Mutation

“ It cannot be demonstrated in all cases, but it is nevertheless certain that this applies also to all the organs of movement, digestion, and reproduction, to all animal groups and also to the differences between them, even although these may not always be obvious adaptations to the conditions of life. What part is left for mutation to play if almost everything is an adaptation? ”

“ The conception of mutation periods producing swarms of species from time to time, among which only a few have a chance of survival, promises to become the basis for speculative pedigree-diagrams, as well as for experimental investigations. ”

“ The theory of mutation on the other hand rests directly upon experimental investigations, and facts concerning the actual descent of one form from another are as yet exceedingly rare. It is always difficult to estimate the validity of conclusions drawn from isolated instances selected from the whole range of contingent phenomena, and this is especially true of the present case. ”

“ It is essential to distinguish the process of evolution toward the mutation–selection balance from an error catastrophe, as the two are completely different. We thus begin by briefly illustrating the dynamic approach to mutation–selection balance. For any combination of mutation rates and fitnesses in our model, a population will evolve until it reaches a unique equilibrium distribution of A1 and A2. Figure 2 illustrates the evolution of two populations—one that initially consisted entirely of A1 and another that consisted almost entirely of A2. ”

“ Variability seems to us to be very general, but very limited. The limits however, are distinctly drawn by the struggle for existence. Of course the chance for useful mutations is a very small one. We have seen that the same mutations are as a rule repeated from time to time by the same species. ”

“ Each of these types breeds true. “Each has arisen independently and suddenly. Every 437part of the body has been affected by one or another of these mutations.” To arrange these mutations arbitrarily into graded series would give the impression of an evolutionary series, but this is directly contrary to the known facts concerning their origin, for each mutation “originated independently from the wild type.” “Evolution has taken place by the incorporation into the race of those mutations that are beneficial to the life and reproduction of the individual.” ”

“ To my mind this work should clear up the strife between the critics and the adherents of evolution by mutation. It is evident that there are variations that are inherited and variations that are not inherited. If we call the one a mutation and the other a fluctuation, we have a distinction that will stand analysis. ”

“ Now mutations come unexpectedly, and if the ancestry of an accidental mutation is to be known, it is of course necessary to keep accounts of all the strains cultivated. It is evident that the required knowledge concerning the ancestry of a supposed mutation, must necessarily nearly all be acquired from the plants in the experimental garden. ”

“ But when an organism is about to undergo mutation, the change is often preluded by frequent and extensive variations. Of these divergent variations, those only persist which are best suited to the conditions of existence. ”

“ The new form may appear suddenly, often in a single generation, or gradually, but in either case it is the result of adaptation that is necessarily advantageous, because it is the result of adjustment to controlling physical factors. Origin by adaptation is perhaps only a special kind of origin by variation, but this might be said with equal truth of mutation. New forms resulting from adaptation are like those produced from mutation, in that they appear suddenly as a rule and without the agency of selection. ”

“ Yet over the eons, creatures have indeed changed, largely through the effects of mutation. If mutations are almost always for the worse, how can one explain that evolution seems to progress toward the better and that out of a primitive form as simple as an amoeba, for instance, there eventually emerged man? ”

“ It has taken millions of years for the long, slow process of evolution to develop specific adaptations that suit dune dwellers to their environment. There are variations between individuals in every form of life. Mostly these are normal inherited variations, such as height or color. But sometimes sudden variations, called “mutations,” occur through accidental changes in the genes controlling inheritance. These are new characteristics not found in other members of the same species. If the mutation is advantageous it may be passed on, and it is in this way that new life forms slowly develop. ”

“ Similarly, I regard mutation as a quite distinct phenomenon, and as par excellence the contribution of the paleontological method; it is the gradual rise of new adaptive characters neither by the selection of accidental variations nor by saltation, but by origin in an obscure and almost invisible form, followed by "direct increase and development in successive generations until a stage of actual usefulness is reached, where perhaps selection may begin to operate. ”

“ The thing, of course, may either be a mutation that is expected—in which case it may occur severally—or an unexpected dominant mutation in which case its spread will occur as the first guy inseminates the race with the seeds of his own being. ”

“ This selection exhibits far-reaching analogies between the artificial and the natural processes, and is in both cases of the very highest importance. In nature the dying out of unfit mutations is the result of the great struggle for life. ”

“ This thesis has become known as the theory of mutation.
At the present time we accordingly imagine that living organisms, such as we see around us, have all descended from older organisms, rather unlike them, of which we still find traces and remnants in the geological strata which have been deposited during past ages. From this stand-point all living organisms might possibly have originated from one single, most simple organism. ”

“ The theory of mutation, as well as that which ascribes the origin of species to the selection of fluctuating varieties, enables us to understand how efficiency and adaptation in organic nature have come about by the mutual interaction of natural processes without the aid of supernatural intervention. ”

“ The experimentalists, represented by Bateson, De Vries, Tower, MacDougal, Davenport and many others, have ascertained that what appear to be new species or races may arise suddenly by a process termed mutation. ”

“ Periods of Gradual Evolution of Groups.—It is certainly a very striking fact that wherever we have been able to trace genetic series, either of invertebrates or vertebrates, in closely sequent geological horizons, or life zones, we find strong proof of evolution through extremely gradual mutation simultaneously affecting many parts of each organism, as set forth above. ”

“ On account of the suddenness with which they appear, mutants do not lend themselves readily to natural experiment, and after they have once been discovered, inquiry into the causes and course of mutation is practicable only by means of habitat and control cultures. Among variable species, those are most promising that show a wide range of variation and are found in abundance over extensive areas. ”

“ It is to be noted here, a fact emphasised by DeVries in his earliest publications on the subject, that in nearly all, if not all cases, a mutation does not consist in a peculiarity of a single organ, but in an alteration of the whole plant in every part. ”

“ The decisive facts and discussions upon which the conclusions are based will be given in every case. Likewise I hope to point out the weak places and the lacunae in our present knowledge, and to show the way in which each of you may try to contribute his part towards the advancement of science in this subject. Lastly I shall try to prove that sudden mutation is the normal way in which nature produces new species and new varieties. ”

“ All evolution and improvement are dependent on crossing, therefore nature has produced more wonderful devices for this purpose than for any other."
"Mutations, or saltations, are often found; that is, fixed forms springing up, generally from unknown causes, forms which are not hybrids, and which remain constant; as, for instance, colored flowers which yield white forms, these yielding white constantly in their progeny. These mutations can be produced at will by any of the various means which disturb the habits of the plant. It comes out when the conditions are ripe. ”

“ In any case, a mutation which suppresses a character is not, as we have seen, a specific change, but merely one of the varietal order, which does not result in the production of a genuine new species. The factorial mutant with a vestigial wing or eye belongs to the same species as its wild or normal parent stock. Moreover, neither disuse nor natural selection has the slightest power to induce mutations of this kind. ”

“ An important assumption made in such experiments is that the machinery of inheritance and mutation is essentially the same in all creatures and that therefore knowledge gained from very simple species (even from bacteria) is applicable to man. ”

“ CHAPTER V The Mutation Theory When with the thoughts suggested in the last chapter we contemplate the problem of Evolution at large the hope at the present time of constructing even a mental picture of that process grows weak almost to the point of vanishing. We are left wondering that so lately men in general, whether scientific or lay, were so easily satisfied. ”

“ This structure and metabolism were handed down to subsequent living things by reproduction, and all variations thereafter resulted from occasional mutation, or changes in the nature of the heredity-transmitting units. One of the most extraordinary of all the attributes of life is its ordered complexity, both in function and structure. ”

“ Yet mutation reduction and lethal mutagenesis may both be feasible strategies for controlling a virus, with the two methods working at opposite ends of the evolutionary spectrum. Mutation suppression may thwart evolutionary exploration of the fitness landscape while mutation enhancement may overwhelm the genome with lethal mutations. ”

“ If we suppose elementary species and varieties originated by sudden leaps or mutations, then the elementary species have mutated in the line of progression, some varieties have mutated in the line of retrogression, while others have diverged from their parental types in a line of depression, or in the way of repetition. ”

“ A critical survey of all the facts of variability of plants in nature as well as under cultivation has led me to the conviction, that Darwin was right in stating that those rare beneficial variations, which from time to time happen to arise,—the now so-called mutations—are the real source of progress in the whole realm of the organic world. ”

“ Some of the most striking cases of adaptation, such as the organs of respiration and circulation in terrestrial Vertebrates, and the asymmetry of Flat-fishes, are developed in the individual by a metamorphosis which is generally regarded as a recapitulation of the ancestral evolution. No cases of mutation or gametogenic variation hitherto described exhibit a similar metamorphosis or recapitulation. ”

“ All in all it seems evident that the duration of life does not comply with the demands of the conception of very slow and continuous evolution. Now it is easily seen, that the idea of successive mutations is quite independent of this difficulty. Even assuming that some thousands of characters must have been acquired in order to produce the higher animals and plants of the present time, no valid objection is raised. The demands of the biologists and the results of [713] the physicists are harmonized on the ground of the theory of mutation. ”

“ Mutations that take place in the ordinary course of nature, without man’s interference, are spontaneous mutations. Most of these arise out of the very nature of the complicated mechanism of gene replication. ”

“ The steps may be surmised to have never been essentially larger than in the mutations now going on under our eyes, and some thousands of them may be estimated as sufficient to account for the entire organization of the higher forms. ”

“ These mutational forms are of several distinct and recognizable types which recur, and several of them breed true from their first appearance. The obvious difficulty, which in my judgment should make us unwilling at present to accept these occurrences as proof of the genesis of new species by mutation, is that we have as yet no certainty that the appearance of the new forms is not an effect of the recombination [Pg 102] of factors, such as is to be seen in so many generations of plants derived from a cross involving many genetic elements. The first question is what is Oenothera Lamarckiana? ”

“ Mutations, sort of unexpected changes, can take place in either the chromosomes or the genes. You see, when this one cell starts dividing, every gene makes a copy of itself; but, sometimes, the copy is a little different from the original. ”

“ Weismann adopts and extends the principle of natural selection. Germinal selection is exhibited in the germ-plasm.
IV. De Vries's Theory of Mutations.
1. The formation of species is due not to gradual changes, but to sudden mutations.
2. Natural selection presides over and improves variations arising from mutation.
Among the other theories of evolution that of Eimer is the most notable. He maintains that variations in organisms take place not fortuitously or accidentally, but follow a perfectly determinate direction. This definitely directed evolution is called orthogenesis. ”