Sunday, November 18, 2007

Simple Males Evolve Faster

A press release titled Males being "simple" evolve faster appeared in the Times of India. It covered a research paper Simpler mode of inheritance of transcriptional variation in male Drosophila melanogaster: published in PNAS. The researchers pointed out that since males are hemizygous i.e . have X and Y sex chromosomes they have more additive genetic variation (variation in traits are due to variation in genes without dominance effects or gene interaction effects), while females with XX sex chromosomes exhibit non-additive genetic variation. Since males have only one X chromosome, they may -unlike females - lack the complementary allele on the corresponding sex (Y) chromosome. In females any beneficial mutation may not necessarily be expressed as the effect of that gene may be masked by the complementary allele, meaning the trait is expressed through dominance. This would mean that it will be hard for that particular version of the gene to increase in frequency since you would need two copies of that gene (recessive) for it to be expressed and subjected to natural selection. In contrast, since males have just one copy of many sex-linked genes, any beneficial mutation will be expressed and face selection, leading to a rapid frequency increase of the gene over few generations. Therefore this type of sex-linked genetic variation in males responds more quickly to selection than the one found in females. More response to selection pressure, faster evolution. Biologist David Rand, of Brown University explains it well "This research shows how recessive and dominant traits are important in determining variation in populations. The best way to think of it is males play with one card, but females get to play one and hold one. If males have got a good trait, it's promoted; something bad, it's eliminated. In females you can have a bad card, but a good card can protect it. As a result, females can carry deleterious traits but not express them."

Interesting stuff. The researchers then add that sexual selection usually operates via males due to this difference in the type of genetic variability. Sexual selection leads to dimorphism, i.e. males and females develop divergent morphologies. In many species it is the males that become bigger or develop large antlers or bright ornamentation. The usual explanation for this: Males have more variable reproductive success than females and tend to be more promiscuous. Females due to a large investment in the gamete are more choosy. So males compete for female attention leading to bigger, stronger, showier males over time. The researchers of this study argue that the simpler genetic inheritance system coupled with the more variable reproductive success of males would help males evolve via sexual selection pressures more rapidly.

Sexual selection though may not be the best example to illustrate the connection between "simple genetic variability" and faster evolution. The reason is the peacock' s tail. This particular study uses the fly Drosophila melanogaster which has an XY (male) / XX (female) sex determination system. But in birds, the system is different. It is the males which are ZZ and females ZW. So, females have a "simple genetic system" yet it is the peacock that evolves the elaborate tail. Even in species where sexual selection operates via XY males, does that really mean males are evolving faster than females as the titles of virtually all press releases implied? Sexual selection in males is fueled by female choice, which is a genetically determined trait. Which means that female discrimination for a mate and male ornamentation will evolve in tandem at the same pace.

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