Under natural selection, species adapt to their environments. When the environment refers to a species' physical habitat, this seems simple enough. If a species lives in the Arctic, it had better evolve some warm fur. Under sexual selection, species adapt too, but they adapt to themselves. Females adapt to males, and males adapt to females. Sexual preferences adapt to the sexual ornaments available, and sexual ornaments adapt to sexual preferences.
This can make things quite confusing. In sexual selection, genes do not code just for the adaptations used in courtship, such as sexual ornaments. They also code for the adaptations used in mate choice, the sexual preferences themselves. What the physical environment is to natural selection, sexual preferences are to sexual selection. They are not only the tastes to which sexual ornaments must appeal, but the environment to which they must adapt.
With sexual selection, genes act as both the fashion models and the fashion critics, both the apostates and the inquisitors. This creates the potential for the same kind of feedback loops that drive progress in high fashion and modern theology. These feedback loops are the source of sexual selection's speed, creativity, and unpredictability. Yet they also raise the classic problem of runaway corruption in autarchies: who watches the watchmen? How can mere genes be trusted as both selectors and selectees in evolution under sexual selection? The world of mate choice plays by its own rules, and though survival is a prerequisite for mating (as it is for scholarship, fashion, and faith), the principles of sexual selection cannot be reduced to the principles of survival. The biologist seems to have no point of entry into this protean wonderland where genes build brains and bodies, which pick the genes that build the next generation's brains and bodies, which in turn pick the genes that pick the genes. . .
Imagine the headaches if natural selection worked that way. Organisms would select which environments exist, as well as environments selecting which organisms exist. Strange, unpredictable feedback loops would arise. Would the feedback loop between polar bears and Arctic tundra result in a tundra of Neptunian frigidity where bears have fur ten feet thick, or a tundra of Brazilian sultriness where bears run nude? Would migratory birds select for more convenient winds, lower gravity, and more intelligible constellations? Or just an ever-full moon that pleasingly resembles an egg? Evolutionary prediction seems impossible under these conditions. Yet this is just what happens with sexual selection: species capriciously transform themselves into their own sexual amusements.
Introducing sexual selection in this way is more than just an attempt to encourage you to share my belief that it is one of the weirdest and more wonderful of nature's phenomena. That I
could achieve simply by presenting the standard catalog of sexual selection's "greatest hits": the peacock's tail, the nightingale's song, the bowerbird's nest, the butterfly's wing, the Irish elk's antlers, the baboon's rump, and the first three Led Zeppelin albums. By presenting sexual selection as a strange world of genes selecting other genes, I have tried to provoke a different question: How could one ever make a science out of sexual selection? Darwin showed that sexual selection exists and documented its effects, but it took another century before biologists had the scientific tools for explaining why sexual selection produces certain kinds of traits and not others. To understand how sexual selection shaped human mental evolution, we need to become familiar with this new toolbox of ideas and models. Let's first have a better look at Fisher's runaway process. It is the best example of how sexual selection exercises a power distinct from natural selection.
Was this article helpful?