The year 2009 is very remarkable, being 200 years since the birth of Charles Darwin and 150 years since the publication of On the origin of species by means of natural selection (Darwin 1859). The phenomena of sexual conflict and sex allocation, which are the subjects of this special issue of Biology Letters, are rooted very firmly in the scientific observations made by Darwin. It is interesting, therefore, to consider what information has become available that has allowed us to extend the initial Darwinian tenets of evolutionary biology into the mature study of conflicts over sexes and sex functions that we have today. There are many technological advances of course that allow ever-greater insights into mechanisms. However, the important advances since the time of Darwin, in terms of understanding evolutionary conflicts of interest, are the incorporation of the mechanism of heredity into evolutionary biology, and the universal explanatory power of taking a gene's eye view of evolution (Hamilton 1964; Dawkins 1976).
It is a mark of the genius of Darwin that he realized when there were general problems that needed to be explained. For example, he noted that the peacock's tail was an example of an obvious ‘non-utilitarian’ trait, which seemed to decrease survival. This was problematic because it was not obvious how such phenotypes could be favoured under natural selection. The problem was resolved, however, when Darwin had the fundamental insight that non-utilitarian phenotypes could be selected if they increased the mating success of their bearers. This idea formed one of the two major themes of The descent of man and selection in relation to sex (Darwin 1871). A similar paradox pertains to the study of sexual conflict: how it is that phenotypes that harm females but benefit males can be selected? The answer comes from adopting, either explicitly or implicitly, a gene-centred view of evolution, a perspective that was first developed in the 1930s by the population geneticists Fisher (1930), Wright (1931) and Haldane (1932). The importance of taking a gene's eye view in the study of behaviour was not realized until the seminal works of Hamilton (1964), Maynard Smith (1964), Trivers (1972, 1985), Dawkins (1976) and Parker (1979), and collectively this research revolutionized the study of social evolution, sex allocation and sexual conflicts. For example, in terms of sexual conflict, it was realized that phenotypes that increase male reproductive success, but at a cost to the current female mating partner, can be selected because they can lead to more efficient transmission of the genes that control those phenotypes. The conflict is ultimately rooted in the different optima for reproductive processes in males and in females (Parker 1979), or in male versus female function (Charnov 1979). The result is that the evolutionary interests of the sexes or sex functions are often not completely aligned (except under complete and lifetime monogamy). The coevolution between males and females that can result from sexual conflict seems to be a particularly potent evolutionary facilitator, leading to the evolution of reproductive novelty, reproductive isolation and, potentially, speciation.
In this special issue, we bring together contributions on sexual conflict and sex allocation. This brief introduction aims to highlight how each of the contributions expand upon major themes that were first laid out in a series of important works that covered sexual conflicts and sex allocation in non-social and social species with and without separate sexes (Fisher 1930; Trivers 1972, 1985; Benford 1978; Charnov 1979; Parker 1979). I draw together where possible, shared themes to look for commonalities and for emergent properties. The effective integration of the study of sexual conflict and sex allocation adds an extra dimension to our understanding of evolutionary conflicts in general. The hope is that the collection of papers in this special issue may help to facilitate this.