Mutability

“ A SINGULARLY instructive field for the study of the mutability or stability of species should be afforded by the lower forms of life, in which organization is reduced to a minimum, they being mere masses of protoplasm without even a containing envelope, while their nourishment is of the simplest. ”

“ This is the fundamental cause of the fixity of species, which means the production of offspring having the same specific characteristics as their parents. How, under these conditions, the mutability of species is brought about must be discussed later. It is first of all necessary to inquire more closely into the composition of the chromatin, and to study the special phenomena of cell-growth in connexion with conjugation, where new and extraordinary features come to light.
A chromosome is not, or is not usually, a simple body. ”

“ Scott did not use these terms. He speaks of 'mutations.' A mutation occurs when one species is formed from another. As it is, 'mutation' is the expression in general use before the days of Darwin, and at first used by Darwin himself. Since it has apparently fallen into disuse, except in paleontology, where it is met with in various authors, always conveying the same meaning, it seems best to continue to use this term. Hence as long as species produce others they are called mutable, and this part of the doctrine of variability is known as mutability. ”

“ Reviewing the above, we find that the mutation theory comprises the following theses. Species originated from others by sudden but small changes, often so small as to be hardly visible to the neophyte. They are constant and true to seed from the first; neither are they connected with the mother species by a series of intermediate forms, nor do they have to pass in their prime, a stage of gradual development. This formation of new forms does not take place continually, but it is only from time to time that a species enters a period of mutability ”

“ 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.” ”

“ Our understanding of gene function is often informed by comparing the phenotypic consequences of mutation with the canonical “wild-type” in a single organism, as well as between mutants of orthologous genes in different organisms. In particular, model organisms have provided great insight into gene function in humans. The importance and need for automating these cross-species comparisons has become imperative as large-scale mutagenesis screens are conducted in model organisms. ”

“ 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. ”

“ In this book, and in this chapter itself, I have discussed adaptations and their origin so much already that I need only refer to these pages for convincing evidence that we cannot think of them as being brought about by the accumulation and augmentation of individually occurring saltatory 'mutations.' Not even if we assume that the leaps of mutation can be increased in the course of generations; in short, even if we say that mutations are all those variations which breed true and lead to the development of species, while variations are those which do not. ”

“ Maybe it was a mutation back in the Old Stone Age who struck two flints together and made himself a fire. Maybe another mutant who dreamed up a wheel and took a stoneboat and changed it to a wagon.
Slowly, he said, it would have to be slowly. Just a little at a time. For if it were too much, if it were noticeable, the other humans would kill off each mutation as it became apparent. For the human race cannot tolerate divergence from the norm, even though mutation is the process by which the race develops.
The race doesn't kill the mutants any more. ”

“ 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. ”

“ 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. ”

“ We might consider a mutation to be either “good” or “bad” in the sense that any change that helps a creature live more easily and comfortably is good and that the reverse is bad.
It seems reasonable that random changes in the gene pattern are almost sure to be bad. Consider that any creature, including man, is the product of millions of years of evolution. In every generation those individuals with a gene pattern that fit them better for their environment won out over those with less effective patterns—won out in the race for food, for mates, and for safety. ”

“ 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. ”

“ Those holding to the theory of descent, evolution, or transmutation, look upon varieties, species, subgenera, genera, families, orders, classes, and sub-kingdoms, as distinctions of merely transient importance, corresponding to the state of our information at the present time; it being assumed that by means of gradual transmutation during the course of ages all organisms have become evolved from a single primitive cell, or from a few primitive types. ”

“ For the present it is well to be clear as to the validity of the general conclusion, that all 'complex,' and especially all 'harmonious,' adaptation, must depend, not upon 'mutation,' but only upon 'variation' guided by selection. As species are essentially complexes of adaptation, originating from a basis of previous complexes of adaptation, there remains, as far as I can see, only the small field of indifferent characters to be determined by mutations, unless indeed we are to include under the term 'mutation' all variations which gain stability, but this would be merely a play upon words. ”

“ We suppose that since life first put in its appearance on the earth there have been uncounted mutations, a vastly greater number than are now represented by determiners. Many of the mutants could not compete with their brothers and sisters of ordinary descent and so promptly died, but occasionally it might happen that a mutant would be as well fitted for life as its relatives, in which case it would establish itself, and in course of time become ancestor to a whole line like itself. ”

“ 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. ”

“ The great difficulty in determining whether there is any true cumulative action of selection which will extend a character beyond the limits of the race or species is met in determining what are and what are not mutations. My experience has led me to conclude that the continuous selection of maximum fluctuations in a certain direction may in some cases lead to the gradual strengthening of the character until finally it may become, more or less suddenly, fully heritable and it would then be recognized as a mutation. ”

“ Since w1 > w2, the constraint on k ensures a higher replacement rate of A1 than of A2 at low mutation rates and the reverse at higher mutation rates. The point at which the lines intersect is the error threshold, beyond which A1 is absent, hence the use of dashes for this part of its replacement rate function. If replacement rate drops below unity, the population goes extinct, so the functions are not extended below one on the vertical axis.Right: the frequency of A1 declines as the mutation rate increases until the error threshold is reached. At higher mutation rates, only A2 is present. ”

“ The concept of a mutation–selection balance is one of the oldest and most fundamental pillars of population genetics: natural selection increases the frequency of fit variants while mutations introduce unfit variants, giving rise to an equilibrium distribution balanced between these two effects. ”

“ 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. ”

“ The innate, inherited vital actions of the protoplasm—that is, the molecular phenomena of motion of the smallest albuminous particles—are therefore more or less modified by it. The phenomenon of Adaptation, or transmutation, depends therefore upon the material influence which the organism experiences from its surroundings, or its conditions of existence; while the transmission by Inheritance is due to the partial identity of the producing and produced organism. These are the real, simple, mechanical foundations of the artificial process of breeding. ”

“ 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. ”

“ 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? ”

“ 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? ”

“ 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. ”

“ The benefit of increasing the network size is shown for three different combinations of forward and back mutation rates.Multiple Error Thresholds on a Binary Sequence of Length 40Each allele has two states—wild-type and mutant. The per base mutation rate from wild-type to mutant is identical for all loci. Back mutation from mutant to wild-type is not allowed. As mutant alleles fix in the population, the proportion of the sequence that can still mutate decreases. Multiple error thresholds exist because the effective mutation rate per genome drops as mutations accumulate. ”

“ 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. ”

“ Mutation.—These new characters are also germinal in origin, because they appear in the teeth, which are structures fully formed beneath the surface before they pierce the gum, and therefore not subsequently modeled by adaptive modification, as the bones, muscles and all the other tissues of the body are. Mutations are found arising according to partly known influences of kinship. They do not, so far as we observe, possess adaptive value when they first appear, but then frequently, if not always, develop into a stage of usefulness. ”