This material is published by Nature (16 March 2000, vol 404, issue 6775, pp. 281-285).

The ecological cost of sex

C.P. Doncaster, G.E. Pound and S.J. Cox

Why sex prevails in nature remains one of the great puzzles of evolution. Sexual reproduction has an immediate cost relative to asexual reproduction, as males only express their contribution to population growth through females. With no males to sustain, an asexual mutant can double its relative representation in the population in successive generations. This is the widely accepted 'twofold cost of males'. Many studies have attempted to explain how sex can recoup this cost from fitness benefits associated with the recombination of parental genotypes, but these require complex biological environments that cycle over evolutionary timescales. In contrast, we have considered the ecological dynamics that govern asexual invasion. Here we show the existence of a threshold growth rate for the sexual population, above which the invasion is halted by intraspecific competition. The asexual population then exerts a weaker inhibitory effect on the carrying capacity of the sexual population than on its own carrying capacity. The stable outcome of this is coexistence on a depleted resource base. Under these ecological circumstances, longer-term benefits of sex may eventually drive out the asexual competitor.

Further details of the work, and Nature Science Update

Correction to article (published in Nature, 405: (6784) 376-376 MAY 18 2000): In Fig. 2a, the right-hand label on the horizontal axis should have read k 2/a 21 and the axis label on the horizontal axis should have been 'Abundance of Predator-1 (N 1)'.

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