Variant

“ 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. ”

“ 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.” ”

“ 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. ”

“ 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. ”

“ How long does it last?And when you’re dealing with variants that clearly might actually be diminished somewhat in their capability of the virus — excuse me, of the vaccine-induced antibodies to essentially be able to neutralize there, you’re in a tenuous zone if you don’t have the full impact. So there is some merit to the arguments that they are made, and we certainly respect that.
But right now, given the number of vaccines we’re able to give every day, literally every day that goes by we get closer and closer to where we want to be. ”

“ And Hispanic persons had lost the second largest life expectancy, dropping 1.9 years.
These findings, though not surprising, are sobering and representative of the continued need to take this pandemic, and actions to stop the spread of COVID-19, seriously.
Now more than ever, with continued spread of variants that stand to threaten the progress we are making, we must recommit to doing our part to protect one another: wear a well-fitting mask, social distant, avoid travel and crowds, practice good hand hygiene, and get vaccinated when the vaccine is available to you. ”

“ All acknowledge the existence of variations and admit that their proximate source is in the fertilized ovum. All admit that the individual is, through its plasticity, in greater or less degree capable of adaptive modification. Transmissionists contend that the effects of modification are somehow transferred to the germinal substance there to give origin to variations. Selectionists deny this transmission and contend that adaptive [p. 735] variations are independent of adaptive modifications.
Now, what is natural selection, at any rate as understood by the master -- Darwin? ”

“ 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. ”

“ 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. ”

“ I intend to give a review of the facts obtained from plants which go to prove the assertion, that species and varieties have originated by mutation, and are, at present, not known to originate in any other way. This review consists of two parts. One is a critical survey of the facts of agricultural and horticultural breeding, as they have accumulated since the time of Darwin. This body of evidence is to be combined with some corresponding experiments [10] concerning the real nature of species in the wild state. ”

“ Progression and regression are seen to go hand in hand everywhere. No large group and probably even no genus or large species has been evolved without the joint agency of these two great principles. In the mutation-period of the evening-primroses the observed facts give direct support to this conclusion, since some of the new species proved, on closer inspection, to be retrograde varieties, while others manifestly owe their origin to progressive steps. ”

“ 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. ”

“ 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. ”

“ It is here suggested that persistent modification through many generations, though not transmitted to the germ, nevertheless affords the opportunity for the occurrence of germinal variation of like nature. 7. Under constant conditions of life, though variations in many directions are occurring in the organisms which have reached harmonious adjustment: to these conditions, yet natural selection eliminates all those which are of such amount as to be disadvantageous, and thus acts as a check on all variations, repressing them to within narrow limits. ”

“ Mutation is not a substitute for natural selection, but a coöperating factor; and neither mutation nor natural selection is a substitute for the doctrine of the continuity of the germ-plasm. Thus we may look forward to a reconciliation between apparently conflicting views, when naturalists by sifting shall have determined the truth embodied in the various theories. ”

“ 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. ”

“ No doubt a large amount of variation is truly indefinite, so that many meaningless or useless variations arise, and in one sense it is a mere coincidence if a particular variation turn out to be useful. But there are several directions in which the field of variation appears to be not only limited but defined in a certain direction. Obviously variations depend on the constitution of the varying organism; a modification, whether it be large or small, is a modification of an already definite and limited structure. ”

“ Speaking at the British Association in 1897 Sedgwick says, "The phenomenon of genetic variation forms the bed rock upon which all the theories of evolution must rest, and it is only by a study of variations; of their nature and cause, that we can ever hope to obtain any real insight into the actual way in which evolution has taken place". ”

“ The causes which give rise to a variation of type have always been sufficiently obscure, whether the origin of species be conceived after the Darwinian or the Mendelian fashion, and the mutation theories have hitherto afforded little light on that question. ”

“ 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. ”

“ As soon as utility itself is supposed to exercise a determinative influence on the direction of variation, we get an insight into the entire process and into much else besides that has hitherto been regarded as a stumbling-block to the theory of selection, and which did indeed present difficulties that for the moment were insuperable—as, for example, the like-directed variation of a large number of already existing similar parts, seen in the origin of feathers from the scales of reptiles. ”

“ Fuller investigations will certainly show that the 'sports' or 'chance' variations come under the same law as that of 'fluctuating' variations, mutations being only fluctuating variations carried beyond the critical point where past fluctuating variations can not withstand the accumulated forces without disintegration, thus bending them in a new direction. ”

“ Conversely, from the accident of collocation or from a misleading resemblance in features we deem essential, forms genetically distinct are often confounded together, and thus the divergence of such forms in their other features, which we declare to be non-essential, passes as an example of variation. ”

“ The present is always found to be the key to the past. It is only by employing a similar method that biologists can hope to attack successfully the problems of Variation and Heredity. By actual observation and experiment they must seek to determine how species are being modified around us to-day and find out what characters are transmissible. The nature and causes of Variation and of Heredity will thus in time come to be understood.
It is only within recent years that any attempt has been made to study variations systematically. ”

“ 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. ”

“ It is however becoming probable that if more knowledge of the chemical and physical structure of organisms is to be attained, the clue will be found through Genetics, and thus that even in the uncoordinated accumulation of facts of Variation we are providing the means of analysis applicable not only to them, but to the problems of normality also.
The only classification that we can yet institute with any confidence among the phenomena of Variation is that which distinguishes on the one hand variations in the processes of division from variations in the nature of the substances divided. ”

“ 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. ”

“ For without primary constituents of the germ, whether they are called as I call them, determinants, or something else, germinal selection, or guidance of variation by personal selection, is impossible; for where all units are alike there can be no struggle, no preference of the best. ”

“ Dear Darwin,—Will you tell me where are Fleeming Jenkin's arguments on the importance of single variation? Because I at present hold most strongly the contrary opinion, that it is the individual differences or general variability of species that enables them to become modified and adapted to new conditions.
Variations or "sports" may be important in modifying an animal in one direction, as in colour for instance, but how it can possibly work in changes requiring co-ordination of many parts, as in Orchids for example, I cannot conceive. ”

“ Darwin draws a broad line of distinction between Variation and Selection, regarding only those variations that are favorable as selected. I conceive that all variations which survive are by that fact of survival, selections, whether favorable or indifferent. A variety is a species in formation; now Selection itself is not a cause, or condition, of variation, it is the expression of variation. Mr. Darwin is at times explicit enough on this head: “It may metaphorically be said that Natural Selection is daily and hourly scrutinizing throughout the world the slightest variations ”

“ Mr.Bateson's work, entitled Materials for the Study of Variation, is an important contribution in this direction. He is not content with the knowledge that variability exists but seeks to find out by detailed investigations of particular cases what variations actually do occur in nature. He brings forward evidence to show that there are variations which cannot be arranged in a continuous series—that such variations are therefore discontinuous. ”

“ The transmissability of acquired characters has been called in question and is totally denied by a great number of leading biologists at the present day.
Darwin took Variation and Heredity as his basis and upon these two pillars he built up his theory of the origin of species and the evolution of organic forms. Biologists are now pushing their enquiries further back; they are not content to take Variation and Heredity as facts which require no further explanation. ”

“ When we thus compare the polymorphism of one species with the fixity of another, and attempt to determine the causes which have led to these extraordinary contrasts, two distinct lines of argument are open to us. We may ascribe the difference either to causes external to the organisms, primarily, that is to say, to a difference in the exigencies of Adaptation under Natural Selection ”