Intra-sexual Selection involves competition between members of the same sex for access to the opposite sex. This often results in the evolution of 'weapons' such as large body size, strength, and aggressive behaviors that allow individuals to dominate rivals.
Inter-sexual Selection involves one sex (the 'choosy' sex) selecting a mate based on specific attractive qualities. This leads to the evolution of 'ornaments' or displays that appeal to the preferences of the opposite sex, such as complex songs or colorful features.
Runaway Selection occurs when a female preference for a specific male trait and the trait itself become genetically linked. Over generations, this can lead to the trait becoming increasingly exaggerated until the survival costs of the trait balance out the reproductive benefits.
| Feature | Intra-sexual Selection | Inter-sexual Selection |
|---|---|---|
| Primary Actor | Competition within one sex | Choice between the sexes |
| Typical Goal | Dominance over rivals | Attraction of a mate |
| Resulting Traits | Weapons, size, aggression | Ornaments, displays, resources |
| Strategy | Quantity (for the non-choosy sex) | Quality (for the choosy sex) |
When analyzing a scenario, first identify which sex is competing and which is choosing to determine if the mechanism is intra-sexual or inter-sexual. Look for keywords like 'combat' or 'territory' for intra-sexual, and 'display' or 'preference' for inter-sexual.
Always link evolutionary explanations back to reproductive success. Answering 'why' a trait exists should involve explaining how it helps the individual pass on their genes to the next generation.
Be prepared to discuss the trade-offs between survival and reproduction. Examiners often look for an understanding that a trait favored by sexual selection (like a heavy tail) might actually decrease an individual's chance of escaping predators.
A common mistake is assuming that sexual selection only applies to males. While males are often the competing sex due to lower parental investment, there are species with sex-role reversal where females compete and males are choosy.
Students often confuse survival with fitness. In evolutionary terms, 'fitness' is strictly about the number of viable offspring produced; an organism that lives a short time but reproduces many times is more 'fit' than one that lives long but never reproduces.
Avoid describing evolution as a conscious 'choice' or 'plan'. Traits do not evolve because an animal 'wants' to be attractive; they evolve because individuals with those traits successfully reproduce more often than those without them.
