Biology Chapter on Selection

Study Notes

### Comparative Vertebrate Biology - Sexual Selection

Selection is the non-random differential survival or reproduction of organisms.
Darwin defined it as "the principle by which each slight variation [of a trait], if useful, is preserved."

Types of selection (directional, disruptive, stabilizing, sexual, artificial)
Mechanisms and examples of sexual selection (same-sex contests, mate choice, intrasexual selection, intersexual selection, coercive mating, sperm competition)
Evolutionary consequences (sexual dimorphism, alternative mating strategies, sexual conflict, human evolution)
Interactive discussion
Conclusions

Directional selection: Favors one extreme of a trait over the other. Example: Giraffe necks lengthening.
Disruptive selection: Favors both extremes of a trait over the intermediate. Example: Light and dark moths.
Stabilizing selection: Favors the average or intermediate trait. Example: Mouse fur color.
Sexual selection: Based on an organism traits appeal to the opposite sex or competition with same sex
Artificial selection: Deliberate selection by humans of a chosen trait or set of traits. Example: Dog breeds.

Same-sex contests: Competition among males. Example: Male elephant seals fighting for females. Male Elk competing for mates.
Scramble competition: Competition to locate and acquire mates.
Endurance rivalry: Competition based on stamina.
Sperm competition: Competition among sperm to fertilize an egg. High in species with multiple males.
Coercion: Forcing copulation.
Mate choice: Example: Female choice of colourful male birds, males advertising their "fitness."

Sexual dimorphism: Observable differences between males and females of a species due to selection pressure. Example: Peacocks, deer antlers.
Alternative mating strategies: Different mating behaviours within a species. Example: Sneaker males in swordtails.
Sexual conflict: Evolutionary struggle between the sexes due to conflicting reproductive strategies. Example: Coercive mating in waterfowl.
Human evolution: Darwin, human beauty standards. Evidence suggests the effects are small but not zero.

Sexual selection is a component of fitness and a potential driver of evolutionary changes.
The specific traits influenced by sexual selection vary greatly across species.
There are examples of mate choice "runaway" selection where preference and the trait related are linked in a positive feedback loop.