A peacock's tail is metabolically expensive to grow, makes the bird conspicuous to predators, and reduces escape speed. Yet peacocks with larger, more elaborate tails mate more frequently, and the trait has become more extreme over generations. What does this tell us about sexual selection's relationship to natural selection?
ASexual selection operates against natural selection; the tail would eventually disappear if natural selection were the dominant force
BThe tail must provide hidden survival benefits — perhaps camouflage or thermoregulation — that compensate for its apparent costs
CSexual selection is a form of natural selection acting through mating success; a trait spreads if reproductive gains sufficiently outweigh survival costs
DThis is an evolutionary paradox: costly traits cannot increase in frequency under Darwinian theory
The key insight is that natural selection acts on total reproductive fitness, not survival alone. Fitness includes both survival probability and mating success. If a tail increases a peacock's matings enough to more than compensate for its survival costs, the net effect on gene transmission is positive, and the trait spreads. Sexual selection is not opposed to natural selection — it is a subset of natural selection, specifically the component acting through differential mating success. Darwin was right to identify it as a distinct mechanism because it can drive traits in the opposite direction from survival-only selection, but the underlying logic is identical: genes that increase representation in the next generation spread.
Question 2 Multiple Choice
What is the key distinction between the 'good genes' hypothesis and the 'runaway selection' hypothesis as explanations for why females prefer costly male ornaments?
AGood genes: females are instinctively attracted to bright colors; runaway: females make deliberate mate assessments
BGood genes: ornaments reliably signal heritable genetic quality, giving choosy females offspring with better fitness; runaway: a preference and the preferred trait coevolve in a self-reinforcing feedback loop that can escalate beyond any indicator value
CGood genes: applies only to birds; runaway: applies only to insects and fish
DGood genes: predicts sexual dimorphism; runaway: predicts sexual monomorphism
Both hypotheses explain female preference for costly ornaments, but through different mechanisms. Under the good genes (indicator) hypothesis, a costly ornament is an honest signal: only genuinely healthy, well-nourished males can afford to produce a brilliant tail, so the tail reveals underlying quality. Females who choose ornamented males get better genes for their offspring. Under runaway selection (Fisherian), the process starts with an arbitrary preference: females who prefer some male trait (for any reason) have sons who inherit the trait and daughters who inherit the preference, so preference and trait coevolve and escalate together. The ornament can become extreme even if it no longer signals any real quality — the preference itself sustains the selection.
Question 3 True / False
Because sexually selected traits often reduce survival, sexual selection works in the opposite direction from natural selection.
TTrue
FFalse
Answer: False
Sexual selection is a component of natural selection, not its opposite. Natural selection encompasses all processes that cause differential reproduction — including survival and mating success. A peacock's tail reduces survival but increases mating success; whether the trait spreads depends on the net effect on total reproductive fitness. Sexual selection can drive traits that harm survival, but only when the mating gain more than compensates for the survival cost. The two components of fitness are not opposing forces; they are additive influences on gene transmission.
Question 4 True / False
Species with more intense sexual selection tend to show greater sexual dimorphism — larger differences in appearance between males and females of the same species.
TTrue
FFalse
Answer: True
Sexual dimorphism is a direct readout of sexual selection intensity. When one sex (typically males) is subject to strong competition for mates or strong female choice, selection drives the divergence of male and female appearance. Elephant seals, where dominant males monopolize harems, show extreme body-size dimorphism. Birds of paradise, with intense female mate choice, show extreme plumage dimorphism. Monogamous species where both sexes choose show minimal dimorphism. This pattern across species is one of the strongest pieces of evidence that sexual selection, not other forces, drives the evolution of sex differences.
Question 5 Short Answer
Why can sexually selected traits that reduce survival (like a peacock's tail or elk antlers) still increase in frequency and become more elaborate over evolutionary time? What two conditions must be met?
Think about your answer, then reveal below.
Model answer: A costly sexually selected trait can spread when: (1) the increase in mating success it provides more than compensates for the reduction in survival — so overall reproductive fitness is positive — and (2) the trait is heritable, so that the mating advantage translates into more offspring that inherit the trait. The net effect on gene transmission across a lifetime must be positive. Traits can escalate over time through intersexual selection (female preferences driving ever-more-elaborate ornaments via runaway selection) or through intrasexual competition (males with larger weapons winning more contests and passing on those traits). The trait stabilizes only when the fitness costs of further elaboration balance the mating gains.
The key to understanding sexual selection is that 'fitness' is not the same as 'survival.' Fitness is the expected number of surviving offspring an individual leaves. A peacock that dies slightly younger but mates four times as often may leave more offspring overall than a survival-optimized peacock that rarely mates. Darwin's insight was that this logic can drive traits in a direction that seems paradoxical from a survival-only perspective — but is perfectly coherent from a total reproductive fitness perspective.