The last five chapters of R. A. Fisher's Genetical Theory of Natural Selection - about a third of the book - are devoted to human evolution. These chapters are seldom quoted and probably seldom read, even by Fisher enthusiasts. [Note 1]

There are some obvious reasons for this neglect. Much of this part of Fisher's book is concerned in a broad sense with eugenics, the very mention of which is sufficient to paralyse rational thought in some quarters. But even for those who are not scared of the e-word, there would be reasons for disregarding these chapters. The evidence on which Fisher relies is thin and out-of-date. His evidence on the heritability of human fertility, which is central to his arguments, depends entirely on studies of the British aristocracy, which is hardly a representative sample of the species. Apart from this, like many of his contemporaries (in the 1930s) Fisher believed that current fertility trends were dysgenic: that Britain (and other western nations) were threatened by a decline in the genetic quality of the population. For example, the psychologist R. B. Cattell estimated in 1937 that average IQ in Britain was falling at a rate of about 1 percent per decade. The snag with the dysgenic hypothesis is that the period since the 1930s has seen a large improvement in almost all measurable aspects of human 'quality': IQ, educational achievement, height, general health, and longevity. The average man or woman in Britain today lives about 20 years longer and has an IQ about 20 points higher than in Fisher's day (by 1930 norms). It is possible to argue, like Richard Lynn [Note 2], that an underlying genetic decline has been masked by an even larger environmental improvement, but from a practical point of view pessimists like Fisher and Cattell have been refuted by events.

Nevertheless, there is much in Fisher's neglected chapters which is interesting and worth reading, and this post is intended as a brief taster....



Why Human Evolution is Special

Fisher complains that the treatment of man in general works on evolution is usually superficial, and emphasises that human evolution is interesting and unusual enough to deserve extended treatment. He points out (p.192) that any animal that has undergone profound changes in its recent evolutionary history should be of special interest to the evolutionist. He mentions some of the more obvious special features of man - big brain, exceptional social organisation, use of artificially constructed tools, symbolic communication - and concludes that 'to the non-human observer mankind would present a number of highly interesting evolutionary inquiries and would raise questions not easily to be answered only by the use of comparisons and analogies'(p.192) . But the distinctive perspective he gives to human evolution is that, in contrast to most other species, natural selection in man operates mainly through differences in fertility, rather than mortality, and that it now operates (at least in 'civilized' man) with exceptional intensity (p.218, 228). He goes on to explore the interaction of social class, fertility, and sexual selection, and concludes that the combination of conditions producing an acceleration of evolutionary changes is 'peculiar to man' (p.269). Far from thinking that in modern man evolution has come to a halt, as some modern evolutionists (e.g. S. J. Gould, Steve Jones) have claimed, Fisher therefore believes that it is exceptionally rapid, and capable of producing significant changes even during recorded historical times.

The Evolution of Fertility

Fisher makes some brief but important general remarks on the evolution of fertility, which are applicable to all species, and not just to man. Despite this, his remarks have been generally overlooked. [Note 3] He argues that fertility, like any other trait, is subject to natural selection, and that the most important factor in determining optimal fertility is the amount of parental expenditure required: 'In organisms in which that degree of parental expenditure, which yields the highest proportionate probability of survival, is large compared to the resources available, the optimal fertility will be relatively low' (p.204). In 'civilized' man, the most important determinants of fertility are psychological. There are factors of temperament which determine the propensity to marry, whether marriage is early or late, and the degree of enthusiasm for children (p.210-13). But there are also social and institutional factors such as prohibitions on infanticide (p.218-21). These factors will themselves be affected by psychological influences which will vary over time, (p.219), since parents who are reluctant to commit infanticide will have more children surviving, and the children will tend to inherit their parents' temperament. Fisher argues that this is responsible for the changing historical views on infanticide, and minimises the role of religious doctrine, which itself (he argues) is responsive to the general mood of the population. In a splendidly Fisherian phrase he remarks: 'It would, I believe, be a fundamental mistake to imagine that the moral attitude of any religious community is to any important extent deducible from the intellectual conceptions of their theology (however much preachers make it their business so to deduce it)' (p.222) .

Man versus Social Insects

In several places (p.199-204, 271-2) Fisher compares and contrasts human and insect societies. He stresses the major difference that in insect societies reproduction is specialised in a reproductive caste, often with a single queen. An insect society therefore 'more resembles a single animal body than a human society' (p.200) and 'selection must in this case act exclusively on the reproductive insects via the prosperity of the societies from which they arise' (p.201). In the light of modern sociobiology this emphasis on the reproductive system may seem blindingly obvious, but in Fisher's time it was not, and even in the 1950s writers like A. E. Emerson still tended to neglect it. Fisher also has a most interesting comment on the origins of insect societies, suggesting that 'as soon as the young adults of any incipient social form took either to performing the preparatory labour for reproduction, or to tending the young, before they themselves had commenced to reproduce, the balance of selective advantage would have been shifted towards favouring the fertility of the foundress of the colony, and towards favouring equally the development of the organs and instincts of workers rather than of queens among her earlier, and possibly less well nourished, offspring' (p.205).

In human societies, in contrast, reproduction remains individualistic, and genetic competition within communities is always present. Fisher does not entirely dismiss the importance of inter-group selection: 'Among small independent competing tribes the elimination of tribes containing an undue proportion of the socially incompetent, and their replacement by branches of the more successful tribes, may serve materially to maintain the average standard of competence appropriate to that state of society' (p.201). But even in this state of society competition within the community is present, and becomes more important as the size of groups increases (p.201). He later points out that 'The selection of whole groups is, however, a much slower process than the selection of individuals, and in view of the length of generation in man the evolution of his higher mental faculties, and especially of the self-sacrificing element in his moral nature, would seem to require the action of group selection over an immense period' (p.264. Incidentally, this is the first use of the exact phrase 'group selection' I have noticed in the literature. Sewall Wright, around the same time, uses 'intergroup selection'.) Fisher concludes that the main force in the evolution of such qualities has been individual selection, but powerfully enhanced by the action of kinship groups and sexual selection, which in the case of man also involves decisions by kinship groups. I will discuss this further in another post.

To be continued, probably after Christmas.....

Note 1: I will give page references to the easily available Dover edition (1958). There are no relevant changes from the first edition. Among Fisher's admirers, W. D. Hamilton does in his very first published paper refer to the 'human' chapters of GTNS (see Narrow Roads of Gene Land, vol. 1, p.8), but elsewhere does not, even when (as in his essay 'Innate social aptitudes of man') they would be highly relevant.

Note 2: Lynn has written a book, Dysgenics, and various articles on this theme. He attributes the increase in average IQ (the Flynn Effect) mainly to improved nutrition. The awkwardness of his position is that his argument for dysgenic effects requires the genetic influence on individual IQ to be large, while his interpretation of the increase in IQ also requires the influence of environment to be large - larger in fact than the entire observed Flynn Effect, since this is the net result of a negative genetic trend and a positive environmental effect. This combination of requirements is not logically impossible, but it is uncomfortable.

Note 3: the modern theory of the selection of optimal fertility is usually credited to David Lack, who gathered empirical evidence for it, but the key concept of optimal parental investment is contained not only in Fisher but in various other writers. Fisher himself credited the concept to Major Leonard Darwin.

Labels: Burbridge, Human Evolution