Why Are Sex and Recombination So Common? Lilach Hadany and Josep M. Comeron Department of Biology, University of Iowa, Iowa City, Iowa, USA The abundance of sex and recombination is still one of the most puzzling questions in the theory of evolution: Most models find that recombination can evolve, but only under a limited range of parameters. Here we review the major models 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- Address for correspondence: Lilach Hadany, Department of Biology, University of Iowa, Iowa City, IA 52242. Voice: 319-335-3669. lilach- hadany@uiowa.edu tive success for the carrier but decreased suc- cess for its partner(s) and often for the entire population (Rice & Chippindale 2001a). Con- flicts can also arise in a sexual population be- tween different genes in the genome because their long-term interests are no longer identi- cal: This includes segregation distorters (Lyttle 1991), conflicts between organelle and genome (Hurst 1993), and between the genome and parasitic DNA such as transposable elements. Altogether, the costs of sex are likely to exceed twofold often, implying that sex is maintained in the world because of a greater than twofold advantage. The evolution of sex and recombination has been modeled at two levels: The first considers their effect on the long-term sur- vival of the population as a whole, asking when a higher rate of sex/recombination would increase the average fitness of the popula- tion at equilibrium (Muller 1964; Crow & Kimura 1965; Lewontin 1971). The second (Nei 1967) considers the fate of an allele mod- ifying the rate of sex or recombination, ask- ing when a new modifier that changes their frequency would increase from rarity within the population because of the forces of natural selection. Ann. N.Y. Acad. Sci. 1133: 26–43 (2008). C  2008 New York Academy of Sciences. doi: 10.1196/annals.1438.011 26