Charles Darwin’s sequel to was The Origin of Species is often known popularly as The Descent of Man. But of course the full title was The Descent of Man , and Selection in Relation to Sex. Darwin was then an early expositor of sexual selection theory, though R. A. Fisher also made his contribution. My own interest in the topic has been partly motivated by the fact that I perceive many people seem use sexual selection as a deus ex machina to explain variation or change where no plausible mechanism can otherwise be provided (recall that this has come up with EDAR). Over a decade ago Geoffrey Miller wrote a book, The Mating Mind, which attempted to take Charles Darwin’s original ideas to heart. I’m not sure how much of the original arguments Miller would stand by today, but it was an entertaining read. My own first encounter with the idea of sexual selection was in the work of Jared Diamond in the early 1990s. In particular, in The Third Chimpanzee he offered that racial variation in human types might be due to sexual selection for aesthetic characters, as opposed to ecological adaptation (see also Peter Frost’s model of the origin of European complexion). But all this conjecture of human variation often strikes me as a touch too speculative. Ultimately what does the theory and the patterns on the tree of life say? More substantively, as a genomicist I’m curious as to the sequence wide signals which one might see presuming a species is subject to sexual selection. Ergo, two weeks ago I blogged The Once and Future Genomics of Sexual Selection.

So naturally I was very interested when this came into my PNAS feed: Sexual selection drives evolution and rapid turnover of male gene expression. Basically the authors looked at differential gene expression and sequence level evolution across a lineage of birds to see if there were patterns correlated with presumed intensity of sexual selection. The figure above illustrates several such trends. Species which were subject to stronger sexual selection on males showed a higher proportion of male-biased genes. I’m not usually very interested in work on transcriptomes, but it strikes me that this is going to be a really big deal in the near future if sexual selection is common and it operates primarily through modifying patterns of gene expression. With only six species the p-values above aren’t the greatest. Perhaps the results won’t stick, but, they open a window toward examining evolutionary processes in a comparative manner which allows us to gauge just how pervasive sexual selection is as a force in driving phenotypic variation.

I’ll finish with this conclusion from the authors:

Taken together, our results indicate that the focus of sexual selection shifts rapidly across lineages. Our results also suggest that sexual selection acts primarily on expression, which may be more labile and less functionally constrained than coding sequence and therefore more likely to be influenced by short-term mating system dynamics among related species. The lability of gene expression evolution is illustrated in recent experimental evolution approaches that found an association between sex-biased gene expression and variations in sex-specific selection (11, 13). Gene expression lability is also clearly illustrated by the rapid turnover of sex-biased genes in our phylogeny (Fig. 2), which has also been observed in other animal clades (6, 46). Furthermore, rank order correlations show that gene expression divergence increases with evolutionary time across the Galloanserae (Fig. 3), again illustrating the lability of gene expression.

Citation: Sexual selection drives evolution and rapid turnover of male gene expression.