Sexually selected novelties of this sort could be called "courtship innovations." Most will be nothing more than a slightly novel design for a penis, a minor variation in mating coloration, or a different style of courtship dance. But from these humble origins, a small proportion of courtship innovations and their side-effects may turn out to have some survival benefits in addition to their courtship benefits. They may then become favored by natural as well as sexual selection. Of these survival adaptations, a small proportion may prove significant enough to allow a species to invade many new environmental niches. They produce adaptive radiations, proving themselves over time as major innovations. The ecological success of major innovations may hide the fact that many of them originated as courtship innovations.
The feathered wing may be a good example of a courtship innovation that proved to have large survival advantages in the long term. Archaeopteryx fossils from 150 million years ago were first found over a century ago, and paleontologist John Ostrom's 1969 theory that birds evolved from small, fast-running theropod dinosaurs has held up fairly well. However, biologists are still not sure how or why feathered wings evolved on dinosaur-type bodies. Many biologists propose that wings always had an aerodynamic function, even in their early stages of evolution. There is the ground-up theory that wings evolved to help small dinosaurs jump and turn quickly to catch prey, and the trees-down theory that wings helped to break their falls (progressing from parachuting to gliding to powered flight). Other biologists point out that the earliest proto-birds (such as the Protarchaeopteryx unearthed in China in the early 1990s) had well-developed wings, but no sign of the lighter skeleton associated with flying, and no sign of the top/bottom asymmetry that gives wings lift. Some have even proposed that feathers originated for insulation, feathered wings helping females to incubate their eggs, as in ostriches. But perhaps wings originated as sexual ornaments, along the following lines. Take a fairly useless dinosaur forelimb. Add a bit of color or an extra skin-flap with a novel mutation. Apply sexual choice and the runaway process. Result: a large surface area ornamented with color, available for display to the opposite sex. Feathers make excellent sexual ornaments—they are light, flexible, and movable. They are still used in courtship displays by male rifle-birds, who snap them open and shut in front of awestruck females. If the male protobirds happened to combine their forelimb displays with energetic jumps during courtship, and if females selected for the bestjumpers, then the transition from a display function to an aerodynamic function would be relatively smooth. Once wings proved useful in other contexts such as escaping predators, then survival selection would start shaping them for flight instead of just sexual ornamentation. This would have led to the well-documented proliferation of bird species well before the extinction of their dinosaur cousins 65 million years ago, and continuing to the present.
Of course, this scenario for wing evolution is just one hypothesis, and it is by no means clear whether it is right. At least this speculative example illustrates the general point that courtship innovations can potentially lead to unanticipated survival advantages. If we want to overcome the threshold problem of how evolution can favor the initial stages of innovations before they show net survival benefits, sexual selection seems to be a very strong candidate.
The human mind can be seen as one of these courtship innovations that happened to show some large survival advantages long after it first evolved. Modern Homo sapiens evolved about 100,000 years ago in Africa. By that time, our ancestors had brains the same size as ours. Yet almost all of the technological process in tool-making came tens of thousands of years later. Agriculture took another 90,000 years to invent, and only after that did the global human population climb above a few million. More than 95,000 years after human language probably evolved, we invented writing and reading, allowing useful information to be transmitted down through generations and across great distances.
Neanderthals had already evolved quite large brains 200,000 years ago, yet showed very limited technological progress and very modest abilities to spread into new habitats. Neanderthals may have had most of the courtship innovations that we call the human mind, yet they did not stumble upon the potential survival advantages conferred by our sort of creative intelligence. Our lineage did, so we imagine those survival advantages as projecting all the way back to the mind's origins.
Every inventor knows that innovation depends a lot on serendipity. A novelty may be invented for one purpose, only to prove its value years later for a completely different purpose. The Chinese invented gunpowder for firework displays, and the Europeans adopted it for warfare. The dinosaurs may have evolved proto-wings as sexual ornaments, and evolved into birds that use them for flight. The human mind may have evolved as a set of fitness indicators and sexual ornaments, and now we use it to make movies, give venture capital to start-up companies, and read books on mental evolution. Each species is free to use its sexually selected adaptations for any non-sexual purpose that it can invent—and as long as that purpose contributes somehow to survival or reproduction, selection can favor such use.
Sexual selection thus works as a natural source of serendipity in evolution. It gives evolution the slack it needs to play around without demanding that every cost incurred now must yield some future economic benefit. As all scientists know and most governments forget, this is the only way that productive research and development happens.
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