Sexual selection

Concept

Sexual selection

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Statement

Traits favoured through mate choice and competition.

Why it matters

natural-selection explains why traits that raise an organism's survival spread through a population, but it does not by itself explain traits, like a peacock's train or a stag's antlers, that plainly cost their bearer in exactly that currency — extra energy to grow and carry, extra visibility to predators, reduced mobility. sexual-selection is the extension that resolves the puzzle: it recognises that fitness-adaptation depends on reproductive success as a whole, and that a trait can be strongly favoured by increasing mating success even while it is mildly disfavoured by reducing survival, provided the net effect on reproduction is still positive.

The concept also feeds forward into speciation: sexually selected traits and the mate preferences that drive them can diverge rapidly between isolated populations, and divergent mate recognition systems are one of the more direct routes by which reproductive isolation, and so a new species, can arise.

Hypotheses

The two sexes commonly differ in how their reproductive success is limited: in species with strong asymmetry in gamete investment (anisogamy) and parental care, one sex's output is limited chiefly by access to resources or offspring capacity, the other's chiefly by access to mates.This asymmetry (formalised as Bateman's principle) is what generates unequal variance in reproductive success between the sexes and hence unequal selection pressure to compete for, or be choosy about, mates; without it, there is no general prediction of which sex should be ornamented or which should be the competitive one. A trait subject to sexual selection increases its bearer's access to mates through one (or both) of two distinct mechanisms: intrasexual selection (competition within one sex for mates) and intersexual selection (choice by one sex among potential mates of the other).These are mechanistically distinct and are not interchangeable: a weapon used in male–male combat is favoured because it wins fights, not because females inspect and prefer it directly, whereas an ornament assessed during courtship is favoured because it influences mate choice, not because it helps win physical contests. A costly ornament can function as a reliable ("honest") signal of underlying quality specifically because its cost scales with condition, so that only genuinely high-quality individuals can bear it without an unsustainable net fitness loss (the handicap principle).Not every sexually selected trait requires this particular explanation: Fisherian runaway selection (Step 5) can drive exaggeration through a purely self-reinforcing genetic correlation between preference and trait, without the trait needing to carry any honest information about quality at all. Both mechanisms are active areas of ongoing empirical work, and real traits may be shaped by a mixture of the two.

Proof

\text{In many anisogamous species, variance in reproductive success is markedly higher among males than among females.}

A female's reproductive output is typically capped by the number of eggs, offspring, or bouts of gestation and care she can physically sustain; a male, producing abundant, comparatively cheap gametes, faces a much looser physiological cap and is instead limited chiefly by how many females he can successfully mate with (Bateman's principle). A

\text{Because male reproductive success is mate-access-limited rather than resource-limited, a trait that increases mating access is favoured even at a net survival cost, provided the reproductive gain outweighs it.}

A male's overall fitness is the product of his survival probability and his expected number of matings given survival; increasing the second factor substantially can more than compensate for a moderate reduction in the first, which is precisely the trade-off a costly ornament or weapon represents. A

\text{Intrasexual selection favours traits that increase success in direct competition for mates: weaponry, body size, and aggression in the competing sex.}

Where mates (or the resources needed to attract them) are a limiting, defensible commodity, individuals that physically out-compete rivals gain disproportionate mating access; the antlers of deer and the canines of many primate males are standard examples of traits shaped predominantly by this route. A

\text{Intersexual selection favours traits assessed and preferred by the choosing sex during courtship, and can favour costly ornaments specifically because their cost makes them an honest signal of quality (the handicap principle).}

If bearing a trait is more costly for a low-quality individual than a high-quality one, only high-quality individuals can display an exaggerated version of it and still survive; choosers that use the trait to infer quality are therefore rewarded with genuinely better mates, on average, than choosers that ignore it, so both the ornament and the preference for it are jointly favoured by selection. B

\text{A preference allele and a trait allele can become statistically correlated across the population and reinforce one another in a positive feedback loop (Fisherian runaway), independent of any honest-signal content.}

Once even a weak initial preference exists, offspring that inherit both the preferred trait (from one parent) and the preference for it (from either parent) tend to be co-inherited together across generations; each generation's increase in trait exaggeration increases the mating advantage of carrying the preference, which increases the frequency of the preference, which further favours trait exaggeration — a runaway process checked only by the trait's opposing survival cost (natural-selection acting against it), reaching an equilibrium where the two opposing pressures balance. B

Result

\text{Sexual selection: differential mating success, via intrasexual competition and/or intersexual choice, can favour a trait even against opposing survival-based natural selection}

Reading. Reproductive fitness has two separable components — surviving to reproduce, and succeeding in competition or choice once able to reproduce — and a trait need only win on the net balance of the two, not on survival alone, to spread through a population.

Scope. Strongest and most extensively documented where reproductive investment is markedly asymmetric between the sexes (Hypotheses); in the minority of species with reversed or near-equal investment (Discussion), the predicted direction of competition and choice reverses or weakens accordingly.

Corollaries & converses

Because intrasexual and intersexual selection typically act on the higher-variance sex (Step 1), pronounced sexual dimorphism — systematic morphological or behavioural difference between males and females of a species — is a common, directly observable signature that sexual selection has been operating, alongside natural-selection acting on both sexes similarly.
evidence-common-descent's use of homology extends naturally here: closely related species with markedly different mating systems (and correspondingly different degrees of dimorphism) offer a comparative test of Bateman's principle, since the degree of dimorphism tracks the degree of variance-asymmetry in reproductive success fairly consistently across many independent lineages.
Converse: in species where parental investment is roughly equal between the sexes, or where the typical sex roles are reversed (Discussion), the predicted direction of competition and ornamentation reverses correspondingly — sexual selection is a mechanism defined by relative investment and mate-access variance, not by which sex happens to be male or female.

Fails without

Drop the reproductive-success variance asymmetry between sexes (Hypotheses): if both sexes faced identical limitation on reproductive output (no anisogamy-driven asymmetry), there would be no general prediction of which sex should evolve costly competitive or ornamental traits; sexual dimorphism in these traits, where it is observed, would need an entirely different explanation.
Drop the cost-scales-with-condition requirement underlying the handicap principle (Hypotheses, Tier 3): if a trait could be displayed at full, exaggerated intensity by low-quality and high-quality individuals alike, at equal cost, it would carry no reliable information about the bearer's underlying quality; choosers gain nothing from attending to it, the selective advantage of choosiness collapses, and the trait's maintenance would then require a different explanation, such as pure Fisherian runaway (Step 5).

Common errors

Treating sexual selection as something separate from, or in opposition to, natural selection generally; it is properly understood as a specific component of natural selection acting via differential mating success rather than differential survival (Why it matters).
Assuming male ornamentation and competition are a universal rule of nature rather than a consequence of typical (not universal) investment asymmetry; in sex-role-reversed species such as seahorses, pipefish, and some phalaropes and jacanas, where males invest more in offspring care, it is females that compete and display (Discussion).
Assuming every costly, conspicuous trait must be an honestly signalling, sexually selected ornament; some conspicuous traits are better explained by other causes (genetic drift, pleiotropic byproducts of a trait selected for a different reason, or simple developmental constraint), and demonstrating a genuine mating-success advantage is required before invoking sexual selection specifically.
Confusing intrasexual selection (competition, typically same-sex rivals) with intersexual selection (choice, typically between the sexes) as though they were the same mechanism; a trait can be shaped by one, the other, or both simultaneously, and the evidence required to demonstrate each differs.

Discussion

Charles Darwin introduced sexual selection in The Descent of Man, and Selection in Relation to Sex (1871), explicitly to explain traits like elaborate plumage, antlers, and vivid ornamentation that seemed to sit awkwardly against the survival-focused account of adaptation he had given in On the Origin of Species (1859). Ronald Fisher later formalised the runaway-selection feedback loop of Step 5 mathematically, showing how a self-reinforcing correlation between a preference and a trait could, in principle, drive elaboration well past whatever a pure survival optimum would predict.

A separate line of explanation, the sensory bias hypothesis, proposes that some mate preferences originate not from any information content in the trait at all, but because a pre-existing bias in the choosing sex's sensory or nervous system (evolved originally for an unrelated function, such as foraging) makes certain signals more detectable or attractive; a trait that happens to exploit an already-existing bias can spread even before it carries any reliable signal of quality, after which honest-signal and runaway dynamics may subsequently take over and further shape it.

Common misconception: that a trait favoured by sexual selection must also be good for the species, or must represent some kind of optimal design. Sexually selected traits are favoured purely by their effect on their bearer's own relative reproductive success, even when they measurably reduce individual survival and, in extreme cases, plausibly increase a population's vulnerability — sexual selection has no mechanism that privileges group-level outcomes over individual reproductive advantage.

Worked examples

\text{Peacock train: costly to grow, costly to carry, increases predation risk, yet persists and is elaborated across generations.}

Peahens preferentially mate with males bearing larger, more symmetric, higher-eyespot-count trains (intersexual selection, Step 4); because only well-fed, parasite-free, generally high-condition males can grow and carry an especially large train without an unsustainable survival cost, train size functions as an honest, hard-to-fake signal of male quality, exactly the handicap-principle logic of Step 4. A

\text{Red deer stags: large antlers and roaring contests determine access to a harem of females, independent of any female inspection of antler size per se.}

This is intrasexual selection (Step 3): antlers function as weapons and status signals used in direct male–male contests over harem control, not primarily as an ornament assessed by choosing females — the same broad outcome (larger, costlier structures favoured in the higher-variance sex) arising through the competition mechanism rather than the choice mechanism. A

\text{Peacock: intersexual selection, honest signalling} \qquad \text{Red deer: intrasexual selection, direct competition}

Reading. The same underlying asymmetry in reproductive-success variance (Step 1) can express itself through either mechanism, or both together in the same species, depending on whether mates are won by out-competing rivals, by being chosen, or by some combination of the two.

Scope. Distinguishing which mechanism (or mixture) is operating in a given species requires direct behavioural evidence — observed fights and dominance hierarchies for intrasexual selection, observed preferential mate choice for intersexual selection — not just the presence of a dimorphic trait alone.

Problems

In a species where males provide all parental care and females compete intensely for mates, predict which sex is expected to be more ornamented and more variable in reproductive success, and explain why using Step 1.
Solution
By Step 1's logic applied with the investment roles reversed, males (the higher-investing, more limiting sex) become the choosier sex and the limiting resource, while females (freed from most parental duties) compete for access to males; females are therefore predicted to be the more ornamented and competitive sex, and to show the higher variance in reproductive success — the opposite pattern from the typical case, but driven by the identical mechanism. This sex-role reversal is observed in several real species, including seahorses and some shorebirds.
Two hypotheses are proposed for an elaborate male courtship display: (a) it is an honest signal of male condition (handicap principle), (b) it arose through Fisherian runaway with no information content. Describe one type of evidence that could help distinguish between them.
Solution
Testing hypothesis (a) directly: measure whether display quality correlates with an independent, objective measure of male condition (parasite load, body condition, survival probability, offspring viability) — a genuine correlation supports the handicap principle; no such correlation, despite strong female preference for the trait, is more consistent with pure Fisherian runaway, where the trait's exaggeration is maintained by the preference-trait genetic correlation alone rather than by any real information about quality.
Explain why a trait that reduces a male's survival probability can still spread through a population under sexual selection, using Step 2.
Solution
By Step 2, overall male fitness is (survival probability) × (expected matings given survival). A trait can reduce the first factor while increasing the second by a proportionally larger amount, so the product — and hence net fitness — still rises; natural selection acting on survival alone would disfavour the trait, but the combined reproductive calculus that sexual selection makes explicit favours it, provided the mating-success gain outweighs the survival cost.