From Stuart Rigby at The Conversation, commenting on a recent study:
Measuring sexual selection…
So, does this then mean that sexual selection does not in fact drive speciation? Not necessarily. In general, sexual selection is stronger when some individuals in a species are better than others at attracting mates and having lots of offspring. To measure this directly, you need to know how many mates and offspring everyone has, which – as you might imagine – is not easy data to gather, especially in the wild.
Studies such as this new one use proxy measures, such as the degree of promiscuity, while other use male testes size, or sex differences in body size, to infer differences in the strength of sexual selection between species. But while promiscuity clearly opens the door to different aspects of sexual selection (such as dispersing to find additional mates) it does not necessarily increase the overall strength of sexual selection.
In fact, under some conditions sexual selection is weaker when females are promiscuous. This is because female promiscuity stops a minority of males from monopolising access to mating, and so all males have a more similar degree of success.
So, promiscuity probably alters the type of sexual selection acting on shorebirds, and it is certainly associated with reduced species diversity. But the broader debate over the relationship between sexual selection and speciation won’t be settled any time soon.More.
In short, we don’t know.
See also: Can sexual selection cause a decline in evolutionary fitness? Probably. There is no law of nature that decrees that choices made by animals will necessarily lead to the thriving of the species. Why should there be?
and
Can sex explain evolution?