The interaction of the three major sexual selection processes can explain sexual ornaments. Less often appreciated is how they can interact with natural selection for survival to produce evolutionary innovations. To understand any specific innovation such as the human mind, it may help us to look at what role sexual choice might play in the evolution of innovations in general.
The history of life on Earth is marked by major evolutionary innovations such as the evolution of DNA, chromosomes, cell nuclei, multicellular bodies, and brains. Classic examples of moderately important innovations include legs, eyes, feathers, eggs, placentas, and flowers. Much more frequent are the minor innovations that distinguish one species from another. These micro-innovations are often no more significant than a different mating call or an unusually shaped penis.
The major innovations give their lineages such an advantage in exploring new niches that they result in a burst of biodiversity called an "adaptive radiation." The first species that suckled its young with milk ended up being the ancestor of all 4,000 species of mammal. The first ape that walked upright became the ancestor of a dozen or so species of hominid, including us. Every major group of organisms (such as a kingdom or phylum) has a major innovation at its root. Every medium-sized group (such as a class or order) has a moderately important innovation at its root. Every species is distinguished by some micro-innovation. The tree of life is a tree of evolutionary innovations.
It remains to be seen how important the human brain is as an evolutionary innovation. If we became extinct tomorrow, it would count as a micro-innovation characteristic of just one species. If our descendants succeeded in colonizing the galaxy and splitting apart into a hundred thousand species millions of years from now, it would count as a macro-innovation. But an innovation's ability to trigger an adaptive radiation millions of years after its origin cannot explain why it evolved. This raises a serious problem that has remained unsolved ever since Darwin: how can innovations emerge through a gradual process like natural selection? This question has three variants of increasing difficulty.
The easy, most general problem is: how can a qualitatively novel structure arise through gradual, quantitative changes? The answer, of course, is that the whole universe unfolds by processes that turn quantitative change into qualitative novelty. The incremental process of gravitational attraction turns interstellar dust clouds into star systems. The incremental processes of capital investment and education turns poor villages into prosperous cities. The incremental process of growth turns a fertilized egg into a human baby. There is nothing special about evolution in this respect. Every thing in the world that we bother to name is a bundle of qualitatively novel properties emerging from an accumulation of quantitative stuff.
The moderately hard problem is: how can a complex innovation emerge that depends on many parts functioning together? Assuming that natural selection can tinker with only one part at a time, it seems difficult for natural selection to construct multipart innovations. What good is the retina of an eye without the lens, or vice versa? This sounds like a lethal argument against incremental Darwinian evolution, but it isn't. If it were, the existence of Microsoft would force one to be a Creationist. The Microsoft Corporation is composed of thousands of employees who must all work together for the corporation to function: management, accounting, personnel, marketing, finance, programming, and so forth. Could Microsoft have arisen through the incremental accumulation of employees, hiring them one by one? It seems logically impossible. If employee number one was a programmer, the corporation couldn't survive, because there would be no one in marketing to sell her product, no one in personnel to pay her, and no one in the legal department to sue software pirates. But if employee number one was in marketing, she wouldn't have any product to sell. And so on. How could a corporation that includes dozens of different kinds of employee possibly have emerged in just twenty years through incremental hiring? The answer is that the early employees were less specialized, and each filled many roles. When Microsoft consisted of just the teenaged Bill Gates and Paul Allen, they split all the corporate responsibilities between them. As more employees were hired, responsibilities were delegated and became more specialized. If one accepts the possibility of growing large, multi-part corporations by hiring one person at a time, perhaps one should not be too bothered by evolution's ability to produce innovations by compiling one genetic mutation after another. As far back as the 1850s, Herbert Spencer was pointing out that gradual growth through progressive differentiation and specialization is the way that both social organizations and biological adaptations must evolve.
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