Sex selection

Sexual selection is the process by which differences in success at attracting mates shape the traits passed into future generations. In The Selfish Gene, it extends the gene-centered argument from survival and parental investment to courtship: a trait can spread not only because it helps its bearer live, but because it helps secure reproduction. The account’s central paradox is that attraction can become a force of its own, favoring conspicuous features even after their practical costs grow substantial.

Mate choice and inherited advantage

The discussion begins with the possibility that females select males for signs of genuinely useful qualities. Strength might indicate an ability to obtain food; long legs might suggest an ability to escape predators. Such traits could benefit offspring of either sex. But a female’s choice can have a further consequence: if a son inherits an attractive quality and wins many matings, he may produce a large number of descendants for his mother. Under those conditions, sexual attractiveness itself becomes advantageous, beyond any original connection to survival or physical utility.

This argument turns mate choice into a feedback process. A female who chooses a highly attractive male may have sons attractive to females in the next generation. Those sons can in turn secure many matings, so the preference and the feature it favors may reinforce one another. The account thus distinguishes the initial usefulness that might make a trait noticeable from the later reproductive advantage of being desired.

Runaway traits and their costs

The male tail of a bird of paradise illustrates the proposed runaway process. A slightly longer-than-average tail might initially have been favored as an indicator of fitness: a shorter tail could have suggested poor condition or an inability to escape predators. If tail length varied heritably, female preference for longer tails could gradually increase the population’s average. Once long tails became the standard of attractiveness, females that did not choose long-tailed males could have fewer attractive sons, giving the preference continuing force even if the tail had become a burden to its bearer.

The development is compared to a fashion trend that gathers momentum. It continues until the disadvantages of the increasingly elaborate feature outweigh its mating advantage. The point is not that a tail must remain useful in an ordinary survival sense, but that inherited variation and mate preference can sustain a trait through reproductive success while its costs accumulate. The example makes sexual selection a possible explanation for the apparently paradoxical features of animals, including the showy tails of birds of paradise.

Rival explanations and evidentiary caution

The account identifies this explanation with sexual selection, first proposed by Charles Darwin and clearly articulated by R. A. Fisher. It also stresses that the theory has attracted skeptics. Amotz Zahavi’s handicap principle offers a competing interpretation: if females can be deceived by false advertisements of quality, selection should favor displays that cannot be produced cheaply by an inferior male. A conspicuous feature that imposes a real cost may therefore serve as evidence that its bearer can withstand that cost. This alternative gives costly display a different logic from runaway preference: the burden itself can make the signal credible.

The contrast clarifies what the explanations share and where they differ. Both address why females might respond to male traits and why those traits may become elaborate. In the runaway account, a preference and an attractive inherited feature can escalate together; in the handicap account, a costly feature demonstrates genuine quality because an inferior bearer could not sustain it. The chapter presents the latter as a rival theory, not as a settled replacement, and treats the origin of elaborate displays as a question requiring explanation rather than as proof of one mechanism.

Sexual selection among competing pressures

Sexual selection also operates alongside the pressure to avoid predators. Bright coloration may make an animal more attractive to potential mates but also more likely to be noticed by predators. In gene-centered terms, genes associated with bright colors may be more likely to be removed through predation, while genes associated with drabness may be less likely to pass on if their bearers have difficulty attracting mates. The resulting phenotype is a compromise between conflicting selection pressures, not the product of a single, universally favorable trait.

The compromise may differ between males and females. The chapter links that difference to reproductive asymmetry: a male produces many millions of sperm for every egg produced by a female, so a male that gains numerous matings may father many offspring even if his conspicuous appearance shortens his life. A drab male, by contrast, may survive for a long time but leave few descendants. Sexual selection therefore does not simply oppose survival; it changes how survival and reproduction weigh against each other for different individuals.

Genes, traits, and the limits of shorthand

The explanation treats genes as the units whose relative representation changes through differential survival and reproduction, but it does not require genes to act as independent designers of particular traits. A survival machine contains thousands of genes whose effects interact: a body part is influenced by many genes, and the effect of any one depends on other genes and the environment. Expressions such as a gene for a behavior or physical feature are shorthand for an influence on development, not a claim that a single gene constructs the trait on its own.

That qualification matters to sexual selection because the argument depends on inherited variation without reducing a complex trait to a simple genetic switch. The proposed runaway increase requires some genetic contribution to natural variation in male tail length; the account does not require every detail of the tail or the female preference to be controlled by one gene. More broadly, the explanation fits the book’s insistence that natural selection works through the differential success of genes in the gene pool, while visible traits emerge from intricate developmental interactions. Sexual selection is thus not an exception to the gene-centered view but an account of how reproductive preferences can alter which heritable variations persist.