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
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
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)?
explanations had been proposed ( Box 2), neither of which are currently well supported (reviewed in
[1,2,4] ). By contrast, evidence is accumulating for
selection influences genital evolution has been extensively developed by
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
from good genes, or from other potential selection pressures involved in
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
selection mechanisms involved are less clear. Here, we discuss recent advances in our understanding
of genital evolution, and the challenge of
selection in the evolution of genital diversity.
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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.