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D4.1.7—Sexual selection

Sexual selection favours traits that increase mating success, even when those traits may reduce survival in evolving populations in evolving populations.

Syllabus
First assessment 2025
Objective
D4.1.7
Level
HL

Exam analysis

Chance of appearing4%of analysed past papers
Latest appearanceNovember 2020
Most common paperPaper3
Typical marks3

Common command terms

  • Explain
  • Suggest
  • Evaluate

Recent exam appearances

November 2020Paper3 ["HL"] · TZ08(b)[ 3 ]D4.1.7—Sexual selection
November 2020Paper3 ["HL"] · TZ08(a)[ 2 ]D4.1.7—Sexual selection
November 2019Paper3 ["HL"] · TZ07(b)[ 3 ]D4.1.7—Sexual selection
November 2018Paper3 ["HL"] · TZ07(a)[ 3 ]D4.1.7—Sexual selection
November 2013Paper3 ["HL"] · TZ06[ 6 ]D4.1.7—Sexual selection

Coverage 2013–2020 · Updated 16 Jul 2026

Sexual Selection Rewards Traits That Increase Access to Mates

Route Selection event Example trait
mate choice one sex preferentially mates with particular partners bright plumage or complex courtship display
mating competition individuals compete directly or indirectly for access body size, weapons, territories or dominance behavior

heritable variation in display or competition trait → unequal mating success → unequal offspring contribution → associated alleles increase

Persistent differences in selected traits between sexes can produce sexual dimorphism, such as bright displaying males and more cryptic females.

A Mating Advantage Can Persist Despite a Survival Cost

Effect of an exaggerated ornament Possible fitness consequence
more visible or energetically costly lower survival through predation or resource cost
attractive display or honest condition signal greater chance of mating
species-specific courtship signal mating isolation from other populations

The trait can spread when its gain in reproductive success exceeds its survival cost. Fitness integrates the whole route to descendants, not survival alone.

Sexual selection names the pressure—competition for mates or mate choice—rather than competition for food or escape from predators.

Endler’s Guppies Reveal a Trade-off Between Predation and Mate Choice

Evidence Interpretation
spots increased before strong predators were added female choice favored conspicuous males
mean spot number fell with C. alta dangerous predators selected against conspicuous males
weak-predator field transplants became more colorful over 15 generations reduced predation allowed sexual selection to dominate
spot size tracked gravel under predation background matching improved camouflage

Natural and sexual selection acted on the same heritable color traits in opposing directions. The observed phenotype reflected the balance of those pressures, not one universal best color.

Guppy treatments K, R and C show spot number rising with no or weak predation and falling under dangerous C. alta predation.

Natural Selection Connects Variation, Environment and Reproduction Across Generations

mutation creates alleles + sexual reproduction reshuffles them → offspring vary → limited resources and abiotic or biotic pressures produce unequal survival and mating → heritable variants reach offspring unequally → population trait and allele frequencies change

Claim to test Evidence needed
adaptation occurred by selection heritable variation plus unequal reproductive contribution across generations
sexual selection acted trait differences predict mating success
environmental pressure caused change controlled manipulation or strong comparative evidence links pressure to fitness
population evolved heritable trait or allele frequency changed, not merely one individual

Avoid purpose-driven language. Populations do not create useful mutations on demand; selection filters inherited variation already produced by mutation and recombination.

Sexual selection

[ 6 ]

Explain what is meant by exaggerated traits and how they may develop in males of a species.

Concept essentials

  • Sexual selection acts through mating success.
  • A trait can spread if it increases reproduction despite survival costs.
  • Mate choice and competition for mates are common sexual selection mechanisms.