Charles Darwin · Science
Why nature builds dazzling, dangerous ornaments that survival alone cannot explain - and Darwin’s second great force, the choice of mates.
Natural selection should ruthlessly trim anything that hurts survival. Yet nature overflows with extravagance that seems to do exactly that: the peacock’s enormous shimmering train, heavy and conspicuous, slowing its escape and flagging it to predators; the stag’s ponderous antlers; the bird of paradise’s absurd plumes. Darwin confessed that the sight of such a feather made him ill, because it appeared to defy his own theory. A trait that lowers survival should be eliminated - so why are the showiest males so common?
Sexual selection works two ways, Darwin saw. The first is combat: males fight or display for access to females, and selection sharpens the weapons of that fight - the stag’s antlers, the elephant seal’s bulk. The second, more radical, is choice: females prefer certain males, and whatever they prefer is amplified generation after generation. If peahens favour the grandest train, then grand trains spread, simply because their bearers are chosen more often. The female’s eye becomes a selective force as real as any predator’s tooth.
Why is it almost always the male who is gaudy and competitive, and the female who is drab and choosy? The answer lies in a deep asymmetry that runs through most of the animal kingdom, rooted in the two kinds of sex cell. An egg is large, costly, and produced in small numbers; sperm is tiny, cheap, and produced in the millions. From this single fact a cascade follows. Because eggs are the scarce, expensive resource, females typically invest far more in each offspring - in many species also bearing, gestating, and rearing the young - while a male’s minimum investment can be almost nothing. This makes females the limiting resource for which males must compete, and it makes females the choosier sex, since each of their few, costly offspring is a far bigger gamble.
So the roles fall out almost inevitably. Males, able to father many offspring if they can only win mates, are selected to compete and to advertise - to fight, to display, to risk everything for access, since the reproductive payoff of winning is enormous and the cost of a short life is offset by many offspring. Females, investing heavily in each of a limited number of young, are selected to choose carefully, since a poor choice is far more costly to them than to a male. The peacock’s tail and the peahen’s plain brown feathers are two faces of the same asymmetry: the male advertises because he competes to be chosen; the female is drab because her advantage lies not in display but in discernment, and often in the camouflage that protects her on the nest. (Where the asymmetry reverses - as in seahorses and some shorebirds, where males do the rearing - the females become the brighter, competing sex, exactly as the logic predicts.)
This is the opening of the lesson. The rest — the dialogue, the primary source, and the recall — is in the app.
You learned that some traits help mating even as they hurt survival. Explain how that tension produces ornaments like the peacock’s tail, in your own words.
Leads to Alfred Russel Wallace.
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