Males have an edge if they're good looking, at least to that potential mate, at least when it comes to birds.

Charles Darwin wrote about evolution in his 1859 book "The Origin of the Species." While Darwin understood the process, he could not explain why peacocks evolved such elaborate tails.

The tail puzzled Darwin, then helped him examine the evolution of beauty.

As much impediment as ornament, the peacock tail is huge (and colorful). It was made so by uncountable generations of selection by peahens who were impressed with glam tails.

The peacock gets to breed, yes, and his genes are passed along. That's the idea. The price he pays for genetic success is a tail that while it pleasing the ladies also makes the peacock slow afoot and a frightful flier. Peacocks are vulnerable to predation. They have expensive dates.

Evolution exists in two forms. Survival of the fittest, genes making the owner more successful, contributing to survival. Or, evolution of characteristics like the peacock's tail, evolution driven by a female's idea of beauty.

This is explained by zoology professor Michael J. Ryan in his book "A Taste for the Beautiful." (Princeton University Press, hard cover, color photographs, $27.95)

Peahens, finding those tails sexy, would choose the males so dressed. Genes for larger tails then move to the next generation, resulting in more lengthened tails.

Nonsurvival traits, particularly those for courtship or battle, like the tail, are most often seen in male birds.

Survival is secondary to sex, Ryan explains. The peacock is more interested in genetic heritage than a long life.

In another example of an extreme adaptation to further procreation, Ryan writes about a particular Central American frog.

The female frog is attracted by the tenor of the male frog's voice. The male has so precisely tuned himself for the mating game that his brain can fit inside his larynx.

The bird's tail and the frog's voice work because the females in each case have in their brains a preference for those traits, according to Ryan. Females have evolved these preferences. Males adapt to take advantage.

The bright red of a cardinal's feathers (or the brilliance of the peacock's tail) indicate that he is a superior bird. He is fit, healthy, better at finding food, at avoiding predation, at defending a nest.

The female takes advantage of characteristics it cannot literally see through its history of developing a yen for what it can see. The bright red is the fitness clue.

"Brains evolve to sense what matters," Ryan tells us.

Sometimes, though, there is little to distinguish one male from another. Ryan uses sage grouse as an example.

Striking differences in male appearance have not evolved in this species. Nothing about a male sage grouse matters except his sperm. He does not defend his mate or provide food for the chicks. He breeds and disappears.

The major difference between one male sage grouse and another, Ryan explains, is that most of them will die virgins, never having a chance to mate.

There are no brighter feathers or longer tails on the males to guide selection by hens. The look-alike males strut their courtship dance while females gather around the dance floor to watch.

How do hens make a choice, lacking fitness clues?

Ryan suggests that one female eventually will choose, for whatever reason. She has now given that male preferred status.

Other hens simply follow her lead. Chosen males become dominant breeders, just because, Ryan believes.

Thus evolution continues.

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