trivers - Parental investment and sexual selectLon 53...

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Parental investment and sexual selectLon 53 Parental investment and sexual selection ROBERT L. TRlVERSt Introduction Charles Darwin's (187 1) treatment of the topic of sexual selection was sometimes confused because he lacked a general framework within which to relate the variables he perceived to be important: sex-linked inheritance, sex ratio at conception, differential mortal- ity, parental care, and the form of the breeding system (mono- gamy, polygyny, polyandry, or promiscuity). This confusion permitted others to attcmpt to show that Darwin's terminology was imprecise, that he misinterpreted the function of some struc- tures, and that the influence of sexual selection was greatly over- rated. Huxley (1938), for cxample, dismisses the importance of female choice without evidence or theoretical argument, and he doubts the prevalence of adaptations in males that decrease their chances of surviving but are selected because they lead to high reproductive success. Some important advances, however, have b&n achieved since Darwin's wbrk. The genetics of sex has now been clarified, and Fisher (1958) has produced a model to explain sex ratios at conception, a model recently extended to include special mechanisms that operate under inbreeding (Hamilton 1967). Data from the laboratory and the field have confirmed that femalcs are capable of very subtle- choices (for example, Petit andEhrman 1969), and Bateman (1948) has suggested a general basis for female choice and male-male competition, and he has produced precise data on one species to support his argument. This paper presents a general framework within which to con- sider sexual selection. In it I attempt to define and interrelate the key variables. No attcmpt is made to review the large, scattered literature relevpit to sexual selection. Instead, arguments are on how one might expect natural selection to act on the sexes, and some data arc presented to support these arguments. t Reprinted by permbrlon from Bernud Campbell: Ssxuol Selection md the Dcrcent of Man (Chiago: Aldine PublisNn~ Company); Copyright O 1972 by Aldlnc Publimhing Company. Variance in rcproductive success Danvin defied sexual selection as (1) competition within one sex for members of the opposite sex and (2) differential choice by members of one sex for members of the opposite sex, and he pointed out that this usually meant males competing with each othcr for femalcs and females choosing some males rather than others. To study these phenomena one needs.accuratc data on differential rcproductive success analysed by sex. Accurate data on female reproductive success are available for many species, but similar data on males are very difficult to gather, even in those species that tend towards monogamy. The human species illustrates this point. In any society it is relatively easy to assign accurately the children to their biological mothers, but an element of uncer- tainty attaches to the assignment of children to their biological fathers. For example, Henry Harpending (personal communication)
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This note was uploaded on 04/12/2008 for the course PSYC 367 taught by Professor Christenson during the Fall '07 term at Tulane.

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trivers - Parental investment and sexual selectLon 53...

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