Mechanisms and Evidence of Genital Coevolution: The Roles of Natural Selection, Mate Choice, and Sexual Conflict

Abstract

Genital coevolution between the sexes is expected to be common because of the direct interaction between male and female genitalia during copulation. Here we review the diverse mechanisms of genital coevolution that include natural selection, female mate choice, male -male competition, and how their interactions generate sexual conflict that can lead to sexually antagonistic coevolution. Natural selection on genital morphology will result in size coevolution to allow for copulation to be mechanically possible, even as other features of genitalia may reflect the action of other mechanisms of selection. Genital coevolution is explicitly predicted by at least three mechanisms of genital evolution: lock and key to prevent hybridization, female choice, and sexual conflict. Although some good examples exist in support of each of these mechanisms, more data on quantitative female genital variation and studies of functional morphology during copulation are needed to understand more general patterns. A combination of different approaches is required to continue to advance our understanding of genital coevolution. Knowledge of the ecology and behavior of the studied species combined with functional morphology, quantitative morphological tools, experimental manipulation, and experimental evolution have been provided in the best-studied species, all of which are invertebrates. Therefore, attention to vertebrates in any of these areas is badly needed.

The evolution of genital complexity and mating rates in sexually size dimorphic spiders

“…Such coevolutionary patterns could also result from the lock and key mechanism, male-male competition, or female choice, or a combination of them [91]. Thus, our discussion of the evidence in support of sexual conflict in spiders does not imply the absence of other mechanisms related to sexual selection.…”

Sexual Cannibalism as a Manifestation of Sexual Conflict

“…Secure attachment will prevent females from ending copulation as evidenced by cases in which male parts, or whole males, remain stuck in the female genital opening after they have been attacked and even died (Knoflach and van Harten 2001;Schneider et al 2001;Foellmer and Fairbairn 2003). Spider species vary widely in their genital morphology and genital coupling mechanisms (Eberhard 2004a), and firmness of the genital coupling may be a result of a sexual conflict over copulation duration in concert with sexual selection and natural selection (see Brennan and Prum 2014).…”

Digest: Ontogenesis and evolutionary allometry shape divergent evolution of genitalia in female cetaceans*

“…Furthermore, Orbach et al's findings are novel and compelling DIGEST enough to provide more weight to the point of view that, even among closely related species, the rapid divergent evolution of genital shape in female cetaceans is facilitated by sexual selection pressures, just as Brennan and Prum (2015) described.…”

“…In contrast, female genitalia are not as well studied, as they are generally thought to be relatively invariant (Ah-King et al 2014). However, it has been shown that the coevolution between male and female genitalia is expected to result from close mechanical interactions during the copulating process (Simmons and Garcia-Gonzalez 2011 adaptations, it is still very important to identify this pattern and confirm the potential evolutionary drivers of genital variation (Brennan and Prum 2015).…”

“…Rapid divergent evolution of male genital shape is a vivid focus of research across animals with internal fertilization in evolutionary biology (Brennan and Prum 2015). Three main hypotheses have been suggested to explain the tremendous diversity of genital shape.…”