An Introduction to Sexual Selection

Sexual selection is an important mechanism of natural selection which may not get enough attention in basic biology courses. The idea dates back to Darwin, who dedicated over half of one of his books (The Descent of Man and Selection in Relation to Sex) to the subject. Sexual selection is based on the struggle to reproduce rather than the struggle to survive; this notion reinforces the different meaning of 'fitness' in evolutionary biology compared with its common usage.

The concept of natural selection is often associated with some kind of struggle to survive. Some individuals in a population do better than others and so have more offspring; standard examples might be prey outrunning a predator (or vice versa) or bacteria developing resistance to an antibiotic. If the reason for their success is heritable (i.e., if it's something that is passed on to their children), then the children will also do better than average and the trait will gradually spread throughout the population. This is one straightforward interpretation of 'survival of the fittest', but restricting our understanding of natural selection to these sorts of cases would be an unfortunate oversimplification.

In order for an organism to be successful, it has to do more than survive; it also has to find a mate and reproduce. Sexual selection is the result of individual differences in successfully attracting mates. It comprises two processes: mate choice, in which members of a sex choose a mate based on displays of courtship signals; and mate competition, in which members of a sex directly compete with each other for access to the other sex. In both cases, an individuals have a higher fitness not because of their ability to survive, but because they are better at finding a mate and reproducing.

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The classic example of mate choice is a peacock's tail. Peahens prefer peacocks with large and colourful tails, so those peacocks get to mate more frequently and have more offspring. The male peachicks will inherit the genes for a long and colourful tail from their father and so will have a similar tail. Peacocks with extravagant tails will have more children, since they are more successful at attracting a mate; as a result, the peacocks in the next generation will, on average, have more extravagant tails. This is where things get interesting: since the tails become more and more impressive from one generation to the next, peacocks constantly need to do better and better to get the attention of a peahen. Each generation, the peahens prefer the peacocks with the most impressive tails, which leads to an increase in the average tail quality in the next generation, when the peahens will again select peacocks with the best tails, leading to an increase in quality, and so on. This positive feedback, which is a hallmark of sexual selection via mate choice, leads to a runaway process where a courtship signal becomes more and more extreme.

The courtship signals used in mate choice are generally costly, either by requiring a great deal of energy or by making the individual more vulnerable to predation (or both). Costly signals are necessarily honest ones, since an individual has to be in good health to be able to maintain them. In other words, size only matters if it's expensive or risky; if the signal is easy to fake, it isn't an honest indicator of good health or good genes. The costs also limit the runaway feedback process of sexual selection. When costs aren't a constraint, a trait might continue becoming more extreme until it hits physical limits.

The other way that sexual selection happens is through competition between individuals of one sex for access to the other sex; for example, in Pacific salmon, the females construct nests and the males fight each other for access to the nesting females. Sexual selection acts on some of the traits used by the males to establish a dominance hierarchy, such as their hooked snout, bright coloration and size. Like mate choice, mate competition can lead to runaway feedback; males need to be above average in order to become successful breeders but their success will lead to an increase in the average level in subsequent generations. Some of the males, however, use an alternative strategy; rather than getting access to mates by being larger than average and dominant, they are small enough to sneak past unnoticed. These 'sneakers' lead to disruptive selection acting on male size: large or small males will succeed by fighting or sneaking, respectively, while intermediate sized males will be unsuccessful at both strategies.

It's important to realize that sexual selection isn't a separate process, but just one of several different mechanisms of natural selection. Nevertheless, it's proven interesting enough to warrant special attention by evolutionary biologists, probably because the feedback inherent in sexual selection can lead to such apparently maladaptive traits as the peacock's tail or the brightly-coloured plumage of a bird of paradise. Other mechanisms, such as selection for survival to reproductive age (viability selection) or differences in fertility (fecundity selection), are generally likely to lead to adaptive traits; these mechanisms are often lumped together under a general term like 'ecological selection', 'adaptive selection' or even just 'natural selection' (which is unfortunate and confusing). This isn't because they aren't important; in fact, the effect of sexual selection can often be overwhelmed by other kinds of selection. For any particular trait, the relative importance of different mechanisms of selection depends on a wide range of factors; unraveling these is usually a complex question which can only be answered by careful research.

I'll close with some examples of traits that might have undergone sexual selection in human evolution. The most obvious examples are our sexual characteristics. Unlike humans, females of other primate species have flat breasts; since the additional fatty tissue in human breasts doesn't actually contribute to milk production, it's been suggested that it might have evolved as a courtship signal. Similarly, human males lack a penis bone and so have to maintain an erection by pumping blood into the penis, unlike other primates. Humans also have a much larger penis than other primates — our close relatives, gorillas and chimps, average 3cm and 8cm, respectively. Rounded breasts and large penises might have served as courtship signals indicative of health and fertility and, like the peacock's tail, they may have been subject to runaway sexual selection.

Sexual selection may have also contributed to the evolution of our unusually large brain. The brain is responsible for about a quarter of our body's consumption of oxygen and glucose. It's also about as big as it can be while fitting through the birth canal; in fact, the female pelvis had to widen during human evolution in order to accommodate a human infant's large skull. Although the brain is capable of amazing feats, it's not necessarily clear that the adaptive benefits from a big brain would justify such extreme costs; after all, other animals with smaller brains can use tools, forage for food and co-ordinate hunting in groups. However, both the costliness and the extreme size would be in keeping with our exceptionally large brain being a product of sexual selection. In The Mating Mind, Geoffrey Miller suggested that things like creativity, music, art and humour might have been used as courtship signals by choosy early humans, leading to runaway selection for these signals driving the evolution of a larger and larger brain. So it might be that your sexiest organ is, in fact, between your ears.

Image credits
The peacock tail image is from a photo by Wikimedia user Jebulon and is distributed under a CC-BY-SA license. The salmon image is by Timothy Knepp of the US Fish and Wildlife Service and is in the public domain.