the problem in another form where theoretically at least the difficulty is lightened even if not entirely removed.
1 Plough has recently reported a case that comes very near to fulfilling these conditions.
Suppose on Bateson's supposition that a germ cell has been produced in a male or in a female that contains a single mutant gene having the required property of forming a sterile hybrid. In order to perpetuate itself, this germ-cell would have to meet a normal germ-cell. The single individual that resulted would by the very conditions imposed be itself sterile ; for, since there are no other individuals of the kind in existence there would be no means of finding out that the single individual would have been fertile with one of its kind. It would, therefore, be lost because of its sterilitv.
The possibilities are not much better even if we assume that the mutation occurred early in the germ-track, so that several or many germ-cells came to contain the mutant gene. When crossed back to the parent stock several sterile individuals would appear, which brings the experiment to a disastrous end as before. If on the other hand two such individuals should by chance mate then a new race miffht be started which tested to the original race would be found to produce offspring that are sterile. Here the conditions are fulfilled, but the chance of recording such a result at the time would be small indeed unless the sterile gene itself carried with it some other landmark, some new character, that would direct attention to it from the beginning. Something of the sort appears, in fact, to have happened in a stock of Drosophila studied by Plough where a new race appeared whose individuals are more fertile inter se than with the parent stock.
The conditions for producing a hybrid sterile race may appear more favorable in a monoecious form, as in a plant with stamens and pistils in the same flower. While a mutation in a single germ-cell would again not improve the situation, yet if mutation occurred early in the germ-track, and both pollen grains and ovules came to contain the gene, self fertilization would start a race that fulfils the requirements. These if inbred would then be found to be fertile infer se and produce fertile offspring, and with the parent type they would produce sterile hybrids. •
It is obvious, as I have said, that the chance of detecting such a mutant type would be veiy small unless its mutation involved some other character than the one under discussion so that it could be at once recorded.
The necessity of putting the Mutation Theory to the test that Bateson calls for, seems to me very doubtful, for while this is one of the possible ways in which a mutant type might spht off at once from the parent type it is by no means the only way, or even, I think, the most probable way in which species have become separated.
I venture also to question the importance ascribed to the sterility of the hybrid as a criterion of the origin of species. The test is arbitrary and not called for by the evidence at hand relating to sterile hybrids between species. There are no such sharp distinctions, as implied in the test, between groups found in nature that are called species.
The interpretation of the infertility between species and the sterility of hybrids that seems to me more probable is very different from that suggested by Bateson. Both phenomena, as I interpret them, are the result of many kinds of differences that have arisen in two species that have been separated for a long time. Each has taken on new characters due to mutational changes of different sorts. There is no one problem of infertility of species and no one problem of the sterility of hybrids, but many problems, each due to differences that have arisen in the germinal material. One or more of these differences may affect the mechanism of fertilization or the process of development, producing some incompatibility.
In order that a species may split up into one or more new species in the way suggested, isolation is implied. Isolation may be due to difference in locality, but it may also come about by individuals within the same area ripening their germ cells at different times, etc.
If species arise in this way we avoid the difficulty raised by Bateson concerning the origin of sterility of hybrids w^hose parents have arisen through mutation. Moreover the difficulty is not one peculiar to the mutation theory, but applies in large part to any theory that postulates the origin of one species from another.
If then we dismiss the problems that have grown out of the historical definitions of species, and turn directly to an examination of the origin of new variations we shall find that some progress has been made since Darwin wrote, and that this new knowledge supports Darwin's view that the variations shown by animals and plants furnish materials for a theory of evolution. There is evidence that new characters suddenly arise by mutation both in domesticated and in wild types, and that these variations are inherited in the same way as are the differences present in wild types that distinguish them from one another. In addition there are variations (fluctuations) due to the action of the environment on the developing individual. These are not inherited and cannot therefore take part in evolution. These statements will be discussed in later chapters after Mendel's laws, and some other laws of heredity discovered since his time, have been examined.
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