Difficulty: Introductory to Intermediate | Prerequisites: Understanding of natural selection and modes of selection.
Sexual reproduction is slower and more costly than asexual reproduction, yet it dominates the natural world. This unit asks why sex exists at all, then explores how the different reproductive investments of males and females drive sexual selection, a form of natural selection that produces some of the most striking traits in the animal kingdom. If you understand directional selection, you already have the toolkit to follow this material.
Sex is costly but maintained because it generates genetic diversity that helps populations survive changing environments. Males and females invest differently in reproduction, which creates sexual dimorphism through intrasexual competition and intersexual mate choice. Several hypotheses (Sexy Son, Good Genes, Handicap, Parasite) explain what drives female choice.
Sexual reproduction
Reproduction involving the fusion of gametes from two parents, producing genetically unique offspring. The key advantage is genetic variation.
Asexual reproduction
Reproduction from a single parent, producing genetically identical offspring (clones). Faster and more efficient, but generates no genetic diversity.
Sexual dimorphism
Observable differences in appearance between males and females of the same species, beyond the reproductive organs themselves. Think of the peacock's tail or the lion's mane.
Fundamental asymmetry of sex
The core principle that males and females invest differently in reproduction. Females typically invest more per offspring (large, nutrient-rich eggs, gestation, nursing), so their reproductive success is limited by resources. Males typically invest less per offspring (small, cheap sperm), so their reproductive success is limited by mating opportunities.
Intrasexual selection
Competition within one sex (usually males) for access to mates. Leads to traits like large body size, weapons (antlers, tusks), and aggressive behaviour.
Intersexual selection
Mate choice exercised by one sex (usually females) based on traits displayed by the other sex. Leads to elaborate ornaments, displays, and courtship behaviours.
Tangled Bank hypothesis
The idea that sex is favoured because it produces genetically diverse offspring, which can exploit a wider range of ecological niches in a complex, variable environment.
Red Queen hypothesis
The idea that sex is favoured because it generates genetic diversity needed to keep up with rapidly evolving parasites and pathogens. Named after the Red Queen in Alice in Wonderland: "It takes all the running you can do, to keep in the same place."
Sexy Son hypothesis
A hypothesis for why females prefer certain male traits: females who mate with attractive males produce sons who inherit those attractive traits, which increases the sons' mating success. The female's preference spreads because her sons attract more mates.
Good Genes hypothesis
A hypothesis proposing that female mate preferences target indicators of male genetic quality. A male who can survive despite carrying a costly ornament must have good genes in other respects.
Handicap hypothesis
A specific version of the Good Genes idea. Costly, exaggerated traits (the peacock's tail, for instance) are reliable signals of male quality precisely because they are expensive to produce and maintain. Only genuinely fit males can afford the handicap.
Parasite hypothesis
A Good Genes variant proposing that females choose mates with traits that signal resistance to parasites. Bright plumage or vigorous displays suggest a male is relatively free of parasites and can pass that resistance to offspring.
Developmental stability
The ability of an organism to develop a consistent, symmetrical phenotype despite environmental and genetic stresses during development. Symmetry is thought to signal genetic quality, and females in some species preferentially mate with more symmetrical males.
Why does sex exist?
Asexual reproduction is faster and avoids the "cost of males" (only half the population produces offspring). Two leading hypotheses explain why sex persists despite these costs:
Tangled Bank hypothesis: a varied environment favours genetically diverse offspring that can partition resources across different niches.
Red Queen hypothesis: parasites and pathogens evolve rapidly. Sexual reproduction reshuffles the host's genome each generation, making it harder for parasites to adapt.
The fundamental asymmetry of sex
This is the engine behind sexual selection. Eggs are large, metabolically expensive, and few. Sperm are small, cheap, and abundant.
Female fitness is limited by resources for producing and rearing offspring.
Male fitness is limited by mating opportunities.
This asymmetry predicts that females will be choosy and males will compete for access.
Intrasexual selection (male-male competition)
Males compete directly with each other for mating access.
Favours large body size, weaponry (antlers, horns, large canines), and aggressive behaviour.
Produces sexual dimorphism in body size and armaments.
When intrasexual selection is strong, a small number of dominant males sire most offspring, creating high variance in male reproductive success.
Intersexual selection (female choice)
Females choose among competing males based on displayed traits.
Favours ornaments, bright colours, elaborate courtship displays.
Produces sexual dimorphism in ornamentation.
Hypotheses for what drives female choice
Sexy Son: females prefer attractive males because their sons will inherit that attractiveness, boosting the sons' reproductive success. This is a self-reinforcing (runaway) process.
Good Genes (including Handicap and Parasite hypotheses): females prefer males whose traits honestly signal genetic quality.
Handicap: the trait is costly enough that only high-quality males can afford it. The cost is the point, as it makes the signal reliable.
Parasite: bright, healthy-looking males signal parasite resistance.
Developmental stability: symmetrical individuals signal the ability to develop normally despite stresses, which correlates with genetic quality.
Sexual selection explains why male birds of paradise have extravagant plumage while females are cryptically coloured. In livestock breeding, understanding the fundamental asymmetry is central to artificial selection programmes: one bull can sire thousands of calves, while each cow produces one per year. The Red Queen hypothesis has direct implications for agriculture, where genetically uniform crops are vulnerable to rapidly evolving pathogens.
Students often think sexual selection is separate from natural selection. Sexual selection is a form of natural selection, specifically acting on traits related to mating success.
A common mistake is assuming that female choice is always about "beauty." Females are selecting for signals of genetic quality or offspring viability, not aesthetics in the human sense.
Students sometimes think the Sexy Son and Good Genes hypotheses are the same thing. Sexy Son is about sons inheriting attractiveness (a self-reinforcing loop). Good Genes is about sons (and daughters) inheriting quality genes for survival.
The fundamental asymmetry is a generalisation. In species where males invest heavily (e.g. seahorses, where males carry the young), the roles can reverse, with males being choosier.
The fundamental asymmetry of sex is a core exam concept. Know the correct characterisation: female fitness limited by resources for producing/rearing young, male fitness limited by mating access.
Be able to distinguish intrasexual from intersexual selection and predict which types of traits each favours.
Expect a question asking you to determine, from a description of sexual dimorphism, whether intrasexual or intersexual selection is the dominant force.
Know the difference between Sexy Son and Good Genes hypotheses.
True or false: Asexual reproduction produces more genetic diversity than sexual reproduction.
Answer: False. Sexual reproduction generates diversity through recombination and independent assortment.
Fill in the blank: The ______ hypothesis explains sexual reproduction as an arms race between hosts and parasites.
Answer: Red Queen.
True or false: Intrasexual selection refers to female choice of male traits.
Answer: False. Intrasexual selection is competition within one sex (usually male-male competition). Female choice is intersexual selection.
Fill in the blank: The Handicap hypothesis predicts that reliable sexual signals must be ______ to produce.
Answer: Costly.
Q: Why does sexual selection often lead to sexual dimorphism? Why are males usually the sex that exhibits exaggerated characteristics?
A: Sexual selection favours different traits in each sex because of the fundamental asymmetry of reproductive investment. Males, whose fitness is limited by mating opportunities, benefit from traits that help them outcompete rivals (intrasexual selection) or attract choosy females (intersexual selection). Females, whose fitness is limited by resources for offspring, benefit from choosing high-quality mates rather than developing costly ornaments. The result is males with exaggerated features and females with more cryptic phenotypes.
Q: Which statement most fully characterises the fundamental asymmetry of sex? (A) Female fitness limited by resources for producing eggs and rearing young, male fitness limited by ability to attract females. (C) Both limited by resources. (D) Both limited by mate attraction.
A: Option A. Female fitness is constrained by the resources needed to produce and rear offspring. Male fitness is constrained by access to mating opportunities (attracting females). This asymmetry drives the different selection pressures on each sex.
Q: You discover a mammal where males are twice the size of females and have canine teeth three times as long. Is it likely that most males produce roughly the same number of offspring?
A: No. The extreme sexual dimorphism in size and weaponry suggests intense intrasexual competition. In such systems, a few dominant males typically monopolise mating, producing high variance in male reproductive success. Most males likely sire few or no offspring.
Q: In gorilla social groups, a new dominant male often kills existing young offspring. Why is this adaptive for the male, and why might a female still mate with him?
A: For the male, infanticide eliminates offspring that carry a rival's genes and causes the female to return to oestrus sooner, so the new male can sire his own offspring more quickly. For the female, mating with the dominant male may be adaptive because his proven ability to take over a group signals strong genes that her sons could inherit (Good Genes / Sexy Son logic), giving her future offspring a competitive advantage despite the immediate cost.
Sexual selection is a specific application of natural selection, so it connects directly to the modes-of-selection material (directional selection on male traits is common). The concept of the fundamental asymmetry also links to the social behaviour unit: parental investment decisions influence whether altruistic or selfish strategies evolve. The Red Queen hypothesis ties back to genetic diversity and, by extension, to why HWE is violated in populations under parasite-driven selection.
Sexual selection, sexual dimorphism, intrasexual selection, intersexual selection, female choice, male-male competition, fundamental asymmetry of sex, Tangled Bank hypothesis, Red Queen hypothesis, Sexy Son hypothesis, Good Genes hypothesis, Handicap hypothesis, Parasite hypothesis, developmental stability, sexual reproduction vs asexual reproduction, BBIO 180, intro biology