Why are sex and recombination so common

Theabundanceof sex and recombination is still one of themost puzzlingquestions in the theory of evolution: Most models find that recombination can evolve, but only under a limited range of parameters.Herewe review themajormodels and supporting evidence, concentrating on recent approaches where more realistic assumptions help explain the evolution of sex and recombination under a wider parameter range: finite populations, selection over long genomes, variation in recombination across the genome, and plas-ticity of sex and recombination. We discuss the similarities and differences between the evolution of sex and that of recombination. Key words: evolution of sex; sexual reproduction; recombination; epistasis; linkage dis-equilibrium; red queen; plasticity Recombination and sexual reproduction are ubiquitous among higher eukaryotes. Yet their adaptive value is not fully understood. The question is particularly hard when we take into account the significant costs of sex: First, if males do not contribute any resources other than genes to the offspring, then—everything else being equal—a sexual population would produce half the number of offspring of an asexual population of the same size in any given generation and is more likely to go ex-tinct than a sexual population. This is known as “the two-fold cost of males ” (Maynard Smith 1978). Second, to reproduce, a sexual organ-ism must find and court a mate. This goal is not always successful and usually requires time and energy while exposing the individual to predation or transmission of pathogens. Last, the mixing of genotypes exposes the sexual species to conflicts (Partridge & Hurst 1998). Conflicts can arise between males and females, where mutations that have a beneficial effect on one sex and a deleterious effect on the other can spread, resulting in increased reproduc