Malthus and Evolution
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[edit] Introduction
- In October 1838, that is, fifteen months after I had begun my systematic inquiry, I happened to read for amusement Malthus on Population, and being well prepared to appreciate the struggle for existence which everywhere goes on from long- continued observation of the habits of animals and plants, it at once struck me that under these circumstances favourable variations would tend to be preserved, and unfavourable ones to be destroyed. The results of this would be the formation of a new species. Here, then I had at last got a theory by which to work. [1]
Wallace, influenced in the same way by Malthus, wrote:
- [T]he most important book I read was Malthus’ An Essay on the Principle of Population [...] It was the first work I had yet read treating any of the problems of philosophical biology, and its main principles remained with me as a permanent possession, and twenty years later gave me the sought-after clue to the effective agent in the evolution of organic species. (My Life, Alfred Russel Wallace [2])
[edit] Malthus and population
In his work An Essay on the Principle of Population , Malthus observed that population, if unchecked, would increase exponentially; that is, given a constant birthrate, there would be a constant period of time over which the human population would double; at the time when he was writing (1798) he conservatively estimated this period at twenty-five years.
Malthus also predicted that the increase in our ability to produce food would not rise in proportion with the population, and that this would ultimately lead to mass starvation, if other factors such as war and disease did not reduce the population. So far, we are pleased to note, Malthus has been wrong: agricultural and technological advances have not only allowed us to increase our food supply, but also to reduce the proportion of the population needed to produce it. In the long run, Malthus will prove to be right: there must be an upper limit to the number of people the Earth can sustain, and as we approach this limit, we shall have to control our population or lose some proportion of it to starvation. Malthus' own solution to the population crisis that he foresaw was to prescribe abstinence from sex and late marriage. When and if the time comes, we can think of more popular measures.
[edit] Darwin and Malthus
Malthus' grim predictions of overpopulation were among the considerations that led Charles Darwin to formulate the theory of evolution. His reasoning was as follows: because species suffer from disease and predation and so forth, natural selection operates on them, favoring those organisms which are good at avoiding predators, fighting off disease, et cetera. However, if such constraints on the population did not exist, and there were no other selective pressures acting on a species, they would still, as a very consequence of the lack of checks on their population, find themselves in competition for finite resources of food, and so natural selection would still operate.
Hence, he concluded, natural selection must be operating all the time on all species.
[edit] Misconceptions
The point Darwin was making is sometimes misunderstood. He does not claim that starvation through overpopulation is the only cause, or even the main cause, of natural selection. His point, as we have stated, is that if there were no other forces (predation, disease, and so forth) acting on a species, then as a consequence of this fact, the species would undergo a Malthusian population explosion in which some must inevitably starve, and so natural selection would still operate.
His argument, then, is not for the ubiquity of starvation as the driving force for natural selection, but simply for the ubiquity of natural selection.
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[edit] Appendix: Darwin on "the doctrine of Malthus"
- A struggle for existence inevitably follows from the high rate at which all organic beings tend to increase. Every being, which during its natural lifetime produces several eggs or seeds, must suffer destruction during some period of its life, and during some season or occasional year, otherwise, on the principle of geometrical increase, its numbers would quickly become so inordinately great that no country could support the product. Hence, as more individuals are produced than can possibly survive, there must in every case be a struggle for existence, either one individual with another of the same species, or with the individuals of distinct species, or with the physical conditions of life. It is the doctrine of Malthus applied with manifold force to the whole animal and vegetable kingdoms; for in this case there can be no artificial increase of food, and no prudential restraint from marriage. Although some species may be now increasing, more or less rapidly, in numbers, all cannot do so, for the world would not hold them.
- There is no exception to the rule that every organic being naturally increases at so high a rate, that if not destroyed, the earth would soon be covered by the progeny of a single pair. Even slow-breeding man has doubled in twenty-five years, and at this rate, in a few thousand years, there would literally not be standing room for his progeny. Linnaeus has calculated that if an annual plant produced only two seeds (and there is no plant so unproductive as this) and their seedlings next year produced two, and so on, then in twenty years there would be a million plants. The elephant is reckoned to be the slowest breeder of all known animals, and I have taken some pains to estimate its probable minimum rate of natural increase: it will be under the mark to assume that it breeds when thirty years old, and goes on breeding till ninety years old, bringing forth three pairs of young in this interval; if this be so, at the end of the fifth century there would be alive fifteen million elephants, descended from the first pair. (Charles Darwin, On the Origin of Species, chapter 3 [3])
