ways in which it might be falsified by natural as well as by artificial ... have arisen in response to natural selection; ... can expect to survive to rear families of their.
NATURAL REGULATION OF NUMBERS IN HUMAN POPULATIONS By C. B. GOODHART M.A., Ph.D., University Museum of Zoology, Cambridge RECENT discussions on the future of the human race have tended to be .A..Xrather discouraging.1' 2 Malthus's fears proved to be unfounded under the quite exceptional conditions of the nineteenth century but it is usually taken for granted that his basic propositions remain unassailable. The population of the world has certainly been increasing geometrically over the past few generations and if this continues it is bound eventually to outgrow the available food supply, when war,'famine or pestilence will intervene to limit any'further increase. It is true that in some animal species there may be other little understood factors which limit population growth3 but these are not thought to have much relevance to man, for whom the only solution appears to be the immediate and universal application of a policy of deliberate family limitation,4 and this is not likely to be either practicable or effective in time to save us from the crash. It is, however, important to remember that the arguments of Malthus, though mathematically sound, are based upon the assumption that the potential rate of population growth in man has some specific and constant value which will be the same in the future as in the past unless it is artificially altered. This assumption remains unproven, and it may be worth while to consider whether there are any possible ways in which it might be falsified by natural as well as by artificial means. Man has, as a matter of fact, a rather unusually low reproductive rate compared with most other animals. Women are fertile for less than half their normal lives, ovulation is inhibited during lactation and multiple births are rare, so that a woman who rears a dozen children in her lifetime is considered highly fertile. This number
could, however, be greatly increased without requiring any fundamental physical modifications to the human body, and there is no reason to doubt that these checks to fertility have arisen in response to natural selection; if there were any real selective advantage in higher fecundity it could easily have been evolved. Is it possible that the mean fertility of man could be still further lowered by natural selection? If it were halved during the next five generations that would probably be enough to solve our problems. At first sight this may seem unlikely since evolution is usually thought of as a slow process working through hundreds of generations. There is, however, one exception to this rule; where natural selection is operating in a population already genetically heterogeneous or polymorphic for the characters involved, and especially where the polymorphism results from a balance between two competing types each having a selective advantage over the other in certain circumstances, any variation in selection pressure will alter the balance and lead quite rapidly to an important change in the mean value by altering the proportions of the two opposing types in the population as a whole. To apply this argument to human populations requires two assumptions; firstly that fecundity in man is under hereditary control, which is more than likely, and secondly that man is genetically heterogeneous for this character, a rather more doubtful suggestion for which however there is some supporting evidence. In man, of course, fertility depends not only on natural fecundity but also on psychological and social factors which may limit the fertility of potentially fecund individuals. Both physiological fecundity and these other factors may well be, at least to some extent, under genetic control. 173
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Increase in World Population The'present rate of increase in the numbers of man is quite exceptional; up to a couple of centuries ago the population of the world was more or less stable or only slowly rising, and it has been estimated at about goo million in i8oo. By I900 it had increased to i,6oo million, at the present day it is estimated at 2,500 million and it seems certain to be 3,000 million before the end of this century. The rate of increase has however not been the same in all races and classes of mankind. Most attention is rightly paid to the peoples of Asia and Africa who are multiplying at such an extraordinary rate, but during the same period many races have had stable or declining numbers and a few seem to be in danger of extinction. The African Bushmen, Australian aborigines, Mongolians and Eskimos should not be forgotten when the present mean rate of increase in the world's population is extrapolated into the, future. A century ago Western Europeans had as high a rate of natural increase as any race before or since, and yet it has been arrested, admittedly partly by artificial means, in four generations. The same could happen to other races as well, and artificial family limitation is not necessarily the only factor involved, even in Europe.
Factors Controlling Fecundity The selective advantages of fecundity are obvious and they are particularly strong in dense populations living under unhygienic conditions subject to a high rate of infant mortality and to acute epidemic disease. Where only a fraction of the children born can expect to survive to rear families of their own a large family has a better chance of being represented in the next generation than a smaller one, irrespective of the quality of the individuals involved; many of the less fertile families will be completely eliminated. It follows, therefore, that in dense disease-ridden populations selection in favour of fecundity and resistance to disease will be relatively stronger than for most other qualities, and if mortality is lowered by improved conditions there will
be a sudden and rapid rise in numbers. However, in primitive races of pastoral nomads or hunters, living widely separated under hygienic conditions, disease may be less important and the principal limiting factor will be periodical shortages of food. Here fecundity loses much of its selective advantage compared to other qualities such as strength and intelligence, and it may even become a liability. In a bad year an Eskimo might be able to find just enough food to keep two or three children from starving but if it had to be shared among ten all would die. It is known that in animals fecundity can be altered by natural selection. Lack5 has shown that in some birds clutch size has become genetically adjusted to the supply of food available for feeding the brood. The same effect is found in mammals, for example the fertility of pigs has been greatly increased by domestication; the European wild pig produces a yearly litter of four or five while a Large White sow will farrow ten to twelve piglets twice a year. In man also there is good evidence of inherited differences in fertility between different families, discounting any environmental effects.7 It is perhaps significant that it is just the primitive and usually strong and healthy nomadic peoples that are unable to replenish themselves after they have been reduced by disease. The often unhealthy and apparently less virile races from dense centres of population seem able to recover their numbers almost immediately after the most disastrous epidemics of disease, and to compete successfully with any other races with whom they are brought into contact. In Fiji, for instance, the Indian settlers, who have come from densely populated and disease-ridden areas, are increasing their numbers very much more rapidly than the native Fijians, although both races enjoy much the same conditions of life and it is unlikely that artificial methods of family limitation are of much importance in either group. These are admittedly rather special cases and the argument does not apply to more civilized races whose declining numbers cannot be explained on the basis of genetical
NATURAL REGULATION OF NUMBERS selection by famine. It is notorious that the most successful nations and classes of mankind are seldom able in the long run to maintain their numbers and it has been suggested that the conditions produced by civilization and a high standard of living may by themselves be unfavourable to fertility. It is certainly true that the more or less stable population in North-Western Europe at the present time owes much to the introduction of artificial methods of family limitation which are only practicable in relatively advanced and prosperous communities, but the almost universal rule of declining numbers in advanced civilizations dates back before the discovery of artificial means of bringing it about, apart from deliberate abortion, which may at some times have been of importance. The possibility that the qualities leading to success in man may have become genetically correlated with relatively low fecundity cannot be disregarded and, although this is another unproven assumption, there are some theoretical reasons for supposing that it may be justified. Under most conditions high fertility has a clear selective advantage in itself, irrespective of any other qualities with which it may be associated, but the less fecund lines will only survive if they have other hereditary qualities of high survival value to offset the disadvantages of low fertility. It has been suggested8 that where there is rigorous selection for one particular genetic character there tends. to be a loss of fitness, by a process of " genetical erosion," in all other characters subject to less intense selection. There is probably an example of this effect in man with milk yield. Women of primitive peoples usually produce a copious flow of milk, provided they receive adequate food, but a considerable proportiorn of more civilized mothers are unable to feed their own babies. This is not likely to be due directly to the easier life led by civilized women, but it is true that, since artificial feeding is now perfectly satisfactory, selection against low milk yield among them has ceased, although in less advanced peoples the only children that are likely to survive
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infancy are still those whose mothers do have an adequate supply of milk. Milk yield in cattle has been greatly increased in quite a short time by deliberate selection under domestication; in civilized man the opposite has occurred, also probably in quite a short time, when selection was relaxed. The same may have happened in man with regard to fecundity. A large family is likely to be represented in the next genera-, tion even though its members are individually of low quality, but a man with a single daughter may easily leave no grandchildren at all if she is ill-favoured, although if she is exceptionally good looking and intelligent she should have no difficulty in finding a husband. A man's choice of a wife is influenced by her appearance, intelligence, sympathetic nature, wealth and many other qualities, and these are not necessarily correlated with high fecundity. If there is this tendency in man for the more desirable qualities to become associated with low fecundity there are mechanisms by which it could become fixed in the genotype. Continuous selection against low fecundity without the other compensating qualities will favour forms in which these characters are genetically associated and in the course of time the association should become closer. There are many examples of this having occurred in polymorphic animal species. If therefore man is polymorphic for fertility (as to some extent he certainly is, since complete sterility is not uncommon) then it is likely that the polymorphism is maintained by a balance between high fecundity on the one hand and other qualities of high survival value associated with low fecundity on the other. Any such equilibrium will be disturbed if the mortality rate in early life is altered, and if it is lowered the relative advantage of high fecundity will be diminished since a larger proportion of the population will survive to an age at which the less fecund can gain the full advantage of their presumed superiority in competition. Thus as the numbers grow the proportion of families of high quality and low fertility should increase in each generation, which will progressively lower the mean fertility of a
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the population as a whole. The well-known example of industrial melanism in moths9 shows the effect of an alteration in selection pressure on such an equilibrium. Where there is predator selection against melanic forms .they remain rare, but as soon as selection pressure is relaxed other compensating qualities associated with the gene for melanism allow it to spread rapidly throughout the population. The relatively higher viability of melanic forms is probably itself a direct result of past predator selection against them, only the most viable will have survived at all. Intelligence and Fertility In advanced human communities it is recognized that the most fertile families are usually below the average in intelligence, and if this character is inherited a progressive decline in the intelligence of the population as a whole is to be expected. In fact no such decline has been found,'0 and a possible explanation of the apparent stability of the mean level of intelligence is that individually the less intelligent may have less chance of leaving descendants even though a few of them have the largest families of all. The observed correlation of low intelligence with high fertility does not necessarily, mean that as a group the less intelligent are the more fecund, the range of family size may be greater, but if there is also a higher proportion of persons who leave few or no surviving children, the mean family size may not be above that for people of normal intelligence. The number of great-great-grandchildren rather than the number of children is the better test of survival value, and a line with a consistently high proportion of smaller families of good quality may well be at an advantage, especially when indiscriminate mortality from disease is low, compared to another of lower quality with a few highly fertile individuals but with the majority leaving no descendants at all. The few highly fertile families are most conspicuous in the pedigree; the aunts and uncles who died childless may often be overlooked. This lowering of the mortality rate in civilized man is having a rather similar effect in
disturbing the sex ratio equilibrium. There has always been a slight excess in the number of boy babies born, but their mortality is higher so that by maturity- the balance has been restored. A fall in mortality has a greater effect on the survival of boys, whose mortality was originally higher, than on girls, and this is leading to an increase in the proportion of males in the whole population. Population Trends in Ireland All this is of course rather speculative, but it is instructive to consider the population problems of Ireland which do suggest that selection against fecundity may have had important results in quite a short period of time.11 Before I750 the Irish lived in conditions of great poverty and must have had a high rate of infant mortality with correspondingly high selection for fecundity. The introduction of the potato then raised the standard of living of the people and there was an extraordinary increase in the population, similar to but greater than that occurring at the same time in England and elsewhere.'2 In I845 the potatoes became infected with blight and, as an inevitable result of relying on a single food crop, there was widespread famine, disease and emigration. In the normal course of events the population, which had shown itself to be highly fertile, should have replenished itself rapidly and continued its growth with only a slight check. But this did not happen and the population of Ireland has continued to decline until it is now less than half what it was in I840, although most neighbouring nations have doubled themselves during the same period., The probable explanation of this decline is that since the famine there has been rather strong selection against large families, although artificial restriction of numbers has probably been less important in Ireland than elsewhere. In the first place large families are likely to have been worse affected by the famine than smaller ones; many small families may have had just enough food to save themselves from starvation, but where there were numerous children to share what little was available all would
NATURAL REGULATION OF NUMBERS
be weakened by hunger and would probably have died. Secondly, and perhaps even more important, emigration is likely to have been selective. An only son who has hopes of inheriting the family property, however impoverished, will be reluctant to leave, but if it is to be shared among half a dozen brothers, all will be more inclined to emigrate during bad times. By I850 the population of Ireland would have lost most of its more fecund members leaving the task of replacement to the genetically less fecund who remained. Nearly five millions of people have probably emigrated from Ireland to the United States, but the number of Americans of Irish descent at the present time is considerably more than that. The emigrants from Ireland must therefore have maintained their fecundity, unlike those left behind, even though it may be true that for social reasons the rate of increase of the Irish in America is now less than that of some other immigrant groups. For various special reasons this process will have been continued since the famine. A girl's chances of marriage in Ireland depended largely on the size of her dowry, which would be inversely related to the number of sisters who had to share it. The peasant economy required that all members of the family should remain on the farm as unpaid workers, and this tended to delay the date of marriage and so reduce the number of children born. That in itself was an important though not a genetical factor in limiting population growth, but it is also likely that the more enterprising and virile young men and the more highly sexed girls would have refused to accept these restrictions and these are the ones that will have left the country. Nor is the influence of the Roman Catholic Church to be neglected; celibate priests and nuns are probably mainly recruited from the larger families where their departure will not affect the running of the family farm and will improve the prospects of inheritance or dowry for those who remain. It would not be surprising to find that the mean number of brothers and sisters to each priest and nun in Ireland is significantly higher than among married people of the same age.
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World Population Trends If this interpretation is correct it suggests that human populations must be geneticaUy heterogeneous for factors determining fertility, for natural selection could only have produced so rapid a transformation from high to low fertility if it were operating in an
already genetically heterogeneous population. The same process may be occurring in other European nations, th6ugh more slowly than in Ireland where it has been encouraged by the special factors that have been mentioned. Throughout much of Europe the very rapid growth of population after the Industrial Revolution has now been replaced by stability, and experience suggests that this may soon be followed by a decline in numbers. Some large increase in the population of Asia and Africa in the immediate future is inevitable, but it may reasonably be doubted whether the present rate of increase will be maintained much longer, and the more that conditions of life can be improved and mortality reduced the faster it should fall. It would indeed be of the greatest importance that conditions should be improved since the factors that have been postulated as limiting the rate of natural increase can only operate when a large proportion of the children born do survive to maturity, thus lessening the overriding selective advantage of fecundity compared with all other inherited characters. Malthus's predictions proved wrong during the nineteenth century because unforeseen improvements in production were able to keep pace with the increase in population'. This cannot go on much longer, but Malthus's other assumption of a constant figure for human fecundity is also open to question; there are other perfectly possible alternatives that can be suggested. The present geometrical increase in the numbers of man certainly could be halted by natural selection and it is satisfactory to realize that famine, disease and war may not necessarily be the only natural checks available, however little reliance is to be placed on the efficacy of human wisdom and self-restraint. Conclusions It is suggested as a hypothesis that human
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populations may be genetically heterogeneous for factors affecting fertility, and that this heterogeneity is maintained by a balance of selective advantage of high fecundity on the one hand and other qualities of high survival value associated with lower fertility on the other. Such an equilibrium will be unstable and the relative advantage of the superior qualities associated with lower fertility will be greatest when mortality is low. If a large proportion of individuals are killed off indiscriminately by disease at an early age before those with potentially superior qualities are able to benefit from the effects of competition with the rest of the population there is a real danger that the superior but relatively infertile lines will become extinct. It is to be expected that in dense disease-ridden populations the mean fertility should be disproportionately high, so that improvements in hygiene and medicine will lead to a dramatic increase in numbers. As soon as the early indiscriminate deathrate from infective disease is reduced, however, the lower fertility lines will begin to benefit in full from their presumed superior competitive qualities, and so will form an increasing proportion of future generations. As a result of this the mean fertility of the population will fall and the original high rate of natural increase should taper off until a new equilibrium is established at a higher absolute level of numbers with the lower
mortality- offset by lowered fertility, and also perhaps a higher mean level of inherited quality. This is contrary to expectation on orthodox Malthusian theory, which assumes that human fecundity is invariable. There are, however, reasons for believing that this may not be so and that fecundity in man is subject to alteration by natural selection, like any other character that is genetically inh&rited.
i.
2. 3. 4.
5. 6.
7.
8. 9. io. ii.
I2.
REFERENCES Cragg, J. B. and Pirie, N. W. (Editors). 1955. The Nutmbers of Man and Animals, Oliver and Boyd. Shlyapentokh, V. E., Priroda. 1954. 3. II8. Chitty, D., in Cragg and Pirie, 1955, pp. 57-67. Darwin, Charles G., I952. The Next Million Years, Hart-Davis. Lack, D., I954. The Natural Regulation of Animal Nuimbers. Clarendon Press, Asdell, S. A., I946. Patterns of Mammalian Reproduction, Ithaca. Fisher, R. A., 1930. The Genetical Theory of Natural Selection, Clarendon Press. Harland, S.C., I955. Advancement of Science. 11, 383. Ford, E. B., 1945. Biol. Revs. 20. 73. The Trend of Scottish Intelligence, I947. I949. U.O.L.P. O'Brien, J. A., I954. The Vanishing Irish, W. H. Allen. Plumb, J. H., 1950. England in the i8th Century. Pelican Books.
