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Review
. 2015 Jul 1;7(7):a017749.
doi: 10.1101/cshperspect.a017749.

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

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Free PMC article
Review

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

Patricia L R Brennan et al. Cold Spring Harb Perspect Biol. .
Free PMC article

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.

Figures

Figure 1.
Figure 1.
Graphical classification of mechanisms of genital evolution and coevolution. Three circles depict the independent and co-occurring actions of natural selection, female choice, and male–male competition. Different specific versions of genital coevolution can occur depending on which of the three broader evolutionary mechanisms are occurring. Sexual conflict (hatched lines) occurs through the simultaneous action of male–male competition and female choice, or male–male competition and natural selection. SAC, sexually antagonistic coevolution. See text for explanation.
Figure 2.
Figure 2.
Female genitalia in Drosophila were previously thought to be invariant, but recent work has described extensive variation and coevolution in genital morphology. (A,C,E) Female oviscapts, and (B,D,F) male epandrial posterior lobes. Scale bars, 50 μm. (Reprinted from Yassin and Orgogozo 2013.)
Figure 3.
Figure 3.
Genital coevolution via sexual conflict. (A) Waterfowl, in which the elaboration of the vagina correlates with penis length (vagina on the left, penis on the right in mallard, harlequin, and long-tailed duck) Scale bars, 2 cm, and (B) seed beetles (Callosobruchus), in which the presence and abundance of spines on the aedagus correlates with the thickness of the wall of the female’s bursa copulatrix. (From Rönn et al. 2007; reprinted, with permission, from the National Academy of Sciences © 2007.)
Figure 4.
Figure 4.
Genital coevolution. (A) Snakes, in which the shape of the inflated male hemipenes corresponds with the shape of the female vagina (Crotalis horridus and Coluber constrictor), and (B) tinamous species without a penis have a simple vagina (Crypturellus undulatus male cloaca with nonintromittent penis and female on the left); the presence of a long penis is associated with an accordion-like vagina (Tinamus major on the right). Scale bars, 2 cm.

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