Human evolution theory utilizing concepts of neoteny & female sexual selection
An etiology of neuropsychological disorders such as autism and dyslexia, and the origin of left handedness.

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R. A. Fisher

Fisher: bibliographical excerpts


"The contrast between non-social and social selective forces was reflected in Darwin's idea that, whereas natural selection would more or less grind to a halt in a constant environment, sexual selection was, in principle, capable of continuing indefinitely on its giddy spiral of ornamental exaggeration: 'In regard to structures acquired through ordinary or natural selection, there is in most cases, as long as the conditions of life remain the same, a limit to the amount of advantageous modification in relation to certain special ends; but in regard to structures adapted to make one male victorious over another, either in fighting or in charming the female, there is no definite limit to the amount of advantageous modification; so that as long as the proper variations arise the work of sexual selection will go on.' (Darwin 1871, i, p. 278) (Cronin, Helena (1992) The Ant and the Peacock: Cambridge Univ. Press, Cambridge p. 233)

"Certain remarkable consequences do, however, follow if some sexual preferences of this kind, determined, for example, by a plumage character, are developed in a species in which the preferences of one sex, in particular the female, have a great influence on the number of offspring left by individual males. In such cases the modification of the plumage character in the cock proceeds under two selective influences. (i) an innitial advantage not due to sexual preference, which advantage may be quite inconsiderable in magnitude, and (ii) an additional advantage conferred by female preference. The intensity of preference will itself by increased by selection so long as the sons of hens exercising the preference most decidedly have any advantage over the sons of other hens, whether this be due to the first or to the second cause. The importance of this situation lies in the fact that the further development of the plumage character will still proceed, by reason of the advantage gained in sexual selection, even after it has passed the point in development at which its advantage in Natural Selection has ceased. The selective agencies other than sexual preference may be opposed to further development, and yet the further development will preceed, so long as the disadvantage is more than counterbalanced by the advantage in sexual selection. Moreover, as long as there is a net advantage in favour of further plumage development, there will also be a net advantage in favour of giving to it a more decided preference. The two characteristics affected by such a process, namely plumage development in the male, and sexual preference for such developments in female, must thus advance together, and so long as the process is unchecked by severe counterselection, will advance with ever-increasing speed. In the total absence of such checks, it is easy to see that the speed of development will be proportional to the development already attained, which will therefore increase with time exponentially, or in geometric progression. There is thus in any bionomic situation, in which sexual selection is capable of conferring a great reproductive advantage, the potentiality of a runaway process, which, however small the beginnings from which it arose, must, unless checked, produce great effects, and in the later stages with great rapidity." (Fisher, R. A. (1930) The Genetical Theory of Natural Selection. Oxford. Clarendon Press pp. 136-7)

"Fisher (1930: 58) used the term 'runaway' sexual selection for situations in which females (usually) begin to favour extremeness of traits in males, that are deleterious in every other respect (Trivers 1972). Within-species social competition is likely to take on 'runaway' sexual aspects for three reasons: (1) the interdependence of the adversarial parties causes the significance of change in one to depend on the traits of the other; (2) the traits involved in the competition are likely to be arbitary (and deleterious) in all other contexts; and (3) within-species groups of an ecologically dominant species such as humans are relatively immune to effects from other selective agents. When one's adversary continually remains similar or identical to one's self in all but the particular trait that is at the moment changing, when changes in one party depend solely upon changes in the other, and when other hostile forces are insignificant, then there are few or no brakes on change in the traits used in the competition, and little extrinsic guidance (cf. West Eberhard 1979, 1983). (Alexander, R.D. (1989) Evolution of the Human Psyche in The Human Revolution. (eds. Mellars & Stringer) Princeton Univ. Press. Princeton p. 471)

"But what reinforces such a fashion, why does it spread? And why does it ever catch on in the first place? The fashion is fuelled by a tie between the preference gene and the ornament gene. Consider a female who has genes for preferring a long-tailed mate. Her offspring will inherit both her preference genes and her mate's long-tail genes, although the preference will be expressed phenotypically only in her daughters and the long tail only in her sons. So her union solders a connection between preferece genes and long-tail genes, a closer connection than would arise form random mating. (A measure of this tie is called the coefficient of linkage disequilibrium.) And the same will happen in subsequent generations. It is this connection that fuels the fashion. The more the females exercise a fashionable preference for long tails, the more the fashion is reinforced, each choice of long-tailed mate automatically being likely to select in favour of copies of genes for that very choice." (Cronin, Helena (1992) The Ant and the Peacock: Cambridge Univ. Press, Cambridge p. 202)

"If males are signallying to females, then those signals are ripe for exploitation by a monstrous regiment of scavergers -- predators, parasites and competing males. ... And that is by no means the only kind of cost of being attractive. Sexual selection for increased body size in male birds invariably brings with it an increase in bill size, in some cases so great that males are forced to exploit suboptimal food niches (Selander 1972). The energetic costs of the males' display may be so high that they are pushed into abandoning safe foraging options for ones that possibly give higher energy returns but are more risky (Vehrencamp and Bradbury 1984). (Cronin, Helena (1992) The Ant and the Peacock: Cambridge Univ. Press, Cambridge p. 227)

"Feedback made this development circular. As neoteny progressed, it became still more necessary for the male to support his mate and the young; as the father became more important for the long-helpless young, there was more competition among females to secure and hold helpful mates, that is, to make herself attractive, which meant more neotenic. It also made the reproductive success of the male dependent not only on insemination but on seeing his children to maturity. If he became fonder of his children and his somewhat childlike mate, the family was complete."(Wesson, Robert (1991) Beyond Natural Selection. MIT Press: Cambridge p. 273)

"Thus, the evolution of social intelligence facilitated human encephalization not by initiating a simple linear trent towards ever-greater social intelligence, but by setting up the preconditions for a sudden, capricious explosion of behavioral courtship displays. So whereas runaway sexual selection in birds tends to affect plumage, runaway selection in higher primates may tend to affect brains and behavior (see Miller, accepted, b)." (Miller, Geoffrey F. (1994) Evolution of the human brain through runaway sexual selection: the mind as a protean courtship device. unpublished thesis. pp. 409)

‘It is argued that psychosis arises as the boundary of a distribution of variation in cerebral structure generated in the course of hominid evolution. Language played a central role, with the critical changes taking place on the basis of a mutation that allowed the two cerebral hemispheres to develop with a degree of independence. Sexual selection (differing criteria in females and males in choosing a mate) acting on this genetic innovation has generated a dimension of competence in social interaction in relation to which there has been a progressive increase in cerebral size by delayed maturation (neoteny). A sexual dimorphism in cerebral asymmetry and the sex difference in age of onset of psychosis can be parsimoniously explained if a gene regulating the relative growth of the two hemispheres is X-Y homologous." (Crow TJ (1995) A Darwinian approach to the origins of psychosis. Br J Psychiatry 167(1):12-25)

"But now let us return to the situation among humans. We have seen that Homo sapiens has a queer arrangement in which both sexes must compete for partners, and both, in turn, must choose. The stage is set for trait-runaway by sexual selection to take place in an unusual two way mode -- acting not only on males, but on females as well. Human runaway sexual selection? At first glance we would seem too sensible a species for anything like that. We don't appear to have been saddled with burdensome exaggerations like antlers or bright tails. Or have we? Consider the greatest exaggeration of them all... our powerful, out-sized brains. Not only do large infant craniums put human mothers in great stress while giving birth, the brains within strike some biologists as extremely perplexing. In their cups, philosophical anthropologists can sometimes be heard wondering why humans "overshot" the mental capacity we needed in order to become masters of the planet -- in other words, competent hunter-gatherers with stone tools and fire. That was enough to remove a lot of environmental stress, and should have led to a period of equilibrium. Instead, change only accelerated, until in short order we produced encephalization capable of conceiving mathematics, spacecraft design, and music more precise than any bird or whale could ever produce." (Brin, D (1995) Neoteny and Two-way Sexual Selection. web link.)

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