Sexual selection and genital evolution
David J. Hosken and Paula Stockley

Genitalia are conspicuously variable, even in closely related taxa that are
otherwise morphologically very similar. Explaining genital diversity is a
longstanding problem that is attracting renewed interest from evolutionary
biologists. New studies provide ever more compelling evidence that sexual
selection is important in driving genital divergence. Importantly, several
studies now link variation in genital morphology directly to male
fertilization success, and modern comparative techniques have confirmed
predicted associations between genital complexity and mating patterns across
species. There is also evidence that male and female genitalia can coevolve
antagonistically. Determining mechanisms of genital evolution is an
important challenge if we are to resolve current debate concerning the
relative significance of mate choice benefits and sexual conflict in sexual
selection.

Extreme variation in male genital morphology is a conspicuous and
comprehensive trend across animals with internal fertilization to the extent
that even closely related species with similar general morphology often have
strikingly different genitalia ( Figure 1). Explaining genital diversity has
been a longstanding problem for evolutionary biologists, and the selective
pressure(s) responsible for this bewildering array has been the subject of
ongoing debate [1-3] . Complicated and divergent morphology is unlikely to
have arisen purely for the relatively simple function of sperm transfer. So
why the enormous variation ( Box 1)?

 Before the recent expansion of studies investigating the role of sexual
selection in genital evolution, two general non-sexual selection
explanations had been proposed ( Box 2), neither of which are currently well
supported (reviewed in [1,2,4] ). By contrast, evidence is accumulating for
a role of sexual selection in genital evolution. The idea that sexual
selection influences genital evolution has been extensively developed by
Eberhard in the context of postcopulatory sexual selection [1,4-6] . In
particular, he has argued that genital divergence is often driven by cryptic
female choice, broadly defined as any postcoupling process or structure
controlled by females that biases paternity towards males having a certain
phenotype when females have mated with more than one male [7]. Eberhard
further argues that the female benefit for this choice is most often
Fisherian ( Box 3). This has generated debate, however, because it can be
difficult to distinguish mechanisms of cryptic female choice for sexy sons
from good genes, or from other potential selection pressures involved in
postcopulatory sexual selection, such as sperm competition and sexual
conflict. The evolutionary effects of purely Fisherian benefits are also
unclear ( Box 3). Moreover, sperm competition probably played at least some
role in the initial evolution of male genitalia from a primordial state
where gametes were released into the environment [8]. Hence, although a new
generation of studies is consistent in supporting Eberhard's hypothesis that
sexual selection is the primary force driving genital divergence, the exact
selection mechanisms involved are less clear. Here, we discuss recent
advances in our understanding of genital evolution, and the challenge of
determining the relative importance of various mechanisms of sexual
selection in the evolution of genital diversity.

Read the rest at Nature
http://gateways.bmn.com/magazine/article?pii=S0169534703003744

Comment:
Exciting as this article seems to be on first reading, the enormous
diversity and accompanying pictures refer to the evolution of Drosophila's
naughty bits.

Posted by
Robert Karl Stonjek.
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