Overpopulation

Overpopulation is the prospect of population growth outrunning the resources available to sustain it. The chapter treats it both as a practical human concern and as a test case for competing explanations of reproduction: whether individuals limit births for the welfare of a group, or whether reproductive outcomes are shaped by the unequal survival of offspring. Its argument turns on a distinction between consciously anticipating a crisis and the evolutionary processes that can regulate population without foresight.

Growth and its arithmetic

For the human population considered as a whole, immigration and emigration cancel out of the account, leaving births and deaths. If the average couple has more than two children who themselves survive to reproduce, population tends to grow by a proportion of its existing size rather than by a fixed number. As the total becomes larger, the increment grows too, making the process accelerate. The chapter therefore presents overpopulation not as a simple matter of adding the same number of people in every generation, but as compounding growth.

The generation interval matters as well as the number of children. Because growth accumulates across generations, spacing generations farther apart slows the rate of increase measured per year. The chapter makes the point by contrasting a limit on family size with a later start to childbearing: population growth depends not only on how many children are born, but also on when they are born.

A deliberately hypothetical calculation gives this acceleration a scale. Starting from a population of roughly 300 million in Latin America, continued growth at the then-current rate would, in less than 500 years, produce a standing human carpet over the continent, even if each person were assumed to be very skinny. A thousand years out, the imagined population would be standing more than a million deep; after two thousand years, the account extends the absurd projection to a human mountain travelling outward at the speed of light and reaching the edge of the known universe. These are not forecasts. Their escalating impossibility is the point: unchecked exponential growth cannot be extrapolated as though practical limits did not exist.

Limits and possible checks

The text names famine, plague, and war as practical checks on the hypothetical expansion, and birth control as a possible alternative. Improvements in agriculture may temporarily increase food production, but they cannot provide a permanent mathematical answer to continuing growth. The chapter further argues that medical advances, by reducing deaths, can intensify the underlying problem if births remain uncontrolled. In its stark formulation, absent effective birth control or mass emigration into space at an impossible scale, excessive birth rates lead to greatly increased death rates.

The discussion connects this prediction to leaders who forbid effective contraceptive methods while preferring what they call natural limits on population. The chapter’s rebuttal is that starvation is itself a natural means of population limitation. The contrast is between deliberate prevention of births and a later reduction in numbers through avoidable deprivation and death. The argument is not that the imagined human carpet will literally appear; it is that no increase in food supply can indefinitely erase the consequences of growth that continues faster than its constraints.

Human foresight and evolutionary explanation

The concern about long-term population growth is framed as concern for the future welfare of humanity as a whole. Some humans can consciously anticipate disastrous consequences and alter their choices accordingly. The chapter contrasts this foresight with the action of genes, which cannot see into the future and cannot be expected to pursue the welfare of an entire species. This contrast helps explain why a policy that seems prudent at the human level does not automatically explain how a corresponding behaviour evolved in animals.

The evolutionary argument takes place within a broader account of genes and behaviour. A survival machine contains many thousands of genes whose effects are interdependent: no single gene builds a whole body, and development depends on interactions among genes and with the environment. Genes can nevertheless influence the nervous systems that make behaviour more likely. In this account, genes are the primary policy-makers and brains the executives: genes shape the machinery, while nervous systems make moment-to-moment decisions and, in some cases, can use learning and prediction. The possibility that humans may refuse to have as many children as they could is presented as a special case of that capacity, not as evidence that genes themselves plan for the future.

For a behaviour, altruistic or selfish, to evolve, the relevant gene must survive in the gene pool more successfully than a rival allele associated with a different behaviour. A gene for altruistic behaviour is therefore shorthand for a gene that influences the development of a nervous system so that altruistic behaviour is more likely. This distinction bears directly on population control: a beneficial consequence for the group does not by itself show that individuals evolved to limit reproduction for the group’s sake. The explanation must account for how the behaviour affects the transmission of genes.

The dispute over animal birth rates

The chapter locates the controversy over group selection chiefly in the explanation of population regulation. Wynne-Edwards proposed that animals deliberately and altruistically reduce their birth rates for the good of the group. On this account, groups whose members restrain their reproduction are less likely to exhaust their food supply and go extinct than groups whose members reproduce rapidly; group selection would consequently favour populations of restrained breeders. This proposal gives animal birth control a group-serving purpose, but the author emphasizes that it must explain how a reduction in individual reproduction can evolve when ordinary natural selection appears to favour reproductive success.

The competing account accepts many of the observations without accepting that explanation. Wild populations do not grow indefinitely. Some remain roughly stable as births and deaths keep pace; others fluctuate sharply. Lemming numbers can pass through explosions, crashes, and near extinction, while estimates of Canadian lynx populations based on pelts sold by the Hudson's Bay Company suggest rhythmic oscillations. In some cases, a population in a local area becomes extinct. These patterns show that population sizes are limited or variable; they do not, by themselves, decide whether regulation evolved for the good of the group or through selection acting on individuals.

The chapter notes that wild animals rarely die of old age: starvation, disease, and predators usually kill them earlier, and many die in childhood or before developing beyond the egg stage. Such deaths are the ultimate checks on indefinite growth. Wynne-Edwards argued that animals could avoid these outcomes by regulating births. The opposing evolutionary view does not deny that birth rates are regulated, or that species tend to have characteristic clutch or litter sizes. The disagreement is over why: whether restraint is altruistic, benefiting the group, or selfish, benefiting the reproducing individual. The distinction is explanatory, not a denial of the observed fact that animals do not produce an infinite number of offspring.

Resources, reproduction, and consequences

The account of David Lack’s theory makes the individual-level alternative concrete. Producing and raising young costs food and energy: a mother invests in eggs, often with a mate’s help in building a nest, and the parents may spend weeks incubating before bringing food to the hatchlings. A parent Great tit, for example, brings an average of one item of food to the nest every 30 seconds of daylight. If limited resources are divided among too many offspring, fewer may be reared than if the parents began with a smaller brood.

On the Lack account, natural selection adjusts initial clutch or litter size to make use of the resources a female or mated pair can actually muster. Parents with too many offspring are penalized not because their whole population necessarily becomes extinct, but because fewer of their own children survive to reproduce. Genes associated with excessive reproduction are consequently passed on in smaller numbers. Birth limitation, in this explanation, can increase the number of surviving descendants an individual leaves; its evolutionary effect is the opposite of sacrificing reproduction for the group’s benefit.

The chapter contrasts this natural resource constraint with modern human arrangements. Where a couple has more children than it can feed, the state may step in and keep the surplus children alive and healthy. The account describes the welfare state as changing the family’s role as an economically self-sufficient unit, and warns that guaranteed support could be abused. In nature, by contrast, parents who have more children than they can support do not leave many grandchildren, because the children carrying the relevant genes are less likely to survive. The comparison is used to explain why the resource-based mechanism that limits reproduction in animal populations does not operate in exactly the same way when collective human support intervenes.

Overpopulation thus has two linked meanings in the chapter: a compounding demographic danger when births continue to outpace replacement, and an evolutionary question about how reproduction is regulated. Human foresight can recognize the danger and seek deliberate limits; natural selection, in the account offered here, requires no such foresight. It can favour reproductive restraint when parents with limited resources leave more surviving descendants by raising fewer young. The distinction between a group’s future welfare and an individual’s reproductive success is what makes population regulation a central test of the book’s gene-centred argument.