The Genetical Theory of Natural Selection
Quotes from the book
"Natural Selection is not evolution."
- opening sentence (p vii)
"No practical biologist interested in sexual reproduction would be led to work out the detailed consequences experienced by organisms having three or more sexes; yet what else should he do if he wishes to understand why the sexes are, in fact, always two?"
- (p ix)
"No efforts of mine could avail to make the book easy reading."
- -- Fisher on the book's difficulty. Most, if not all, readers probably wish he had made more of an effort. (p x)
We may consequently state the fundamental theorem of Natural Selection in the form : The rate of increase in fitness of any organism at any time is equal to its genetic variance in fitness at that time.
- Defining the fundamental theorem of natural selection (p 35)
Professor Eddington has recently remarked that 'The law that entropy always increases -- the second law of thermodynamics -- holds, I think, the supreme position among the laws of nature'. It is not a little instructive that so similar a law [the fundamental theorem of natural selection] should hold the supreme position among the biological sciences.
- Fisher on his fundamental theorem of natural selection (p 36)
[We are now] in a position to judge of the validity of the objection which has been made, that the principle of Natural Selection depends on a succession of favourable chances. The objection is more in the nature of an innuendo than of a criticism, for it depends for its force upon the ambiguity of the word chance, in its popular uses. The income derived from a Casino by its proprietor may, in one sense, be said to depend upon a succession of favourable chances, although the phrase contains a suggestion of improbability more appropriate to the hopes of the patrons of his establishment. It is easy without any very profound logical analysis to perceive the difference between a succession of favourable deviations from the laws of chance, and on the other hand, the continuous and cumulative action of these laws. It is on the latter that the principle of Natural Selection relies.
- Fisher on the objection (still often made by creationists) that the theory of evolution predicts evolution occurs "only by chance" (p 37)
In organisms of all kinds the young are launched upon their careers endowed with a certain amount of biological capital derived from their parents. This varies enormously in amount in different species, but, in all, there has been, before the offspring is able to lead an independent existence, a certain expenditure of nutriment in addition, almost universally, to some expenditure of time or activity, which the parents are induced by their instincts to make for the advantage of their young. Let us consider the reproductive value of these offspring at the moment when this parental expenditure on their behalf has just ceased. If we consider the aggregate of an entire generation of such offspring it is clear that the total reproductive value of the males in this group is exactly equal to the total value of all the females, because each sex must supply half the ancestry of all future generations of the species. From this it follows that the sex ratio will so adjust itself, under the influence of Natural Selection, that the total parental expenditure incurred in respect of children of each sex, shall be equal; for if this were not so and the total expenditure incurred in producing males, for instance, were less than the total expenditure incurred in producing females, then since the total reproductive value of the males is equal to that of the females, it would follow that those parents, the innate tendencies of which caused them to produce males in excess, would, for the same expenditure, produce a greater amount of reproductive value; and in consequence would be the progenitors of a larger fraction of future generations than would parents having a congenital bias towards the production of females. Selection would thus raise the sex-ratio until the expenditure upon males became equal to that upon females.
The evolution of distastefulness
An important question raised by both the Batesian and the Mullerian theories of mimicry concerns the process by which nauseous flavours, as a means of defence, have been evolved. Most other means of defence such as stings, or disagreeable secretions and odours, are explicable by increasing the chance of life of the individuals in which they are best developed, or of the social community to which they belong. With distastefulness, however, although it is obviously capable of giving protection to the species as a whole, through its effect upon the instinctive or acquired responses of predators, yet since any individual tasted would seem almost bound to perish, it is difficult to perceive how individual increments of the distasteful quality, beyond the average level of the species, could confer any individual advantage.
The gregarious habit of certain larvae supplies a possible solution of the problem, if we are willing to accept the view that the distasteful quality of the imago, which warning colours are so well adapted to advertise, is itself merely a by-product due to the persistence of nauseous substances acquired through the protection afforded to the larva. For, although with the adult insect the effect of increased distastefulness upon the actions of the predator will be merely to make that individual predator avoid all members of the persecuted species, and so, unless the individual attacked possibly survives, to confer no advantage upon its genotype, with gregarious larvae the effect will certainly be to give the increased protection especially to one particular group of larvae, probably brothers and sisters of the individual attacked. The selective potency of the avoidance of brothers will of course be only half as great as if the individual itself were protected ; against this is to be set the fact that it applies to the whole of a possibly numerous brood. There is thus no doubt of the real efficacy of this form of selection, though it may well be doubted if all cases of insect distastefulness can be explained by the same principle.
- Fisher anticipates, but does not fully explore what since Hamilton (1964) is now recognised as the important concept of kin selection (pp 158-9).
Quotes on the book
A book that I rate only second in importance in evolution theory to Darwin's Origin (this as joined with its supplement Of Man), and also rate as undoubtedly one of the greatest books of the twentieth century
- W.D. Hamilton, on the cover of the Variorum Edition (1999)
"This is perhaps the most important book on evolutionary genetics ever written"