vol. 165, supplement the american naturalist may 2005 Quantitative Genetic Models of Sexual Conﬂict Based on Interacting Phenotypes Allen J. Moore 1,* and Tommaso Pizzari 2,† 1. Faculty of Life Sciences, University of Manchester, Oxford Road, Manchester M13 9PT, United Kingdom; 2. School of Biological Sciences, University of Leeds, Leeds LS6 9JT, United Kingdom abstract: Evolutionary conﬂict arises between reproductive part- ners when alternative reproductive opportunities are available. Sexual conﬂict can generate sexually antagonistic selection, which mediates sexual selection and intersexual coevolution. However, despite in- tense interest, the evolutionary implications of sexual conﬂict remain unresolved. We propose a novel theoretical approach to study the evolution of sexually antagonistic phenotypes based on quantitative genetics and the measure of social selection arising from male-female interactions. We consider the phenotype of one sex as both a ge- netically inﬂuenced evolving trait as well as the (evolving) social environment in which the phenotype of the opposite sex evolves. Several important points emerge from our analysis, including the relationship between direct selection on one sex and indirect effects through selection on the opposite sex. We suggest that the proposed approach may be a valuable tool to complement other theoretical approaches currently used to study sexual conﬂict. Most importantly, our approach highlights areas where additional empirical data can help clarify the role of sexual conﬂict in the evolutionary process. Keywords: indirect genetic effects, interacting phenotypes, sexual conﬂict, sexual selection, sexually antagonistic selection, social selection. The theoretical predictions for evolutionary outcomes aris- ing from sexual conﬂict have been extensively investigated. Evolutionary conﬂict between reproductive partners arises over multiple reproductive decisions whenever reproduc- tion is costly and alternative reproductive opportunities are available (Trivers 1972; Dawkins 1976; Parker 1979; Holland and Rice 1999). When this happens, the ﬁtness * E-mail: allen.j.moore@manchester.ac.uk. † Present address: Edward Grey Institute, Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, United Kingdom; e-mail: tommaso.pizzari@zoology.oxford.ac.uk. Am. Nat. 2005. Vol. 165, pp. S88–S97.  2005 by The University of Chicago. 0003-0147/2005/1650S5-40798$15.00. All rights reserved. interests of partners diverge, and by increasing its own ﬁtness, an individual may simultaneously reduce that of its partner (Trivers 1972; Dawkins 1976; Parker 1979). This (inter)sexual conﬂict may generate sexually antagonistic selection promoting sexually antagonistic traits that confer a ﬁtness advantage to members of one sex at the expense of their partners’ ﬁtness (Parker 1979; Chapman et al. 2003; Pizzari and Snook 2003), that is, trait z in sex i is antagonistic if and , where Cov ( z , q ) 1 0 Cov ( z , q ) ! 0 i i i j q i and q j are the ﬁtness of members of the ith and jth sex. This creates potential for an evolutionary arms race be- tween the sexes driven by the rapid coevolution of sex- limited, counteracting, antagonistic traits (Parker 1979; Rice 1996; Alexander et al. 1997; Rice and Holland 1997; Holland and Rice 1998). The occurrence of sexually antagonistic coevolution de- pends on whether natural selection on female resistance overrides indirect effects determined by opposing sexual selection on male manipulation (Parker 1979; Cordero and Eberhard 2003; Eberhard and Cordero 2003; Kokko et al. 2003; Pizzari and Snook 2003). For example, whether the spread of a male allele conferring a reproductive advantage to males while imposing a cost on females favors female alleles coding for resistance to antagonistic coevolution or for preference for such a male mutant in granddaughters and subsequent generations (Parker 1979). While the mag- nitude of indirect selection may be relatively small (Kirk- patrick and Barton 1997), males have typically higher var- iance in ﬁtness than females, suggesting that sexual selection on male stimulation and indirect selection on female response to such stimulation may exceed natural selection on female response in intensity (Shuster and Wade 2003; Pizzari and Snook 2004). Sexually antagonistic coevolution provides a critical al- ternative to traditional models of sexual selection that view sexual coevolution as an essentially mutualistic process, where female preference for a male type is directly and/ or indirectly selected (Fisher 1915; Andersson 1994; Mead and Arnold 2004). Testing the extent to which sexual co- evolution is antagonistic is therefore crucial to understand- ing the functional signiﬁcance of female preference and