Males are Simple Creatures
The secret to why male organisms evolve faster than their female counterparts comes down to this: Males are simple creatures.
In nearly all species, males seem to ramp up glitzier garbs, more graceful dance moves and more melodic warbles in a never-ending vie to woo the best mates. Called sexual selection, the result is typically a showy male and a plain-Jane female. Evolution speeds along in the males compared to females.
The idea that males evolve more quickly than females has been around since 19th century biologist Charles Darwin observed the majesty of a peacock’s tail feather in comparison with those of the drab peahen.
How and why males exist in evolutionary overdrive despite carrying essentially the same genes as females has long puzzled scientists.
New research on fruit flies, detailed online last week in the journal Proceedings of the National Academy of Sciences, finds males have fewer genetic obstacles to prevent them from responding quickly to selection pressures in their environments.
"It’s because males are simpler," said lead author Marta Wayne, a zoologist at the
The finding could also shed light on why diseases show up differently in men and women.
Wayne and her colleagues examined more than 8,500 genes shared by both sexes of the fruit fly Drosophila melanogaster. Of those genes, about 7,600 have different expressions (alleles) that do different jobs in males and females.
The flies were identical genetically, except for their sex chromosomes. In flies and humans, thousands of genes made up of DNA are packaged into tiny units called chromosomes. Each parent contributes one set of 23 chromosomes to offspring, resulting in little ones with 23 father-given chromosomes and 23 mother-chromosomes—46 total.
One pair of these is called the sex chromosome. In this case, the females have two X chromosomes (XX) and males, XY. Many genes are found on the X chromosome, whereas few are associated with the Y chromosome.
For female fruit flies, the X-chromosome genes can come in two flavors called alleles that not only interact with each other but also with other genes.
That's not the case with males. "We find direct evidence that the expression of the genes on the X has this covering behavior in females whereas in males they're out in the open," said study team member Lauren McIntyre, also of UF.
Males only have one X chromosome, so what you see is what you get. If that particular gene gives the male a boost in terms of sexual selection, say a gene responsible for fluffier feathers, the gene would be selected for in the game of natural selection over successive generations. But if the gene is no good for males, it would get selected against over time.
"Having one X means your genes are more open to selection in males," UF researcher Marina Telonis-Scott said in a telephone interview. "So in a female if you have a recessive allele that confers a sickness, it can be concealed within the two X's but if you've only got one, such as the male, you're more open to selection."
"The X function is thought to be quite different in flies than humans," McIntyre told LiveScience. In humans, one of the X chromosomes gets inactivated in females, though research is finding this inactivation isn't always absolute.
However, the results could help explain differences in symptoms and responses to diseases in men and women, the authors say. Sexual selection does occur in humans, they note. In addition, fruit flies and humans share an evolutionary history, the authors point out, which is the reason why we share more than 65 percent of our genes with the tiny insects.
"If we see a mechanism in flies it may also be true in everything that shares that evolutionary history," McIntyre said. On a basic level, the genetic machinery works in a similar manner in flies and us.
"There's a health aspect in figuring out differences in gene expression between the sexes,"
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