Swamping argument

The swamping argument is an objection against Darwinism made by Fleeming Jenkin. He asserted that an accidentally-appearing profitable variety cannot be preserved by natural selection in the population, but should be 'swamped' with ordinary traits. Later, population genetics helped to overcome this logical difficulty.

Jenkin published his article "The Origin of Species" in the North British Review in June 1867.

... Suppose a white man to have been wrecked on an island inhabited by negroes.... Our shipwrecked hero would probably become king; he would kill a great many blacks in the struggle for existence; he would have a great many wives and children, while many of his subjects would live and die as bachelors.... Our white's qualities would certainly tend very much to preserve him to good old age, and yet he would not suffice in any number of generations to turn his subjects' descendants white.... In the first generation there will be some dozens of intelligent young mulattoes, much superior in average intelligence to the negroes. We might expect the throne for some generations to be occupied by a more or less yellow king; but can any one believe that the whole island will gradually acquire a white, or even a yellow population...?

Here is a case in which a variety was introduced, with far greater advantages than any sport every heard of, advantages tending to its preservation, and yet powerless to perpetuate the new variety.^[1]

Darwin agreed that a variation originating in a single individual would not spread across a population, and would invariably be lost. In the fifth edition of On the Origin of Species he responded:

I saw, also, that the preservation in a state of nature [as opposed to under domestication] of any occasional deviation of structure, such as a monstrosity, would be a rare event; and that, if preserved, it would generally be lost by subsequent intercrossing with ordinary individuals. Nevertheless, until reading an able and valuable article in the 'North British Review' (1867), I did not appreciate how rarely single variations, whether slight or strongly-marked could be perpetuated.^[2]

Darwin concluded that natural selection must instead act upon the normal small variations in any given characteristic across all the individuals in the population.

The swamping argument was secondary to Jenkin's central thesis. Jenkin asserted that the population-average of any characteristic of an organism could be modified by selection (natural or human), but only within certain definite bounds. He further asserted that once selective pressure was removed, the population would revert to its original condition. Jenkin then introduced the swamping argument to deny the possibility that an occasional monstrous individual could supply an escape from this state of affairs.

Notes

See also

• Haldane's dilemma

External links

• Bulmer, Michael. Did Jenkin’s swamping argument invalidate Darwin’s theory of natural selection? // The British Journal for the History of Science (2004), 37:3:281-297 Cambridge University Press