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to understanding human evolution but general issue in Darwinian theory. The classic case is the peacock’s both Darwin and his early critics well understood. Some initial reluctance by otherwise sympathetic scientists to accept the Origin of Species stemmed from Darwin’s failure to explain nature’s excesses: for example, plumage and songs of birds. Darwin was acutely aware of this difficulty. In a letter written to Asa Gray the year after the Origin of Species published, he remarked on it. Before he developed the theory evolution by natural selection, Darwin wrote to Gray, “the thought eye made me cold all over.” He had eventually resolved the puzzle how eyes can evolve, but he was then faced with a new obstacle. Now, confided, the “sight of a feather in a peacock’s tail, whenever I gaze makes me sick!” As well it might, because the peacock’s tail throws defiant challenge to the fundamental principles of natural selection. It is expensive to grow, requiring energy that could be otherwise utilized organism. The weight alone of this panoply of feathers with their bird evolved. Natural selection ought to have long since eliminated peacock’s tail; in fact, natural selection should never have allowed develop in the first place.

In addition to its costliness and danger, another aspect of the peacock’s tail is inconsistent with principles of evolution as Darwin originally mapped them out. Given the ways in which environments, food sources, and predators present themselves to animals, natural selection actually pushes toward uniformity, with the constant reinvention of same general adaptations. As Geoffrey Miller puts it, “Natural selection ecological utility tends to produce convergent evolution, where many lineages inde pendently evolve the same efficient, low-cost solution same environmental problems—traits such as wings, eyes, teeth, claws, hearts, and lungs.” Natural selection should lead us to expect birds evolved in the same environment to have much the same for example, and to differ in such respects as size or beak shape only in terms of the particular niche each species occupies. But animal species frequently violate this pattern. Birds can exhibit in the same habitat an endless variety of striking, colorful plumage arrays.

In the nineteenth century this variety was usually explained as a matter of males signaling to females that they were members of the same species. (This is implausible: members of the same species need to recognize each other, but nothing like the feathers of an oriole or a peacock needed for that.) As with the central thesis of the Origin of Species, Darwin worked slowly over years developing his solution to the riddle the peacock, which he published in its most developed form in book in 1871, The Descent of Man, and Selection in Relation to Sex. this work he puts forward a new, powerful, versatile principle, the second great driving force in the evolution of animal physiology and psychology: sexual selection by mate choice.

Sexual selection is, like natural selection, easily described, and its gross effects on the appearance and behavior of animals are often obvious human observation. The peacock’s tail, according to sexual selection, fitness indicator, a signal of health and high-quality genes. A large, colorful, symmetrical tail functions as an advertisement to peahens, proclaiming, “See what a strong, healthy peacock I am.” The difficulty of growing and will not, therefore, readily find mates. Indeed, the accuracy peahen preferences in terms of fitness has been experimentally demonstrated: peacocks with the finest tails do possess relatively better genes. In addition to being a fitness indicator for the bird that has one, a tail that attractive to peahens points toward another reproductive virtue: the male offspring of a union with a splendidly outfitted male will themselves more likely to have splendid tails, which will in their turn be attractive next generation of peahens—thereby ensuring that the couple’s genes be passed through to future generations. At the same time, this pro ensures that