Sexual Selection, Mating Systems, and Reproductive Conflict
Selection Through Reproduction
Sexual selection
Sexual selection is selection on traits that increase mating success. It can favor ornaments, displays, weapons, courtship behavior, and gamete competition even when those traits reduce survival.
Mechanisms
Intersexual selection involves mate choice, while intrasexual selection involves competition between members of the same sex. Sexual selection can generate strong dimorphism and rapid trait divergence.
Trade-off
A trait may be costly for survival but valuable for reproduction, so evolution optimizes total fitness rather than survival alone.
Which example best illustrates intrasexual selection?
Competition within a sex for access to mates is intrasexual selection.
Correct answer: Males competing physically for access to females
Why can sexual selection produce exaggerated traits that seem maladaptive for survival?
Selection favors net reproductive success, not survival in isolation.
Correct answer: Because traits that increase mating success can outweigh survival costs in terms of overall fitness.
Mating-system effects
Monogamy
- Often reduces intense competition for mates
- May reduce sexual dimorphism
Polygyny
- Can intensify competition among males
- Often increases sexual selection on male traits
Reproductive conflict
Conflict can occur among sexes, among mates, and between nuclear and cytoplasmic genomes. Examples include sperm competition, mate guarding, and sexually antagonistic selection, where a trait benefits one sex but harms the other.
Which outcome is most likely under strong mate choice for a display trait?
Mate choice can amplify and exaggerate display traits over time.
Correct answer: Increased trait elaboration
Analyzing sexual selection cases
-
1
Identify whether mate choice or competition is involved.
-
2
Ask which sex bears the stronger selection pressure.
-
3
Consider survival costs versus mating benefits.
-
4
Predict whether dimorphism or signal exaggeration should increase.