Fitness indicators are pointless unless individuals vary in their fitness. If we take fitness to mean the possession of good genes that can be inherited by offspring, then it seems hard to understand how evolution can allow any variation in fitness to remain. Selection is supposed to maximize fitness, driving it ever upwards. It is not supposed to permit fitness variation to persist in species just to provide an incentive for sexual choice.
To follow this argument, it is crucial to understand the difference between "inherited" and "heritable." All traits that depend on genes are inherited. But the term "heritable" is much more restrictive: it refers to the proportion of individual differences in a trait that are due to genetic differences between individuals. The concept of heritability applies only to traits that differ between individuals. If a trait exists in precisely the same form across all individuals, it may be inherited, but it cannot be heritable. It should come as no surprise that fitness is inherited, because fitness clearly depends on genes. The surprising thing is that fitness still varies between individuals in most species, and that the variation often seems to depend on genetic differences.
To see why the heritability of fitness is surprising, consider what happens in species that mate in large aggregations called "leks." Lek is Swedish for a playful game or party. Some birds like sage grouse congregate in these leks to choose their sexual partners. The males display as vigorously as they can, dancing, strutting, and cooing. The females wander around inspecting them, remembering them, and coming back to copulate with their favorite after they have seen enough. Leks resemble music festivals where mostly male rock bands compete to attract female groupies. In species that lek, the males usually contribute nothing but their genes. The females may never see them again, and raise their offspring as single mothers. Leks create a situation where sexual selection is extremely strong. The most attractive male sage grouse may mate with thirty females in one morning; average males usually mate with none. It is a winner-takes-all contest, and it should spread the most attractive male's genes very quickly through the population.
If the lekking females choose males for good genes generation after generation, all the males should end up being perfectly fit and identically attractive. Males of lower apparent fitness will have died unmated, their mutations having died with them. After a few generations, all the mutations that show up in fitness indicators should be gone. Only the good genes should be left. If every male has the same high fitness, there is no variation for fitness indicators to reveal. If there is no variation in genetic quality and if genes are all that females get, there is no longer any incentive for females to be choosy about their mates. Instead of spending time and energy wandering around the lek admiring male displays, the females might as well pick randomly. The reasons for mate choice should disappear as the heritable variation in fitness disappears. According to this evolutionary logic, leks should be temporary phenomena. Yet leks still exist. Presumably, sage grouse have been gathering in leks for thousands of generations. Biologists call this the "lek paradox."
The lek paradox is the most extreme case of a general problem with the heritability of fitness. Any form of sexual selection for fitness indicators should even out genetic variation in fitness. If female choice in our species favored tall males, all males should be equally tall. If male choice favored large breasts, all females should be equally large-breasted. If both sexes favored high intelligence and beautiful faces, all humans should be equally bright and beautiful. Yet we are not. The differences remain, and they are still genetically heritable. So why would selection allow such differences to persist?
Once biologists agreed that the lek paradox was a problem, the hunt was on for evolutionary forces that could maintain variation in fitness. Two major candidates emerged. One emphasized that fitness is environment-relative; the other emphasized the ubiquity of harmful mutations that erode fitness.
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