A new study explains why we aren't all born with Brad Pitt's perfectly chiseled features or Angelina Jolie's pouty lips.

A long-standing thorn in the side of biologists has been the difficulty in accounting for the enormous variation between individuals, when, from an evolutionary standpoint, sexual selection by females for the most attractive mates should quickly spread the "best" genes through a population.

"It is a major problem for evolutionary biology," said study team leader Marion Petrie of Newcastle University in England.

The lek paradox

For some species, females select the most attractive males to mate with. Female peacocks will choose males with the longest tail feathers — the peacock version of George Clooney.

These more attractive features usually indicate some other level of genetic fitness, such as disease resistance, that the female's offspring will then also inherit.

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According to this method of sexual selection, if females only bred with the most attractive males, then all males should be equally attractive and sexual selection could not take place.

(In the peacocks' case, all males would have similarly long tails.)

But clearly this isn't the case. For every Johnny Depp out there, there's a George Costanza — in humans, birds and other animals alike.

This so-called "lek paradox" (a lek is a group of males congregated for mating) has plagued evolutionary biologists for decades.

"If you had no variation at all, you wouldn't get evolution," Petrie told LiveScience.

But a group of British scientists think they have found the answer to the paradox in the human body's "DNA repair kits."

DNA repair kits

A cell's DNA repair kit is not really a kit, but a set of molecular processes that routinely repair the damage done to the cell's DNA that results in genetic mutations, under normal conditions.

Mutations can be harmful and cause tissue to degenerate, malfunction or develop cancers. Other mutations are beneficial, such as those in the part of the genome responsible for disease defense that make an individual more resistant to attacks from bacteria and viruses.

Some mutations affect the repair processes themselves and make them less efficient, which results in more mutations as the damage goes unrepaired.

"You can raise or lower your own mutation rate," Petrie said.

A higher mutation rate creates more diversity in a population.

Using a computer model, Petrie found that the greater genetic diversity created by mutations that affect DNA repair outweighed the decrease in diversity arising from sexual selection.

In some of her earlier work, Petrie showed that men with greater genetic diversity in areas of the genome relating to disease defense were also rated as more attractive by women.

So because these males with greater mutation rates are more likely to mate, they will pass that higher rate on to their offspring, allowing sexual selection to propagate variation through the population, a finding that likely applies to all organisms.

This propagation means that peacocks' tail feathers will vary in length and that some of us definitely won't be America's next top model.

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