Otto.AmNat.09 - v ol. 174, supplement the american...

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vol. 174, supplement the american naturalist july 2009 The Evolutionary Enigma of Sex Sarah P. Otto * Department of Zoology, University of British Columbia, 6270 University Boulevard, Vancouver, British Columbia V6T 1Z4, Canada abstract: Sexual reproduction entails a number of costs, and yet the majority of eukaryotes engage in sex, at least occasionally. In this article, I review early models to explain the evolution of sex and why they failed to do so. More recent efforts have attempted to account for the complexities of evolution in the real world, with selection that varies over time and space, with differences among individuals in the tendency to reproduce sexually, and with populations that are limited in size. These recent efforts have clariFed the conditions that are most likely to explain why sex is so common, as exempliFed by the articles in this symposium issue of the American Naturalist . Introduction Sexual reproduction is a costly endeavor. In order to out- cross, an individual has to Fnd a potential partner, attract it, risk contracting sexually transmitted diseases, hazard predation while mating (sometimes by the mate itself), and forego opportunities to gather resources. ±or many facultatively sexual species, there is an additional cost in- volved in switching from mitotic to meiotic reproduction. ±or example, in Saccharomyces cerevisiae , mitotic replica- tion occurs in 90 min, but the induction of meiosis takes days. All of this effort would make sense if sex were a more efFcient means of transmitting genes to future generations, but it is not. A sexual parent transmits only 50% of its genes to the next generation, compared with 100% for an asexual parent. Thus, unless sexuals produce twice as many offspring per individual, sexuality suffers from a trans- mission disadvantage, a problem so acute that it has been labeled the cost of sex (Bell 1982). Last, but not least, sexual reproduction breaks apart favorable combinations of genes built by past selection. To hammer this point home, consider an analogy. Imagine entering a poker hall after a game has been played. If you were to offer the winners (holding, say, a 3 ,4 ,5 ,6 , 7 straight at one table, a three-queen hand Q l ,Q K , Q ; ,2 K ,8 l at another, etc.) the opportunity to keep their hands or to shuf²e their cards with those of another, * E-mail: Am. Nat. 2009. Vol. 174, pp. S1–S14. q 2009 by The University of Chicago. 0003-0147/2009/1740S1-50665$15.00. All rights reserved. DOI: 10.1086/599084 everybody would hold his or her cards. Winning hands— those that have “survived” previous rounds—have cards that work well with one another. Shuf²ing these cards together produces descendant hands with no guarantee of success (creating, e.g., a lousy hand of 3 ; K , 8 l ). In all card games of interest, it is not enough to know the suit and number of each card in isolation; rather, the interactions among cards are what determine whether the card is in a winning hand or a losing hand. Similarly, genes do not work in isolation; the interactions among an
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Otto.AmNat.09 - v ol. 174, supplement the american...

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