Males answer the call of selection because they’re simpler… I see…

Ok, first let me get the extremely sloppy wording in this story on EurekAlert out of the way [Simple reason helps males evolve more quickly]:

“No matter the species, males apparently ramp up flashier features and more melodious warbles in an eternal competition to win the best mates, a concept known as sexual selection.”

“Researchers believe this relatively uncomplicated genetic pathway helps males respond to the pressures of sexual selection, ultimately enabling them to win females and produce greater numbers of offspring.”

“It turns out that the extra X in females may make answering the call of selection more complicated.”

The story suggests that traits like elaborate ornamentation evolve more readily in males than in females because males have “simpler” genetic systems, not having that second X chromosome and all. This is based on a study of Drosophila melanogaster, which the story notes has an XY (male) / XX (female) chromosomal sex determination system, similar in broad outline to the situation in mammals.

To my mind, we don’t need any additional genetic explanation. Where sexual selection leads to elaborate characteristics in one sex, it is usually males because they make the least investment (at least at the gamete level, and often in terms of parental care), have more variable reproductive success among individuals, and have their reproductive output determined in large part by the number of females with whom they mate. For females, this is most often not the case. So males compete for females and females are choosy in many species, leading to traits in males that are used in combat with other males or are favoured by females. This goes back to Darwin in 1871, with important contributions from the likes of R.A. Fisher in the ’30s.

Here’s the problem with the “it’s the extra chromosome, stupid” hypothesis. In some groups (e.g., seahorses, some birds) the males raise the young and are choosy and the females are the ornamented ones. As far as I know, they have the same sex determination system as related species that have the more typical sexual selection processes.

If that weren’t enough, just consider that birds — which provide some of the best known outcomes of sexual selection like peacocks’ tails — have a ZZ (male) / ZW (female) chromosomal sex determination system. That’s right — female birds have the so-called “simpler” genetic arrangement.



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