The evolutionary outcome of sexual conflict

Abstract

Inter-locus sexual conflict occurs by definition when there is sexually antagonistic selection on a trait so that the optimal trait value differs between the sexes. As a result, there is selection on each sex to manipulate the trait towards its own optimum and resist such manipulation by the other sex. Sexual conflict often leads additionally to the evolution of harmful behaviour and to self-reinforcing and even perpetual sexually antagonistic coevolution. In an attempt to understand the determinants of these different outcomes, I compare two groups of traits-those related to parental investment (PI) and to mating-over which there is sexual conflict, but which have to date been explored by largely separate research traditions. A brief review suggests that sexual conflict over PI, particularly over PI per offspring, leads less frequently to the evolution of manipulative behaviour, and rarely to the evolution of harmful behaviour or to the rapid evolutionary changes which may be symptomatic of sexually antagonistic coevolution. The chief determinants of the evolutionary outcome of sexual conflict are the benefits of manipulation and resistance, the costs of manipulation and resistance, and the feasibility of manipulation. All three of these appear to contribute to the differences in the evolutionary outcome of conflicts over PI and mating. A detailed dissection of the evolutionary changes following from sexual conflict exposes greater complexity than a simple adaptation-counteradaptation cycle and clarifies the role of harm. Not all of the evolutionary changes that follow from sexual conflict are sexually antagonistic, and harm is not necessary for sexually antagonistic coevolution to occur. In particular, whereas selection on the trait over which there is conflict is by definition sexually antagonistic, collateral harm is usually in the interest of neither sex. This creates the opportunity for palliative adaptations which reduce collateral harm. Failure to recognize that such adaptations are in the interest of both sexes can hinder our understanding of the evolutionary outcome of sexual conflict.

Yolk androgens do not appear to mediate sexual conflict over parental investment in the collared flycatcher Ficedula albicollis

“…Unfortunately, empirical testing of the hypothesis that past manipulation by females has selected males to ignore certain offspring signals may prove very difficult. However, Lessells (2006) pointed out that, in general, manipulative behaviour in sexual conflict over parental investment may be rare, as the benefits may be small and the costs could be substantial. Thus, sexual conflict over parental investment may not generate rapid evolutionary change and antagonistic coevolution.…”

“…In species with biparental care, females and males are in conflict over the amount of parental care they provide to their offspring (Trivers, 1972; reviewed by Arnquist and Rowe, 2005;Houston et al, 2005;Lessells, 2006;Hartley and Royle, 2007). For each parent, increased investment reduces the parent's future reproductive success and survival prospects (e.g.…”

“…A parent could thus try to manipulate the investment of its partner by changing either offspring traits or his or her own behaviour (e.g. Slagsvold et al, 1995;Westneat and Sargent, 1996;Arnquist and Rowe, 2005;Lessells, 2006).…”

“…The effects of yolk androgens thus seem to vary, but there is evidence that they can affect offspring traits. It has therefore been proposed that by allocating more yolk androgens into the egg, a female could affect offspring trait(s) to which males respond, and this way increase the relative feeding effort of the male to her benefit (Groothuis et al, 2005a;Michl et al, 2005;Gil et al, 2006;Lessells, 2006;Moreno-Rueda, 2007;Müller et al, 2007a).…”

Introduction. Sexual conflict: a new paradigm?

“…It may be divided up into: (i) the fitness disadvantage due to an individual being shifted away from its optimum value for a trait over which conflict occurs (the 'shared trait' (Rowe & Day 2005) or 'conflict trait' (Lessells 2006), an example might be the energetic costs to females of mating more frequently than is optimal; and (ii) harm which reduces fitness through direct effects on traits that are not subject to sexually antagonistic selection, for instance female lifespan may be reduced by seminal fluid chemicals, but lifespan is not actually a trait over which there is any conflict of interest. This second type of harm can be further divided into 'adaptive harm', where it is the costs of the harm inflicted that leads to a female changing the value of the shared trait, and 'collateral harm' which is a negative side effect in one individual of a conflict driven adaptation in another.…”