Sexual Selection: The Logical Imperative

Abstract

Modern sexual selection theory, developed from Darwin's original intuition, is a cornerstone of evolutionary theory and represents the most parsimonious and robust explanation for a bewildering array of evolutionary patterns and diversity. Here we first outline the principles of modern sexual selection theory and discuss their heuristic value. Second, we review empirical demonstrations of the operation of sexual selection through the case study of the yellow dung fly. Finally, we propose that a sequence of evolutionary events flows inevitably from the early evolution of sexual recombination and gametes, to anisogamy and in dioecious organisms, to the unity sex ratio via Fisher's principle. As Darwin and Bateman predicted, it was the primary sexual difference-anisogamy-that became an almost obligatory, irreversible transition favouring socio-ecological conditions that ultimately generated secondary differentiation of sexual strategies between the sexes, and typically plays a strong part in their maintenance (though sex roles can, rather rarely, be reversed). When considered within the broader context, sexual selection emerges deductively as the logical consequence of this evolutionary succession. We conclude by highlighting aspects integral to sexual selection theory that are currently the focus of on-going discussion.

The evolution of expenditure on testes

“…A counterintuitive phenomenon exists in many immobile or weakly mobile broadcast spawners in which the sexes are separate: male GSI (i.e. RTS) is sometimes higher than female GSI (Parker & Pizzari, ). Such species often show little sexual size dimorphism and there is little reason to suppose that female resources for reproduction should be less than that of males; how male GSI can exceed female GSI is puzzling.…”

“…The opposing view is that there have been huge phylogenetic changes across evolutionary time in GSI, particularly in RTS, and that these changes relate strongly to sexual selection (Parker, ; Parker & Pizzari, ). In particular, they relate to changes in sexual selection in the sequential developments in sexual strategy that have taken place through evolutionary time (the ‘sexual cascade’; Parker, ).…”

“…At this stage, sperm competition and possibly also sperm limitation may have been intense, favouring extreme expenditure on testes, since both pressures act to increase sperm output (Lehtonen & Kokko, ; Lehtonen & Parker, ) and in the absence of mobility, somatic investment would have been directed mainly towards survival and somatic maintenance. Very high RTS in cnidarians of up to 17%, echinoderms up to 43%, and molluscs up to 35% have been reported (Parker & Pizzari, ), though many are lower.…”

“…But what selective forces determine GSI, and the ratio of female to male GSI? Both can show enormous variation, even within one taxon (see Parker & Pizzari, ), and this variation deserves adaptive interpretation. For example, both sexes of some animals can exhibit GSI in excess of 40% during the reproductive season (e.g.…”

What do isogamous organisms teach us about sex and the two sexes?

“…For example, males tend to be more competitive for fertilization opportunities than females (e.g. [91,92]). Theory suggests that this can be driven by the numerical imbalance that results from anisogamy: the producers of the more abundant gamete type have more scope to increase the fertilization probability of their gametes, which can select for such 'competitive' traits at both the pre-and post-ejaculatory stages [20].…”