Darwin’s third premise

Darwinian natural selection is widely considered to rest on three premises (Alcock, 1998; Darwin, 1859; Davies, Krebs, and West, 2012; Freeman and Herron, 1998). First, genetic variability within a population. Second, differential reproduction within the population; and third, the overproduction of offspring greatly in excess of their possibility of survival. It is the last of these, known as Darwin’s third premise, which I consider here. To quote Darwin in the Origin of Species (Darwin, 1859), p3:

In the next chapter the Struggle for Existence amongst all organic beings throughout the world, which inevitably follows from the high geometrical ratio of their increase, will be considered. This is the doctrine of Malthus, applied to the whole animal and vegetable kingdoms. As many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurrent struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected. From the strong principle of inheritance, any selected variety will tend to propagate in its new and modified form.

The above premise hinges on the axioms that, within a population, individuals with the greatest number of offspring pass on more of their genes to subsequent generations. This, the number of surviving offspring, is one definition of fitness, (Dawkins, 1982), p183. Hence it is selection at the level of the gene that drives the production of offspring. The necessary consequence of a drive to produce offspring is competition between individuals for resources. If the severity of competition is density dependent, as is assumed, then the outcome is that density decreases variability by eliminating less fit genetic variants. In other words, density promotes normalising selection in that it removes extremes. And since natural selection requires genetic differences to choose between, it must be less strong at high densities (Williams, 1966), p.32. Much of the classical work of E. B. Ford recognises the importance of population density effects on natural selection (Ford, 1964). Thus we have a paradox which might appear to undermine the Darwinian theory of evolution, depending heavily as it does on density dependent competition in the Malthusian model.

Indeed, it has been shown, both theoretically and empirically, that animals do not often, perhaps never, overproduce offspring but instead produce only as many as can survive. For example, the work of (Lack, 1966), described by (Krebs and Davies, 1993), pp 16 – 20, shows elegantly that in Wytham Wood in Oxford, the population of great tits closely adjust clutch size, from year to year, to the number of chicks that can be fed by the parents. In other words, parents control the number of chicks that survive. This is in direct contrast to Darwin’s third premise. Peter Medawar (Medawar, 1957), p14, makes the same point: …”So far from producing a vastly excessive number of offspring, most organisms produce just about that number which is sufficient and necessary…..”.

Does this mean that selection is active only when competition is week of absent? Lying at the heart of evolution theory as it does, questions over the role of competition is highly significant, but evidently largely overlooked conclusion of George Williams, cited above. Indeed, in the words of John Endler (Endler, 1986)…. “There are almost no examples showing competition or density dependent selection among phenotypes, p. 161. A well worked out theory of evolution that does no depend heavily on competition, is what is missing. What we need is a re-think and re-framing of natural selection based not on competition but instead on something else, perhaps one driven primarily by genetical variability. By extension, the removal of competition from its central place means that evolution itself depends on variability rather than density. An emphasis on variability has been made many times before but what is required is that it replace competition at the centre of natural selection theory.

All this is not to say that the Malthusian effect is not important in evolution. After all, provided ever sexually reproducing pair produce more than two surviving offspring, we have exponential population growth. For a pair producing three offspring the progression in succeeding generation is; 3, 9, 18 etc. My aim is to shift emphasis from population growth to genetical variance within the population. 

References

Alcock, J. (1998). Animal Behaviour. (6 ed.): Sinauer Associates.

Darwin, C. D. (1859). The origin of species by means of natural selection, or the preservation of favoured races in the struggle of life. (Fascimile 1901 ed.). London: John Murray.

Davies, N. B., Krebs, J. R., and West, S. A. (2012). An Introduction to Behavioural Ecology. (4 ed.). Oxford: Wiley-Blackwell.

Dawkins, R. (1982). The Extended Phenotype. (1999 edition ed.). Oxford: Oxford University Press

Endler, J. (1986). Natural Selection in the wild. Princeton, New Jersey.: Princeton University Press.

Freeman, S., and Herron, J. C. (1998). Evolutionary Analysis. New Jersey: Prentis - Hall.
Ford, E. B. (1964). Ecological genetics. The Broadwater Press Ltd, Welyn Garden City

Krebs, J. R., and Davies, N. B. (1993). An Introduction to Behavioural Ecology. Oxford: Blackwell Scientific Publications.

Lack, D. (1966). Population Studies of Birds. Oxford: Clarendon Press.

Medawar, P. (1957). The Uniqueness of the Individual. London: Methuen.

Williams, G. C. (1966). Adaptation and Natural Selection. Princeton: Princeton University Press.