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Transcriber's note: A few typographical errors have been corrected: they are listed at the end of the text.
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Princeton University
THE LOUIS CLARK VANUXEM FOUNDATION LECTURES FOR 1915-1916
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The Louis Clark Vanuxem Foundation of Princeton University
was established in 1912 with a bequest of $25,000 under the will of Louis Clark Vanuxem, of the Class of 1879. By direction of the executors of Mr. Vanuxem's estate, the income of the foundation is to be used for a series of public lectures delivered in Princeton annually, at least one half of which shall be on subjects of current scientific interest. The lectures are to be published and distributed among schools and libraries generally.
The following lectures have already been published or are in press:
1912-13 The Theory of Permutable Functions, by Vito Volterra
1913-14 Lectures delivered in connection with the dedication of the Graduate College of Princeton University by Emile Boutroux, Alois Riehl, A. D. Godley, and Arthur Shipley
1914-15 Romance, by Sir Walter Raleigh
1915-16 A Critique of the Theory of Evolution, by Thomas Hunt Morgan
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LOUIS CLARK VANUXEM FOUNDATION
A CRITIQUE
OF THE
THEORY OF EVOLUTION
BY
THOMAS HUNT MORGAN
PROFESSOR OF EXPERIMENTAL ZOOLOGY IN COLUMBIA UNIVERSITY
LECTURES DELIVERED AT PRINCETON UNIVERSITY FEBRUARY 24, MARCH 1, 8, 15, 1916
PRINCETON UNIVERSITY PRESS PRINCETON LONDON: HUMPHREY MILFORD OXFORD UNIVERSITY PRESS 1916
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Copyright, 1916, by PRINCETON UNIVERSITY PRESS Published October, 1916
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PREFACE
Occasionally one hears today the statement that we have come to realize that we know nothing about evolution. This point of view is a healthy reaction to the over-confident belief that we knew everything about evolution. There are even those rash enough to think that in the last few years we have learned more about evolution than we might have hoped to know a few years ago. A critique therefore not only becomes a criticism of the older evidence but an appreciation of the new evidence.
In the first lecture an attempt is made to put a new valuation on the traditional evidence for evolution. In the second lecture the most recent work on heredity is dealt with, for only characters that are inherited can become a part of the evolutionary process. In the third lecture the physical basis of heredity and the composition of the germ plasm stream are examined in the light of new observations; while in the fourth lecture the thesis is developed that chance variation combined with a property of living things to manifold themselves is the key note of modern evolutionary thought.
T. H. MORGAN
July, 1916
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TABLE OF CONTENTS
CHAPTER I A REVALUATION OF THE EVIDENCE ON WHICH THE THEORY OF EVOLUTION WAS BASED
PAGE PREFACE v
1. THREE KINDS OF EVOLUTION 1-7
2. THE EVIDENCE FOR ORGANIC EVOLUTION 7-27 a. The Evidence from Comparative Anatomy 7-14 b. The Evidence from Embryology 14-23 c. The Evidence from Paleontology 24-27
3. THE FOUR GREAT HISTORICAL SPECULATIONS 27-39 a. The Environment 27-31 Geoffroy St. Hilaire b. Use and Disuse 31-34 From Lamarck to Weismann c. The Unfolding Principle 34-36 Naegeli and Bateson d. Natural Selection 36-39 Darwin
CHAPTER II THE BEARING OF MENDEL'S DISCOVERY ON THE ORIGIN OF HEREDITY CHARACTERS
1. Mendel's First Discovery—Segregation 41-52
2. Mendel's Second Discovery—Independent Assortment 52-59
3. The Characters of Wild Animals and Plants Follow the Same Laws of Inheritance as do the Characters of Domesticated Animals and Plants 59-84 a. Sexual Dimorphism 61-64 Eosin eye color of Drosophila 61-62 Color of the Clover Butterfly, Colias philodice 62-63 Color of Papilio turnus 63 Color pattern of Papilio polytes 63-64 b. Duplication of parts 65-66 Thorax of Drosophila 65 Legs of Drosophila 65-66 c. Loss of characters 66-68 "Eyeless" of Drosophila 66-67 Vestigial wings of Drosophila 67 Bar eye of Drosophila 67-68 d. Small changes of characters 68-70 "Speck" 68 Bristles of "club" 70 e. Manifold effects of same factor 71 f. Constant but trivial effects may be the product of factors having other vital aspect 73 g. Sex-linked inheritance 75-80 in Drosophila ampelophila 75-76 in the wild species D. repleta 76 in man 77 in domesticated Fowls 77-78 in the wild moth, Abraxas 78-80 h. Multiple allelomorphs 81-84 in the wild Grouse Locust 81-83 in domesticated mice and rabbits 83 in Drosophila ampelophila 84
4. MUTATION AND EVOLUTION 84-88
CHAPTER III THE FACTORIAL THEORY OF HEREDITY AND THE COMPOSITION OF THE GERM PLASM
1. THE CELLULAR BASIS OF ORGANIC EVOLUTION AND HEREDITY 89-98
2. THE MECHANISM OF MENDELIAN HEREDITY DISCOVERED IN THE BEHAVIOR OF THE CHROMOSOMES 98-102
3. THE FOUR GREAT LINKAGE GROUPS OF DROSOPHILA AMPELOPHILA 103-118 a. Group I. 104-109 b. Group II. 109-112 c. Group III. 112-115 d. Group IV. 115-118
4. LOCALIZATION OF FACTORS IN THE CHROMOSOMES 118-142 a. The Evidence from Sex Linked Inheritance 118-137 b. The Evidence from Interference 137-138 c. The Evidence from Non-Disjunction 139-142
5. HOW MANY GENETIC FACTORS ARE THERE IN THE GERM-PLASM OF A SINGLE INDIVIDUAL? 142-143
6. CONCLUSIONS 144
CHAPTER IV SELECTION AND EVOLUTION
1. THE THEORY OF NATURAL SELECTION 145-161
2. HOW HAS SELECTION IN DOMESTICATED ANIMALS AND PLANTS BROUGHT ABOUT ITS RESULTS? 161-165
3. ARE FACTORS CHANGED THROUGH SELECTION? 165-187
4. HOW DOES NATURAL SELECTION INFLUENCE THE COURSE OF EVOLUTION? 187-193
5. CONCLUSIONS 193-194
INDEX 195-197
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CHAPTER I
A REVALUATION OF THE EVIDENCE ON WHICH THE THEORY OF EVOLUTION WAS BASED
We use the word evolution in many ways—to include many different kinds of changes. There is hardly any other scientific term that is used so carelessly—to imply so much, to mean so little.
THREE KINDS OF EVOLUTION
We speak of the evolution of the stars, of the evolution of the horse, of the evolution of the steam engine, as though they were all part of the same process. What have they in common? Only this, that each concerns itself with the history of something. When the astronomer thinks of the evolution of the earth, the moon, the sun and the stars, he has a picture of diffuse matter that has slowly condensed. With condensation came heat; with heat, action and reaction within the mass until the chemical substances that we know today were produced. This is the nebular hypothesis of the astronomer. The astronomer explains, or tries to explain, how this evolution took place, by an appeal to the physical processes that have been worked out in the laboratory, processes which he thinks have existed through all the eons during which this evolution was going on and which were its immediate causes.
When the biologist thinks of the evolution of animals and plants, a different picture presents itself. He thinks of series of animals that have lived in the past, whose bones (fig. 1) and shells have been preserved in the rocks. He thinks of these animals as having in the past given birth, through an unbroken succession of individuals, to the living inhabitants of the earth today. He thinks that the old, simpler types of the past have in part changed over into the more complex forms of today.
He is thinking as the historian thinks, but he sometimes gets confused and thinks that he is explaining evolution when he is only describing it.
A third kind of evolution is one for which man himself is responsible, in the sense that he has brought it about, often with a definite end in view.
His mind has worked slowly from stage to stage. We can often trace the history of the stages through which his psychic processes have passed. The evolution of the steam-boat, the steam engine, paintings, clothing, instruments of agriculture, of manufacture, or of warfare (fig. 2) illustrates the history of human progress. There is an obvious and striking similarity between the evolution of man's inventions and the evolution of the shells of molluscs and of the bones of mammals, yet in neither case does a knowledge of the order in which these things arose explain them. If we appeal to the psychologist he will probably tell us that human inventions are either the result of happy accidents, that have led to an unforeseen, but discovered use; or else the use of the invention was foreseen. It is to the latter process more especially that the idea of purpose is applied. When we come to review the four great lines of evolutionary thought we shall see that this human idea of purpose recurs in many forms, suggesting that man has often tried to explain how organic evolution has taken place by an appeal to the method which he believes he makes use of himself in the inorganic world.
What has the evolution of the stars, of the horse and of human inventions in common? Only this, that in each case from a simple beginning through a series of changes something more complex, or at least different, has come into being. To lump all these kinds of changes into one and call them evolution is no more than asserting that you believe in consecutive series of events (which is history) causally connected (which is science); that is, that you believe in history and that you believe in science. But let us not forget that we may have complete faith in both without thereby offering any explanation of either. It is the business of science to find out specifically what kinds of events were involved when the stars evolved in the sky, when the horse evolved on the earth, and the steam engine was evolved from the mind of man.
Is it not rather an empty generalization to say that any kind of change is a process of evolution? At most it means little more than that you want to intimate that miraculous intervention is not necessary to account for such kinds of histories.
We are concerned here more particularly with the biologists' ideas of evolution. My intention is to review the evidence on which the old theory rested its case, in the light of some of the newer evidence of recent years.
Four great branches of study have furnished the evidence of organic evolution. They are:
Comparative anatomy. Embryology. Paleontology. Experimental Breeding or Genetics.
The Evidence from Comparative Anatomy
When we study animals and plants we find that they can be arranged in groups according to their resemblances. This is the basis of comparative anatomy, which is only an accurate study of facts that are superficially obvious to everyone.
The groups are based not on a single difference, but on a very large number of resemblances. Let us take for example the group of vertebrates.
The hand and the arm of man are similar to the hand and arm of the ape. We find the same plan in the forefoot of the rat, the elephant, the horse and the opossum. We can identify the same parts in the forefoot of the lizard, the frog (fig. 3), and even, though less certainly, in the pectoral fins of fishes. Comparison does not end here. We find similarities in the skull and back bones of these same animals; in the brain; in the digestive system; in the heart and blood vessels; in the muscles.
Each of these systems is very complex, but the same general arrangement is found in all. Anyone familiar with the evidence will, I think, probably reach the conclusion either that these animals have been created on some preconceived plan, or else that they have some other bond that unites them; for we find it difficult to believe that such complex, yet similar things could have arisen independently. But we try to convince our students of the truth of the theory of evolution not so much by calling their attention to this relation as by tracing each organ from a simple to a complex structure.
I have never known such a course to fail in its intention. In fact, I know that the student often becomes so thoroughly convinced that he resents any such attempt as that which I am about to make to point out that the evidence for his conviction is not above criticism.
Because we can often arrange the series of structures in a line extending from the very simple to the more complex, we are apt to become unduly impressed by this fact and conclude that if we found the complete series we should find all the intermediate steps and that they have arisen in the order of their complexity. This conclusion is not necessarily correct. Let me give some examples that have come under my own observation. We have bred for five years the wild fruit fly Drosophila ampelophila (fig. 4) and we have found over a hundred and twenty-five new types that breed true. Each has arisen independently and suddenly. Every part of the body has been affected by one or another of these mutations. For instance many different kinds of changes have taken place in the wings and several of these involve the size of the wings. If we arrange the latter arbitrarily in the order of their size there will be an almost complete series beginning with the normal wings and ending with those of apterous flies. Several of these types are represented in figure 5. The order in which these mutations occurred bears no relation to their size; each originated independently from the wild type.
The wings of the wild fly are straight (fig. 4). Several types have arisen in which the wings are bent upwards and in the most extreme type the wings are curled over the back, as seen in figure 54 (g), yet there is no historical connection between these stages.
Mutations have occurred involving the pigmentation of the body and wings. The head and thorax of the wild Drosophila ampelophila are grayish yellow, the abdomen is banded with yellow and black, and the wings are gray. There have appeared in our cultures several kinds of darker types ranging to almost black flies (fig. 20) and to lighter types that are quite yellow. If put in line a series may be made from the darkest flies at one end to the light yellow flies at the other. These types, with the fluctuations that occur within each type, furnish a complete series of gradations; yet historically they have arisen independently of each other.
Many changes in eye color have appeared. As many as thirty or more races differing in eye color are now maintained in our cultures. Some of them are so similar that they can scarcely be separated from each other. It is easily possible beginning with the darkest eye color, sepia, which is deep brown, to pick out a perfectly graded series ending with pure white eyes. But such a serial arrangement would give a totally false idea of the way the different types have arisen; and any conclusion based on the existence of such a series might very well be entirely erroneous, for the fact that such a series exists bears no relation to the order in which its members have appeared.
Suppose that evolution "in the open" had taken place in the same way, by means of discontinuous variation. What value then would the evidence from comparative anatomy have in so far as it is based on a continuous series of variants of any organ?
No one familiar with the entire evidence will doubt for a moment that these 125 races of Drosophila ampelophila belong to the same species and have had a common origin, for while they may differ mainly in one thing they are extremely alike in a hundred other things, and in the general relation of the parts to each other.
It is in this sense that the evidence from comparative anatomy can be used I think as an argument for evolution. It is the resemblances that the animals or plants in any group have in common that is the basis for such a conclusion; it is not because we can arrange in a continuous series any particular variations. In other words, our inference concerning the common descent of two or more species is based on the totality of such resemblances that still remain in large part after each change has taken place. In this sense the argument from comparative anatomy, while not a demonstration, carries with it, I think, a high degree of probability.
The Evidence from Embryology
In passing from the egg to the adult the individual goes through a series of changes. In the course of this development we see not only the beginnings of the organs that gradually enlarge and change into those of the adult animal, but also see that organs appear and later disappear before the adult stage is reached. We find, moreover, that the young sometimes resemble in a most striking way the adult stage of groups that we place lower in the scale of evolution.
Many years before Darwin advanced his theory of evolution through natural selection, the resemblance of the young of higher animals to the adults of lower animals had attracted the attention of zoologists and various views, often very naive, had been advanced to account for the resemblance. Among these speculations there was one practically identical with that adopted by Darwin and the post-Darwinians, namely that the higher animals repeat in their development the adult stages of lower animals. Later this view became one of the cornerstones of the theory of organic evolution. It reached its climax in the writings of Haeckel, and I think I may add without exaggeration that for twenty-five years it furnished the chief inspiration of the school of descriptive embryology. Today it is taught in practically all textbooks of biology. Haeckel called this interpretation the Biogenetic Law.
It was recognized, of course, that many embryonic stages could not possibly represent ancestral animals. A young fish with a huge yolk sac attached (fig. 6) could scarcely ever have led a happy, free life as an adult individual. Such stages were interpreted, however, as embryonic additions to the original ancestral type. The embryo had done something on its own account.
In some animals the young have structures that attach them to the mother, as does the placenta of the mammals. In other cases the young develop membranes about themselves—like the amnion of the chick (fig. 7) and mammal—that would have shut off an adult animal from all intercourse with the outside world. Hundreds of such embryonic adaptations are known to embryologists. These were explained as adaptations and as falsifications of the ancestral records.
At the end of the last century Weismann injected a new idea into our views concerning the origin of variations. He urged that variations are germinal, i.e. they first appear in the egg and the sperm as changes that later bring about modifications in the individual. The idea has been fruitful and is generally accepted by most biologists today. It means that the offspring of a pair of animals are not affected by the structure or the activities of their parents, but the germ plasm is the unmodified stream from which both the parent and the young have arisen. Hence their resemblance. Now, it has been found that a variation arising in the germ plasm, no matter what its cause, may affect any stage in the development of the next individuals that arise from it. There is no reason to suppose that such a change produces a new character that always sticks itself, as it were, on to the end of the old series. This idea of germinal variation therefore carried with it the death of the older conception of evolution by superposition.
In more recent times another idea has become current, mainly due to the work of Bateson and of de Vries—the idea that variations are discontinuous. Such a conception does not fall easily into line with the statement of the biogenetic "law"; for actual experience with discontinuous variation has taught us that new characters that arise do not add themselves to the end of the line of already existing characters but if they affect the adult characters they change them without, as it were, passing through and beyond them.
I venture to think that these new ideas and this new evidence have played havoc with the biogenetic "law". Nevertheless, there is an interpretation of the facts that is entirely compatible with the theory of evolution. Let me illustrate this by an example.
The embryos of the chick (fig. 8) and of man (fig. 9) possess at an early stage in their development gill-slits on the sides of the neck like those of fishes. No one familiar with the relations of the parts will for a moment doubt that the gill slits of these embryos and of the fish represent the same structures. When we look further into the matter we find that young fish also possess gill slits (fig. 10 and 11)—even in young stages in their development. Is it not then more probable that the mammal and bird possess this stage in their development simply because it has never been lost? Is not this a more reasonable view than to suppose that the gill slits of the embryos of the higher forms represent the adult gill slits of the fish that in some mysterious way have been pushed back into the embryo of the bird?
I could give many similar examples. All can be interpreted as embryonic survivals rather than as phyletic contractions. Not one of them calls for the latter interpretation.
The study of the cleavage pattern of the segmenting egg furnishes the most convincing evidence that a different explanation from the one stated in the biogenetic law is the more probable explanation.
It has been found that the cleavage pattern has the same general arrangement in the early stages of flat worms, annelids and molluscs (fig. 12). Obviously these stages have never been adult ancestors, and obviously if their resemblance has any meaning at all, it is that each group has retained the same general plan of cleavage, possessed by their common ancestor.
Accepting this view, let us ask, does the evidence from embryology favor the theory of evolution? I think that it does very strongly. The embryos of the mammal, bird, and lizard have gill slits today because gill slits were present in the embryos of their ancestors. There is no other view that explains so well their presence in the higher forms.
Perhaps someone will say, Well! is not this all that we have contended for! Have you not reached the old conclusion in a roundabout way? I think not. To my mind there is a wide difference between the old statement that the higher animals living today have the original adult stages telescoped into their embryos, and the statement that the resemblance between certain characters in the embryos of higher animals and corresponding stages in the embryos of lower animals is most plausibly explained by the assumption that they have descended from the same ancestors, and that their common structures are embryonic survivals.
The Evidence from Paleontology
The direct evidence furnished by fossil remains is by all odds the strongest evidence that we have in favor of organic evolution. Paleontology holds the incomparable position of being able to point directly to the evidence showing that the animals and plants living in past times are connected with those living at the present time, often through an unbroken series of stages. Paleontology has triumphed over the weakness of the evidence, which Darwin admitted was serious, by filling in many of the missing links.
Paleontology has been criticised on the ground that she cannot pretend to show the actual ancestors of living forms because, if in the past genera and species were as abundant and as diverse as we find them at present, it is very improbable that the bones of any individual that happened to be preserved are the bones of just that species that took part in the evolution. Paleontologists will freely admit that in many cases this is probably true, but even then the evidence is, I think, still just as valuable and in exactly the same sense as is the evidence from comparative anatomy. It suffices to know that there lived in the past a particular "group" of animals that had many points in common with those that preceded them and with those that came later. Whether these are the actual ancestors or not does not so much matter, for the view that from such a group of species the later species have been derived is far more probable than any other view that has been proposed.
With this unrivalled material and splendid series of gradations, paleontology has constructed many stages in the past history of the globe. But paleontologists have sometimes gone beyond this descriptive phase of the subject and have attempted to formulate the "causes", "laws" and "principles" that have led to the development of their series. It has even been claimed that paleontologists are in an incomparably better position than zoologists to discover such principles, because they know both the beginning and the end of the evolutionary series. The retort is obvious. In his sweeping and poetic vision the paleontologist may fail completely to find out the nature of the pigments that have gone into the painting of his picture, and he may confuse a familiarity with the different views he has enjoyed of the canvas with a knowledge of how the painting is being done.
My good friend the paleontologist is in greater danger than he realizes, when he leaves descriptions and attempts explanation. He has no way to check up his speculations and it is notorious that the human mind without control has a bad habit of wandering.
When the modern student of variation and heredity—the geneticist—looks over the different "continuous" series, from which certain "laws" and "principles" have been deduced, he is struck by two facts: that the gaps, in some cases, are enormous as compared with the single changes with which he is familiar, and (what is more important) that they involve numerous parts in many ways. The geneticist says to the paleontologist, since you do not know, and from the nature of your case can never know, whether your differences are due to one change or to a thousand, you can not with certainty tell us anything about the hereditary units which have made the process of evolution possible. And without this knowledge there can be no understanding of the causes of evolution.
THE FOUR GREAT HISTORICAL SPECULATIONS
Looking backward over the history of the evolution theory we recognize that during the hundred and odd years that have elapsed since Buffon, there have been four main lines of speculation concerning evolution. We might call them the four great cosmogonies or the four modern epics of evolution.
THE ENVIRONMENT
Geoffroy St. Hilaire
About the beginning of the last century Geoffroy St. Hilaire, protege, and in some respects a disciple of Buffon, was interested as to how living species are related to the animals and plants that had preceded them. He was familiar with the kind of change that takes place in the embryo if it is put into new or changed surroundings, and from this knowledge he concluded that as the surface of the earth slowly changed—as the carbon dioxide contents in the air altered—as land appeared—and as marine animals left the water to inhabit it, they or their embryos responded to the new conditions and those that responded favorably gave rise to new creations. As the environment changed the fauna and flora changed—change for change. Here we have a picture of progressive evolution that carries with it an idea of mechanical necessity. If there is anything mystical or even improbable in St. Hilaire's argument it does not appear on the surface; for he did not assume that the response to the new environment was always a favorable one or, as we say, an adaptation. He expressly stated that if the response was unfavorable the individual or the race died out. He assumed that sometimes the change might be favorable, i.e., that certain species, entire groups, would respond in a direction favorable to their existence in a new environment and these would come to inherit the earth. In this sense he anticipated certain phases of the natural selection theory of Darwin, but only in part; for his picture is not one of strife within and without the species, but rather the escape of the species from the old into a new world.
If then we recognize the intimate bond in chemical constitution of living things and of the world in which they develop, what is there improbable in St. Hilaire's hypothesis? Why, in a word is not more credit given to St. Hilaire in modern evolutionary thought? The reasons are to be found, I think, first, in that the evidence to which he appealed was meagre and inconclusive; and, second, in that much of his special evidence does not seem to us to be applicable. For example the monstrous forms that development often assumes in a strange environment, and with which every embryologist is only too familiar, rarely if ever furnish combinations, as he supposed, that are capable of living. On the contrary, they lead rather to the final catastrophe of the organism. And lastly, St. Hilaire's appeal to sudden and great transformations, such as a crocodile's egg hatching into a bird, has exposed his view to too easy ridicule.
But when all is said, St. Hilaire's conception of evolution contains elements that form the background of our thinking to-day, for taken broadly, the interaction between the organism and its environment was a mechanistic conception of evolution even though the details of the theory were inadequate to establish his contention.
In our own time the French metaphysician Bergson in his Evolution Creatrice has proposed in mystical form a thought that has at least a superficial resemblance to St. Hilaire's conception. The response of living things is no longer hit in one species and miss in another; it is precise, exact; yet not mechanical in the sense at least in which we usually employ the word mechanical. For Bergson claims that the one chief feature of living material is that it responds favorably to the situation in which it finds itself; at least so far as lies within the possible physical limitations of its organization. Evolution has followed no preordained plan; it has had no creator; it has brought about its own creation by responding adaptively to each situation as it arose.
But note: the man of science believes that the organism responds today as it does, because at present it has a chemical and physical constitution that gives this response. We find a specific chemical composition and generally a specific physical structure already existing. We have no reason to suppose that such particular reactions would take place until a specific chemical configuration had been acquired. Where did this constitution come from? This is the question that the scientist asks himself. I suppose Bergson would have to reply that it came into existence at the moment that the first specific stimulus was applied. But if this is the answer we have passed at once from the realm of observation to the realm of fancy—to a realm that is foreign to our experience; for such a view assumes that chemical and physical reactions are guided by the needs of the organism when the reactions take place inside living beings.
USE AND DISUSE
From Lamarck to Weismann
The second of the four great historical explanations appeals to a change not immediately connected with the outer world, but to one within the organism itself.
Practice makes perfect is a familiar adage. Not only in human affairs do we find that a part through use becomes a better tool for performing its task, and through disuse degenerates; but in the field of animal behavior we find that many of the most essential types of behavior have been learned through repeated associations formed by contact with the outside.
It was not so long ago that we were taught that the instincts of animals are the inherited experience of their ancestors—lapsed intelligence was the current phrase.
Lamarck's name is always associated with the application of the theory of the inheritance of acquired characters. Darwin fully endorsed this view and made use of it as an explanation in all of his writings about animals. Today the theory has few followers amongst trained investigators, but it still has a popular vogue that is widespread and vociferous.
To Weismann more than to any other single individual should be ascribed the disfavor into which this view has fallen. In a series of brilliant essays he laid bare the inadequacy of the supposed evidence on which the inheritance of acquired characters rested. Your neighbor's cat, for instance, has a short tail, and it is said that it had its tail pinched off by a closing door. In its litter of kittens one or more is found without a tail. Your neighbor believes that here is a case of cause and effect. He may even have known that the mother and grandmother of the cat had natural tails. But it has been found that short tail is a dominant character; therefore, until we know who was the father of the short-tailed kittens the accident to its mother and the normal condition of her maternal ancestry is not to the point.
Weismann appealed to common sense. He made few experiments to disprove Lamarck's hypothesis. True, he cut off the tails of some mice for a few generations but got no tailless offspring and while he gives no exact measurements with coefficients of error he did not observe that the tails of the descendants had shortened one whit. The combs of fighting cocks and the tails of certain breeds of sheep have been cropped for many generations and the practice continues today, because their tails are still long. While in Lamarck's time there was no evidence opposed to his ingenious theory, based as it was on an appeal to the acknowledged facts of improvement that take place in the organs of an individual through their own functioning (a fact that is as obvious and remarkable today as in the time of Lamarck), yet now there is evidence as to whether the effects of use and disuse are inherited, and this evidence is not in accord with Lamarck's doctrine.
THE UNFOLDING PRINCIPLE
Naegeli and Bateson
I have ventured to put down as one of the four great historical explanations, under the heading of the unfolding principle, a conception that has taken protean forms. At one extreme it is little more than a mystic sentiment to the effect that evolution is the result of an inner driving force or principle which goes under many names such as Bildungstrieb, nisus formativus, vital force, and orthogenesis. Evolutionary thought is replete with variants of this idea, often naively expressed, sometimes unconsciously implied. Evolution once meant, in fact, an unfolding of what pre-existed in the egg, and the term still carries with it something of its original significance.
Naegeli's speculation written several years after Darwin's "Origin of Species" may be taken as a typical case. Naegeli thought that there exists in living material an innate power to grow and expand. He vehemently protested that he meant only a mechanical principle but as he failed to refer such a principle to any properties of matter known to physicists and chemists his view seems still a mysterious affirmation, as difficult to understand as the facts themselves which it purports to explain.
Naegeli compared the process of evolution to the growth of a tree, whose ultimate twigs represent the living world of species. Natural selection plays only the role of the gardener who prunes the tree into this or that shape but who has himself produced nothing. As an imaginative figure of speech Naegeli's comparison of the tree might even today seem to hold if we substituted "mutations" for "growth", but although we know so little about what causes mutations there is no reason for supposing them to be due to an inner impulse, and hence they furnish no justification for such a hypothesis.
In his recent presidential address before the British Association Bateson has inverted this idea. I suspect that his effort was intended as little more than a tour de force. He claims for it no more than that it is a possible line of speculation. Perhaps he thought the time had come to give a shock to our too confident views concerning evolution. Be this as it may, he has invented a striking paradox. Evolution has taken place through the steady loss of inhibiting factors. Living matter was stopped down, so to speak, at the beginning of the world. As the stops are lost, new things emerge. Living matter has changed only in that it has become simpler.
NATURAL SELECTION
Darwin
Of the four great historical speculations about evolution, the doctrine of Natural Selection of Darwin and Wallace has met with the most widespread acceptance. In the last lecture I intend to examine this theory critically. Here we are concerned only with its broadest aspects.
Darwin appealed to chance variations as supplying evolution with the material on which natural selection works. If we accept, for the moment, this statement as the cardinal doctrine of natural selection it may appear that evolution is due, (1) not to an orderly response of the organism to its environment, (2) not in the main to the activities of the animal through the use or disuse of its parts, (3) not to any innate principle of living material itself, and (4) above all not to purpose either from within or from without. Darwin made quite clear what he meant by chance. By chance he did not mean that the variations were not causal. On the contrary he taught that in Science we mean by chance only that the particular combination of causes that bring about a variation are not known. They are accidents, it is true, but they are causal accidents.
In his famous book on "Animals and Plants under Domestication", Darwin dwells at great length on the nature of the conditions that bring about variations. If his views seem to us today at times vague, at times problematical, and often without a secure basis, nevertheless we find in every instance, that Darwin was searching for the physical causes of variation. He brought, in consequence, conviction to many minds that there are abundant indications, even if certain proof is lacking, that the causes of variation are to be found in natural processes.
Today the belief that evolution takes place by means of natural processes is generally accepted. It does not seem probable that we shall ever again have to renew the old contest between evolution and special creation.
But this is not enough. We can never remain satisfied with a negative conclusion of this kind. We must find out what natural causes bring about variations in animals and plants; and we must also find out what kinds of variations are inherited, and how they are inherited. If the circumstantial evidence for organic evolution, furnished by comparative anatomy, embryology and paleontology is cogent, we should be able to observe evolution going on at the present time, i.e. we should be able to observe the occurrence of variations and their transmission. This has actually been done by the geneticist in the study of mutations and Mendelian heredity, as the succeeding lectures will show.
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CHAPTER II
THE BEARING OF MENDEL'S DISCOVERY ON THE ORIGIN OF HEREDITARY CHARACTERS
Between the years 1857 and 1868 Gregor Mendel, Augustinian monk, studied the heredity of certain characters of the common edible pea, in the garden of the monastery at Bruenn.
In his account of his work written in 1868, he said:
"It requires indeed some courage to undertake a labor of such a far-reaching extent; it appears, however, to be the only right way by which we can finally reach the solution of a question the importance of which cannot be over-estimated in connection with the history of the evolution of organic forms."
He tells us also why he selected peas for his work:
"The selection of the plant group which shall serve for experiments of this kind must be made with all possible care if it be desired to avoid from the outset every risk of questionable results."
"The experimental plants must necessarily
1. Possess constant differentiating characters.
2. The hybrids of such plants must, during the flowering period, be protected from the influence of all foreign pollen, or be easily capable of such protection."
Why do biologists throughout the world to-day agree that Mendel's discovery is one of first rank?
A great deal might be said in this connection. What is essential may be said in a few words. Biology had been, and is still, largely a descriptive and speculative science. Mendel showed by experimental proof that heredity could be explained by a simple mechanism. His discovery has been exceedingly fruitful.
Science begins with naive, often mystic conceptions of its problems. It reaches its goal whenever it can replace its early guessing by verifiable hypotheses and predictable results. This is what Mendel's law did for heredity.
MENDEL'S FIRST DISCOVERY—SEGREGATION
Let us turn to the demonstration of his first law—the law of segregation. The first case I choose is not the one given by Mendel but one worked out later by Correns. If the common garden plant called four o'clock (Mirabilis jalapa) with red flowers is crossed to one having white flowers, the offspring are pink (fig. 13). The hybrid, then, is intermediate in the color of its flowers between the two parents. If these hybrids are inbred the offspring are white, pink and red, in the proportion of 1:2:1. All of these had the same ancestry, yet they are of three different kinds. If we did not know their history it would be quite impossible to state what the ancestry of the white or of the red had been, for they might just as well have come from pure white and pure red ancestors respectively as to have emerged from the pink hybrids. Moreover, when we test them we find that they are as pure as are white or red flowering plants that have had all white or all red flowering ancestors.
Mendel's Law explains the results of this cross as shown in figure 14.
The egg cell from the white parent carries the factor for white, the pollen cell from the red parent carries the factor for red. The hybrid formed by their union carries both factors. The result of their combined action is to produce flowers intermediate in color.
When the hybrids mature and their germ cells (eggs or pollen) ripen, each carries only one of these factors, either the red or the white, but not both. In other words, the two factors that have been brought together in the hybrid separate in its germ cells. Half of the egg cells are white bearing, half red bearing. Half of the pollen cells are white bearing, half red bearing. Chance combinations at fertilization give the three classes of individuals of the second generation.
The white flowering plants should forever breed true, as in fact they do. The red flowering plants also breed true. The pink flowering plants, having the same composition as the hybrids of the first generation, should give the same kind of result. They do, indeed, give this result i.e. one white to two pink to one red flowered offspring.
Another case of the same kind is known to breeders of poultry. One of the most beautiful of the domesticated breeds is known as the Andalusian. It is a slate blue bird shading into blue-black on the neck and back. Breeders know that these blue birds do not breed true but produce white, black, and blue offspring.
The explanation of the failure to produce a pure race of Andalusians is that they are like the pink flowers of the four o'clock, i.e., they are a hybrid type formed by the meeting of the white and the black germ cells. If the whites produced by the Andalusians are bred to the blacks (both being pure strains), all the offspring will be blue (fig. 15); if these blues are inbred they will give 1 white, to 2 blues, to 1 black. In other words, the factor for white and the factor for black separate in the germ cells of the hybrid Andalusian birds (fig. 16).
The third case is Mendel's classical case of yellow and green peas (fig. 17). He crossed a plant belonging to a race having yellow peas with one having green peas. The hybrid plants had yellow seeds. These hybrids inbred gave three yellows to one green. The explanation (fig. 18) is the same in principle as in the preceding cases. The only difference between them is that the hybrid which contains both the yellow and the green factors is in appearance not intermediate, but like the yellow parent stock. Yellow is said therefore to be dominant and green to be recessive.
Another example where one of the contrasted characters is dominant is shown by the cross of Drosophila with vestigial wings to the wild type with long wings (fig. 19). The F_1 flies have long wings not differing from those of the wild fly, so far as can be observed. When two such flies are inbred there result three long to one vestigial.
The question as to whether a given character is dominant or recessive is a matter of no theoretical importance for the principle of segregation, although from the notoriety given to it one might easily be misled into the erroneous supposition that it was the discovery of this relation that is Mendel's crowning achievement.
Let me illustrate by an example in which the hybrid standing between two types overlaps them both. There are two mutant races in our cultures of the fruit fly Drosophila that have dark body color, one called sooty, another which is even blacker, called ebony (fig. 20). Sooty crossed to ebony gives offspring that are intermediate in color. Some of them are so much like sooty that they cannot be distinguished from sooty. At the other extreme some of the hybrids are as dark as the lightest of the ebony flies. If these hybrids are inbred there is a continuous series of individuals, sooties, intermediates and ebonies. Which color here shall we call the dominant? If the ebony, then in the second generation we count three ebonies to one sooty, putting the hybrids with the ebonies. If the dominant is the sooty then we count three sooties to one ebony, putting the hybrids with the sooties. The important fact to find out is whether there actually exist three classes in the second generation. This can be ascertained even when, as in this case, there is a perfectly graded series from one end to the other, by testing out individually enough of the flies to show that one-fourth of them never produce any descendants but ebonies, one-fourth never any but sooties, and one-half of them give rise to both ebony and sooty.
MENDEL'S SECOND DISCOVERY—INDEPENDENT ASSORTMENT
Besides his discovery that there are pairs of characters that disjoin, as it were, in the germ cells of the hybrid (law of segregation) Mendel made a second discovery which also has far-reaching consequences. The following case illustrates Mendel's second law.
If a pea that is yellow and round is crossed to one that is green and wrinkled (fig. 21), all of the offspring are yellow and round. Inbred, these give 9 yellow round, 3 green round, 3 yellow wrinkled, 1 green wrinkled. All the yellows taken together are to the green as 3:1. All the round taken together are to the wrinkled as three to one; but some of the yellows are now wrinkled and some of the green are now round. There has been a recombination of characters, while at the same time the results, for each pair of characters taken separately, are in accord with Mendel's Law of Segregation, (fig. 22). The second law of Mendel may be called the law of independent assortment of different character pairs.
We can, as it were, take the characters of one organism and recombine them with those of a different organism. We can explain this result as due to the assortment of factors for these characters in the germ cells according to a definite law.
As a second illustration let me take the classic case of the combs of fowls. If a bird with a rose comb is bred to one with a pea comb (fig. 23), the offspring have a comb different from either. It is called a walnut comb. If two such individuals are bred they give 9 walnut, 3 rose, 3 pea, 1 single. This proportion shows that the grandparental types differed in respect to two pairs of characters.
A fourth case is shown in the fruit fly, where an ebony fly with long wings is mated to a grey fly with vestigial wings (fig. 24). The offspring are gray with long wings. If these are inbred they give 9 gray long, 3 gray vestigial, 3 ebony long, 1 ebony vestigial (figs. 24 and 25).
The possibility of interchanging characters might be illustrated over and over again. It is true not only when two pairs of characters are involved, but when three, four, or more enter the cross.
It is as though we took individuals apart and put together parts of two, three or more individuals by substituting one part for another.
Not only has this power to make whatever combinations we choose great practical importance, it has even greater theoretical significance; for, it follows that the individual is not in itself the unit in heredity, but that within the germ-cells there exist smaller units concerned with the transmission of characters.
The older mystical statement of the individual as a unit in heredity has no longer any interest in the light of these discoveries, except as a past phase of biological history. We see, too, more clearly that the sorting out of factors in the germ plasm is a very different process from the influence of these factors on the development of the organism. There is today no excuse for confusing these two problems.
If mechanistic principles apply also to embryonic development then the course of development is capable of being stated as a series of chemico-physical reactions and the "individual" is merely a term to express the sum total of such reactions and should not be interpreted as something different from or more than these reactions. So long as so little is known of the actual processes involved in development the use of the term "individuality", while giving the appearance of profundity, in reality often serves merely to cover ignorance and to make a mystery out of a mechanism.
THE CHARACTERS OF WILD ANIMALS AND PLANTS FOLLOW THE SAME LAWS OF INHERITANCE AS DO THE CHARACTERS OF DOMESTICATED ANIMALS AND PLANTS.
Darwin based many of his conclusions concerning variation and heredity on the evidence derived from the garden and from the stock farm. Here he was handicapped to some extent, for he had at times to rely on information much of which was uncritical, and some of which was worthless.
Today we are at least better informed on two important points; one concerning the kinds of variations that furnish to the cultivator the materials for his selection; the other concerning the modes of inheritance of these variations. We know now that new characters are continually appearing in domesticated as well as in wild animals and plants, that these characters are often sharply marked off from the original characters, and whether the differences are great or whether they are small they are transmitted alike according to Mendel's law.
Many of the characteristics of our domesticated animals and cultivated plants originated long ago, and only here and there have the records of their first appearance been preserved. In only a few instances are these records clear and definite, while the complete history of any large group of our domesticated products is unknown to us.
Within the last five or six years, however, from a common wild species of fly, the fruit fly, Drosophila ampelophila, which we have brought into the laboratory, have arisen over a hundred and twenty-five new types whose origin is completely known. Let me call attention to a few of the more interesting of these types and their modes of inheritance, comparing them with wild types in order to show that the kinds of inheritance found in domesticated races occur also in wild types. The results will show beyond dispute that the characters of wild types are inherited in precisely the same way as are the characters of the mutant types—a fact that is not generally appreciated except by students of genetics, although it is of the most far-reaching significance for the theory of evolution.
A mutant appeared in which the eye color of the female was different from that of the male. The eye color of the mutant female is a dark eosin color, that of the male yellowish eosin. From the beginning this difference was as marked as it is to-day. Breeding experiments show that eosin eye color differs from the red color of the eye of the wild fly by a single mutant factor. Here then at a single step a type appeared that was sexually dimorphic.
Zoologists know that sexual dimorphism is not uncommon in wild species of animals, and Darwin proposed the theory of sexual selection to account for the difference between the sexes. He assumed that the male preferred certain kinds of females differing from himself in a particular character, and thus in time through sexual selection, the sexes came to differ from each other.
In the case of eosin eye color no such process as that postulated by Darwin to account for the differences between the sexes was involved; for the single mutation that brought about the change also brought in the dimorphism with it.
In recent years zoologists have carefully studied several cases in which two types of female are found in the same species. In the common clover butterfly, there is a yellow and a white type of female, while the male is yellow (fig. 26). It has been shown that a single factor difference determines whether the female is yellow or white. The inheritance is, according to Gerould, strictly Mendelian.
In Papilio turnus there exist, in the southern states, two kinds of females, one yellow like the male, one black (fig. 27). The evidence here is not so certain, but it seems probable that a single factor difference determines whether the female shall be yellow or black.
Finally in Papilio polytes of Ceylon and India three different types of females appear, (fig. 28 to right) only one of which is like the male. Here the analysis of the breeding data shows the possibility of explaining this case as due to two pairs Mendelian factors which give in combination the three types of female.
Taking these cases together, they furnish a much simpler explanation than the one proposed by Darwin. They show also that characters like these shown by wild species may follow Mendel's law.
There has appeared in our cultures a fly in which the third division of the thorax with its appendages has changed into a segment like the second (fig. 29). It is smaller than the normal mesothorax and its wings are imperfectly developed, but the bristles on the upper surface may have the typical arrangement of the normal mesothorax. The mutant shows how great a change may result from a single factor difference.
A factor that causes duplication in the legs has also been found. Here the interesting fact was discovered (Hoge) that duplication takes place only in the cold. At ordinary temperatures the legs are normal.
In contrast to the last case, where a character is doubled, is the next one in which the eyes are lost (fig. 30). This change also took place at a single step. All the flies of this stock however, cannot be said to be eyeless, since many of them show pieces of the eye—indeed the variation is so wide that the eye may even appear like a normal eye unless carefully examined. Formerly we were taught that eyeless animals arose in caves. This case shows that they may also arise suddenly in glass milk bottles, by a change in a single factor.
I may recall in this connection that wingless flies (fig. 5 f) also arose in our cultures by a single mutation. We used to be told that wingless insects occurred on desert islands because those insects that had the best developed wings had been blown out to sea. Whether this is true or not, I will not pretend to say, but at any rate wingless insects may also arise, not through a slow process of elimination, but at a single step.
The preceding examples have all related to recessive characters. The next one is dominant.
A single male appeared with a narrow vertical red bar (fig. 31) instead of the broad red oval eye. Bred to wild females the new character was found to dominate, at least to the extent that the eyes of all its offspring were narrower than the normal eye, although not so narrow as the eye of the pure stock. Around the bar there is a wide border that corresponds to the region occupied by the rest of the eye of the wild fly. It lacks however the elements of the eye. It is therefore to be looked upon as a rudimentary organ, which is, so to speak, a by-product of the dominant mutation.
The preceding cases have all involved rather great changes in some one organ of the body. The following three cases involve slight changes, and yet follow the same laws of inheritance as do the larger changes.
At the base of the wings a minute black speck appeared (fig. 32). It was found to be a Mendelian character. In another case the spines on the thorax became forked or kinky (fig. 52b). This stock breeds true, and the character is inherited in strictly Mendelian fashion.
In a certain stock a number of flies appeared in which the wing pads did not expand (fig. 33). It was found that this peculiarity is shown in only about twenty per cent of the individuals supposed to inherit it. Later it was found that this stock lacked two bristles on the sides of the thorax. By means of this knowledge the heredity of the character was easily determined. It appears that while the expansion of the wing pads fails to occur once in five times—probably because it is an environmental effect peculiar to this stock,—yet the minute difference of the presence or absence of the two lateral bristles is a constant feature of the flies that carry this particular factor.
In the preceding cases I have spoken as though a factor influenced only one part of the body. It would have been more accurate to have stated that the chief effect of the factor was observed in a particular part of the body. Most students of genetics realize that a factor difference usually affects more than a single character. For example, a mutant stock called rudimentary wings has as its principle characteristic very short wings (fig. 34). But the factor for rudimentary wings also produces other effects as well. The females are almost completely sterile, while the males are fertile. The viability of the stock is poor. When flies with rudimentary wings are put into competition with wild flies relatively few of the rudimentary flies come through, especially if the culture is crowded. The hind legs are also shortened. All of these effects are the results of a single factor-difference.
One may venture the guess that some of the specific and varietal differences that are characteristic of wild types and which at the same time appear to have no survival value, are only by-products of factors whose most important effect is on another part of the organism where their influence is of vital importance.
It is well known that systematists make use of characters that are constant for groups of species, but which do not appear in themselves to have an adaptive significance. If we may suppose that the constancy of such characters may be only an index of the presence of a factor whose chief influence is in some other direction or directions, some physiological influence, for example, we can give at least a reasonable explanation of the constancy of such characters.
I am inclined to think that an overstatement to the effect that each factor may affect the entire body, is less likely to do harm than to state that each factor affects only a particular character. The reckless use of the phrase "unit character" has done much to mislead the uninitiated as to the effects that a single change in the germ plasm may produce on the organism. Fortunately, the expression "unit character" is being less used by those students of genetics who are more careful in regard to the implications of their terminology.
There is a class of cases of inheritance, due to the XY chromosomes, that is called sex linked inheritance. It is shown both by mutant characters and characters of wild species.
For instance, white eye color in Drosophila shows sex linked inheritance. If a white eyed male is mated to a wild red eyed female (fig. 35) all the offspring have red eyes. If these are inbred, there are three red to one white eyed offspring, but white eyes occur only in the males. The grandfather has transmitted his peculiarity to half of his grandsons, but to none of his granddaughters.
The reciprocal cross (fig. 36) is also interesting. If a white eyed female is bred to a red eyed male, all of the daughters have red eyes and all of the sons have white eyes. We call this criss-cross inheritance. If these offspring are inbred, they produce equal numbers of red eyed and white eyed females and equal numbers of red eyed and white eyed males. The ratio is 1: 1: 1: 1, or ignoring sex, 2 reds to 2 whites, and not the usual 3:1 Mendelian ratio. Yet, as will be shown later, the result is in entire accord with Mendel's principle of segregation.
It has been shown by Sturtevant that in a wild species of Drosophila, viz., D. repleta, two varieties of individuals exist, in one of which the thorax has large splotches and in the other type smaller splotches (fig. 37). The factors that differentiate these varieties are sex linked.
Certain types of color blindness (fig. 38) and certain other abnormal conditions in man such as haemophilia, are transmitted as sex linked characters.
In domestic fowls sex linked inheritance has been found as the characteristic method of transmission for at least as many as six characters, but here the relation of the sexes is in a sense reversed. For instance, if a black Langshan hen is crossed to a barred Plymouth Rock cock (fig. 39), the offspring are all barred. If these are inbred half of the daughters are black and half are barred; all of the sons are barred. The grandmother has transmitted her color to half of her granddaughters but to none of her grandsons.
In the reciprocal cross (fig. 40) black cock by barred hen, the daughters are black and the sons barred—criss-cross inheritance. These inbred give black hens and black cocks, barred hens and barred cocks.
There is a case comparable to this found in a wild species of moth, Abraxas grossulariata. A wild variation of this type is lighter in color and is known as A. lacticolor. When these two types are crossed they exhibit exactly the same type of heredity as does the black-barred combination in the domestic fowl. As shown in figure 41, lacticolor female bred to grossulariata male gives grossulariata sons and daughters. These inbred give grossulariata males and females and lacticolor females. Reciprocally lacticolor male by grossulariata female, (fig. 42) gives lacticolor daughters and grossulariata sons and these inbred give grossulariata males and females and lacticolor males and females.
It has been found that there may be even more than two factors that show Mendelian segregation when brought together in pairs. For example, in the southern States there are several races of the grouse locust (Paratettix) that differ from each other markedly in color patterns (fig. 43). When any two individuals of these races are crossed they give, as Nabours has shown, in F_2 a Mendelian ratio of 1: 2: 1. It is obvious, therefore, that there are here at least nine characters, any two of which behave as a Mendelian pair. These races have arisen in nature and differ definitely and strikingly from each other, yet any two differ by only one factor difference.
Similar relations have been found in a number of domesticated races. In mice there is a quadruple system represented by the gray house mouse, the white bellied, the yellow and the black mouse (fig. 44). In rabbits there is probably a triple system, that includes the albino, the Himalayan, and the black races. In the silkworm moth there have been described four types of larvae, distinguished by different color markings, that form a system of quadruple allelomorphs. In Drosophila there is a quintuple system of factors in the sex chromosome represented by eye colors, a triple system of body colors, and a triple system of factors for eye colors in the third chromosome.
MUTATION AND EVOLUTION
What bearing has the appearance of these new types of Drosophila on the theory of evolution may be asked. The objection has been raised in fact that in the breeding work with Drosophila we are dealing with artificial and unnatural conditions. It has been more than implied that results obtained from the breeding pen, the seed pan, the flower pot and the milk bottle do not apply to evolution in the "open", nature "at large" or to "wild" types. To be consistent, this same objection should be extended to the use of the spectroscope in the study of the evolution of the stars, to the use of the test tube and the balance by the chemist, of the galvanometer by the physicist. All these are unnatural instruments used to torture Nature's secrets from her. I venture to think that the real antithesis is not between unnatural and natural treatment of Nature, but rather between controlled or verifiable data on the one hand, and unrestrained generalization on the other.
If a systematist were asked whether these new races of Drosophila are comparable to wild species, he would not hesitate for a moment. He would call them all one species. If he were asked why, he would say, I think, "These races differ only in one or two striking points, while in a hundred other respects they are identical even to the minutest details." He would add, that as large a group of wild species of flies would show on the whole the reverse relations, viz., they would differ in nearly every detail and be identical in only a few points. In all this I entirely agree with the systematist, for I do not think such a group of types differing by one character each, is comparable to most wild groups of species because the difference between wild species is due to a large number of such single differences. The characters that have been accumulated in wild species are of significance in the maintenance of the species, or at least we are led to infer that even though the visible character that we attend to may not itself be important, one at least of the other effects of the factors that represent these characters is significant. It is, of course, hardly to be expected that any random change in as complex a mechanism as an insect would improve the mechanism, and as a matter of fact it is doubtful whether any of the mutant types so far discovered are better adapted to those conditions to which a fly of this structure and habits is already adjusted. But this is beside the mark, for modern genetics shows very positively that adaptive characters are inherited in exactly the same way as are those that are not adaptive; and I have already pointed out that we cannot study a single mutant factor without at the same time studying one of the factors responsible for normal characters, for the two together constitute the Mendelian pair.
And, finally, I want to urge on your attention a question that we are to consider in more detail in the last lecture. Evolution of wild species appears to have taken place by modifying and improving bit by bit the structures and habits that the animal or plant already possessed. We have seen that there are thirty mutant factors at least that have an influence on eye color, and it is probable that there are at least as many normal factors that are involved in the production of the red eye of the wild fly.
Evolution from this point of view has consisted largely in introducing new factors that influence characters already present in the animal or plant.
Such a view gives us a somewhat different picture of the process of evolution from the old idea of a ferocious struggle between the individuals of a species with the survival of the fittest and the annihilation of the less fit. Evolution assumes a more peaceful aspect. New and advantageous characters survive by incorporating themselves into the race, improving it and opening to it new opportunities. In other words, the emphasis may be placed less on the competition between the individuals of a species (because the destruction of the less fit does not in itself lead to anything that is new) than on the appearance of new characters and modifications of old characters that become incorporated in the species, for on these depends the evolution of the race.
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CHAPTER III
THE FACTORIAL THEORY OF HEREDITY AND THE COMPOSITION OF THE GERM PLASM
The discovery that Mendel made with edible peas concerning heredity has been found to apply everywhere throughout the plant and animal kingdoms—to flowering plants, to insects, snails, crustacea, fishes, amphibians, birds, and mammals (including man).
There must be something that these widely separated groups of plants and animals have in common—some simple mechanism perhaps—to give such definite and orderly series of results. There is, in fact, a mechanism, possessed alike by animals and plants, that fulfills every requirement of Mendel's principles.
THE CELLULAR BASIS OF ORGANIC EVOLUTION AND HEREDITY
In order to appreciate the full force of the evidence, let me first pass rapidly in review a few familiar, historical facts, that preceded the discovery of the mechanism in question.
Throughout the greater part of the last century, while students of evolution and of heredity were engaged in what I may call the more general, or, shall I say, the grosser aspects of the subject, there existed another group of students who were engaged in working out the minute structure of the material basis of the living organism. They found that organs such as the brain, the heart, the liver, the lungs, the kidneys, etc., are not themselves the units of structure, but that all these organs can be reduced to a simpler unit that repeats itself a thousand-fold in every organ. We call this unit a cell (fig. 45).
The egg is a cell, and the spermatozoon is a cell. The act of fertilization is the union of two cells (fig. 47, upper figure). Simple as the process of fertilization appears to us today, its discovery swept aside a vast amount of mystical speculation concerning the role of the male and of the female in the act of procreation.
Within the cell a new microcosm was revealed. Every cell was found to contain a spherical body called the nucleus (fig. 46a). Within the nucleus is a network of fibres, a sap fills the interstices of the network. The network resolves itself into a definite number of threads at each division of the cell (fig. 46 b-e). These threads we call chromosomes. Each species of animals and plants possesses a characteristic number of these threads which have a definite size and sometimes a specific shape and even characteristic granules at different levels. Beyond this point our strongest microscopes fail to penetrate. Observation has reached, for the time being, its limit.
The story is taken up at this point by a new set of students who have worked in an entirely different field. Certain observations and experiments that we have not time to consider now, led a number of biologists to conclude that the chromosomes are the bearers of the hereditary units. If so, there should be many such units carried by each chromosome, for the number of chromosomes is limited while the number of independently inherited characters is large. In Drosophila it has been demonstrated not only that there are exactly as many groups of characters that are inherited together as there are pairs of chromosomes, but even that it is possible to locate one of these groups in a particular chromosome and to state the relative position there of the factors for the characters. If the validity of this evidence is accepted, the study of the cell leads us finally in a mechanical, but not in a chemical sense, to the ultimate units about which the whole process of the transmission of the hereditary factors centers.
But before plunging into this somewhat technical matter (that is difficult only because it is unfamiliar), certain facts which are familiar for the most part should be recalled, because on these turns the whole of the subsequent story.
The thousands of cells that make up the cell-state that we call an animal or plant come from the fertilized egg. An hour or two after fertilization the egg divides into two cells (fig. 47). Then each half divides again. Each quarter next divides. The process continues until a large number of cells is formed and out of these organs mould themselves.
At every division of the cell the chromosomes also divide. Half of these have come from the mother, half from the father. Every cell contains, therefore, the sum total of all the chromosomes, and if these are the bearers of the hereditary qualities, every cell in the body, whatever its function, has a common inheritance.
At an early stage in the development of the animal certain cells are set apart to form the organs of reproduction. In some animals these cells can be identified early in the cleavage (fig. 48).
The reproductive cells are at first like all the other cells in the body in that they contain a full complement of chromosomes, half paternal and half maternal in origin (fig. 49). They divide as do the other cells of the body for a long time (fig. 49, upper row). At each division each chromosome splits lengthwise and its halves migrate to opposite poles of the spindle (fig. 49 c).
But there comes a time when a new process appears in the germ cells (fig 49 e-h). It is essentially the same in the egg and in the sperm cells. The discovery of this process we owe to the laborious researches of many workers in many countries. The list of their names is long, and I shall not even attempt to repeat it. The chromosomes come together in pairs (fig. 49 a). Each maternal chromosome mates with a paternal chromosome of the same kind.
Then follow two rapid divisions (fig. 49 f, g and 50 and 51). At one of the divisions the double chromosomes separate so that each resulting cell comes to contain some maternal and some paternal chromosomes, i.e. one or the other member of each pair. At the other division each chromosome simply splits as in ordinary cell division.
The upshot of the process is that the ripe eggs (fig. 51) and the ripe spermatozoa (fig. 50) come to contain only half the total number of chromosomes.
When the eggs are fertilized the whole number of chromosomes is restored again.
THE MECHANISM OF MENDELIAN HEREDITY DISCOVERED IN THE BEHAVIOR OF THE CHROMOSOMES
If the factors in heredity are carried in the chromosomes and if the chromosomes are definite structures, we should anticipate that there should be as many groups of characters as there are kinds of chromosomes. In only one case has a sufficient number of characters been studied to show whether there is any correspondence between the number of hereditary groups of characters and the number of chromosomes. In the fruit fly, Drosophila ampelophila, we have found about 125 characters that are inherited in a perfectly definite way. On the opposite page is a list of some of them.
It will be observed in this list that the characters are arranged in four groups, Groups I, II, III and IV. Three of these groups are equally large or nearly so; Group IV contains only two characters. The characters are put into these groups because in heredity the members of each group tend to be inherited together, i.e., if two or more enter the cross together they tend to remain together through subsequent generations. On the other hand, any member of one group is inherited entirely independently of any member of the other groups; in the same way as Mendel's yellow-green pair of characters is inherited independently of the round-wrinkled pair.
Group I Group II Group III Group IV Abnormal Antlered Band Bent Bar Apterous Beaded Eyeless Bifid Arc Cream III Bow Balloon Deformed Cherry Black Dwarf Chrome Blistered Ebony Cleft Comma Giant Club Confluent Kidney Depressed Cream II Low crossing over Dot Curved Maroon Eosin Dachs Peach Facet Extra vein Pink Forked Fringed Rough Furrowed Jaunty Safranin Fused Limited Sepia Green Little crossover Sooty Jaunty Morula Spineless Lemon Olive Spread Lethals, 13 Plexus Trident Miniature Purple Truncate intensifier Notch Speck Whitehead Reduplicated Strap White ocelli Ruby Streak Rudimentary Trefoil Sable Truncate Shifted Vestigial Short Skee Spoon Spot Tan Truncate intensifier Vermilion White Yellow
If the factors for these characters are carried by the chromosomes, then we should expect that those factors that are carried by the same chromosome would be inherited together, provided the chromosomes are definite structures in the cell.
In the chromosome group of Drosophila, (fig. 52) there are four pairs of chromosomes, three of nearly the same size and one much smaller. Not only is there agreement between the number of hereditary groups and the number of the chromosomes, but even the size relations are the same, for there are three great groups of characters and three pairs of large chromosomes, and one small group of characters and one pair of small chromosomes.
THE FOUR GREAT LINKAGE GROUPS OF DROSOPHILA AMPELOPHILA
The following description of the characters of the wild fly may be useful in connection with the account of the modifications of these characters that appear in the mutants.
The head and thorax of the wild fly are grayish-yellow, the abdomen is banded with alternate stripes of yellow and black. In the male, (fig. 4 to right), there are three narrow bands and a black tip. In the female there are five black bands (fig. 4 to left). The wings are gray with a surface texture of such a kind that at certain angles they are iridescent. The eyes are a deep, solid, brick-red. The minute hairs that cover the body have a very definite arrangement that is most obvious on the head and thorax. There is a definite number of larger hairs called bristles or chaetae which have a characteristic position and are used for diagnostic purposes in classifying the species. On the foreleg of the male there is a comb-like organ formed by a row of bristles; it is absent in the female. The comb is a secondary sexual character, and it is, so far as known, functionless.
Some of the characters of the mutant types are shown in figures 53, 54, 55, 56. The drawing of a single fly is often used here to illustrate more than one character. This is done to economize space, but of course there would be no difficulty in actually bringing together in the same individual any two or more characters belonging to the same group (or to different groups). Without colored figures it is not possible to show many of the most striking differences of these mutant races; at most dark and light coloring can be indicated by the shading of the body, wings, or eyes.
Group I
In the six flies drawn in figure 53 there are shown five different wing characters. The first of these types (a) is called cut, because the ends of the wings look as though they had been cut to a point. The antennae are displaced downward and appressed and their bristle-like aristae are crumpled.
The second figure (b) represents a fly with a notch in the ends of the wings. This character is dominant, but the same factor that produces the notch in the wings is also a recessive lethal factor; because of this latter effect of the character no males of this race exist, and the females of the race are never pure but hybrid. Every female with notch wings bred to a wild male, will produce in equal numbers notch winged daughters and daughters with normal wings. There will be half as many sons as daughters. The explanation of this peculiar result is quite simple. Every notch winged female has one X chromosome that carries the factor for notch and one X chromosome that is "normal". Daughters receiving the former chromosomes are notched because the factor for notch is dominant, but they are not killed since the lethal effect of the notch factor is recessive to the normal allelomorph carried by the other chromosome that the daughters get from their father. This normal factor is recessive for notch but dominant for life. This same figure (b) is used here to show three other sex linked characters. The spines on the thorax are twisted or kinky, which is due to a factor called "forked". The effect is best seen on the thorax, but all spines on the body are similarly modified; even the minute hairs are also affected. Ruby eye color might be here represented—if the eyes in the figure were colored. The lighter color of the body and antennae is intended to indicate that the character tan is also present. The light color of the antennae is the most certain way of identifying tan. The tan flies are interesting because they have lost the positive heliotropism that is so marked a feature in the behavior of D. ampelophila. As this peculiarity of the tan flies is inherited like all the other sex linked characters, it follows that when a tan female is bred to a wild male all the sons inherit the recessive tan color and indifference to light, while the daughters show the dominant sex linked character of their father, i.e., they are "gray", and go to the light. Hence when such a brood is disturbed the females fly to the light, but the males remain behind.
One of the first mutants that appeared in D. ampelophila was called rudimentary on account of the condition of the wings (c). The same mutation has appeared independently several times. In the drawing (c) the dark body color is intended to indicate "sable" and the lighter color of the eyes is intended to indicate eosin. This eye color, which is an allelomorph of white, is also interesting because in the female the color is deeper than in the male. In other cases of sex linked factors the character is the same in the two sexes.
In the fourth figure (d) the third and fourth longitudinal veins of the wing are fused into one vein from the base of the wing to the level of the first cross-vein and in addition converge and meet near their outer ends. The shape of the eye is represented in the figure as different from the normal, due to another factor called "bar". This is a dominant character, the hybrid condition being also narrow, but not so narrow as the pure type. Vermilion eye color might also be here represented—due to a factor that has appeared independently on several occasions.
In the fifth figure (e) the wings are shorter and more pointed than in the wild fly. This character is called miniature. The light color of the drawing may be taken to represent yellow body color, and the light color of the eye white eye color.
In the last figure (f) the wings are represented as pads, essentially in the same condition that they are in when the fly emerges from the pupa case. Not all the flies of this stock have the wings in this condition; some have fully expanded wings that appear normal in all respects. Nevertheless, about the same percentage of offspring show the pads irrespective of whether the parents had pads or expanded wings.
The flies of this stock show, however, another character, which is a product of the same factor, and which is constant, i.e., repeated in all individuals. The two bristles on the sides of the thorax are constantly absent in this race. The lighter color of the eye in the figure may be taken to indicate buff—a faint yellowish color. The factor for this eye color is another allelomorph of white.
There are many other interesting characters that belong to the first group, such as abnormal abdomen, short legs, duplication of the legs, etc. In fact, any part of the body may be affected by a sex-linked factor.
Group II
In the first figure (a) of figure 54 that contains members of Group II the wings are almost entirely absent or "vestigial". This condition arose at a single step and breeds true, although it appears to be influenced to some extent by temperature, also by modifiers that sometimes appear in the stock. Purple eye color belongs in Group II; it resembles the color of the eye of the wild fly but is darker and more translucent.
In the second figure (b) the wing is again long and narrow and sometimes bent back on itself, as shown here. In several respects the wing resembles strap (d) but seems to be due to another factor, called antler, insufficiently studied as yet.
In the third figure (c) the wings turn up at the end. This is brought about by the presence of the factor called jaunty.
In the fourth figure the wings are long and narrow and several of the veins are unrepresented. This character, "strap", is very variable and has not yet been thoroughly studied. On the thorax there is a deep black mark called trefoil. Even in the wild fly there is a three pronged mark on the thorax present in many individuals. Trefoil is a further development and modification of this mark and is due to a special factor.
In the fifth figure (e) the wings are arched. The factor is called arc. The dark color of the body, and especially of the wings, indicates the factor for black.
The sixth figure (f) shows the wings "curved" downwards. In addition there is present a minute black speck at the base of each wing, due to another factor called speck.
In the seventh figure (g) the wing is truncate. Its end is obliquely squared instead of rounded; it may be longer than the body, or shorter when other modifying factors are present. The mutation that produces this type of wing is of not infrequent occurrence. It has been shown by Muller and Altenburg that there are at least two factors that modify this character—the chief factor is present in the second chromosome; alone it produces the truncate wing in only a certain percentage of cases, but when the modifiers are also present about ninety percent of the individuals may show the truncate condition of the wing. But the presence of these factors makes the stock very infertile, so that it is difficult to maintain.
In the eighth figure (h) the legs are shortened owing to the absence of a segment of the tarsus. The stock is called dachs—a nickname given to it because the short legs suggested the dachshund.
Group III
In figure 55, (a), a mutant type called bithorax is shown. The old metathorax is replaced by another mesothorax thrust in between the normal mesothorax and the abdomen. It carries a pair of wings that do not completely unfold. On this new mesothorax the characteristic arrangement of the bristles is shown. Thus at a single step a typical region of the body has doubled. The character is recessive.
The size of the adult fly of D. ampelophila varies greatly according to the amount of nourishment obtained by the larva. After the fly emerges its size remains nearly constant, as in many insects. Two races have, however, been separated by Bridges that are different in size as a result of a genetic factor. The first of these, called dwarf, is represented by figure 55, (b).
The race is minute, although of course its size is variable, depending on food and other conditions. The same figure shows the presence of another factor, "sooty", that makes the fly very dark. Maroon eye color might be here represented, due to still another factor.
In the third figure (c) the other mutation in size is shown. It is called "giant". The flies are twice the size of wild flies. An eye color, called peach, might here be represented. It is an allelomorph of pink. |
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