Sexual Selection

Intrasexual Selection

Competition within one sex (usually males) for control over mates or resources (territory/nesting).

Intersexual Selection

Individuals of one sex (usually females) choose the "best" advertisement (songs/colors/dances) among the other sex.

Male-Male Competition

Leads to selection for weaponry (armor, horns, claws), body size, and tactical intelligence to establish dominance.

Marine Iguana Size

Exhibits stabilizing selection for medium size due to metabolic limits; larger iguanas die in "lean" years because they can't maintain weight.

Iguana Sexual Dimorphism

Males violate survival selection (staying larger than the metabolic "max") because larger size wins more fights and mates.

Iguana Investment

Females invest 20% body mass in eggs/nesting (limited by production); males invest cheap sperm (limited by mate access).

Mate Choice & Hardy-Weinberg

Selective mating for displays is a type of nonrandom mating that represents a direct violation of Hardy-Weinberg equilibrium.

Heritability of Displays

If a display trait and the preference for it are heritable, alleles for "attractive" traits increase even if they hurt survival.

Red-Collared Widowbirds

Experiment showing long tails cause weight loss and poor nesting, but are maintained because they attract significantly more females.

Gray Tree Frog Choice

Females prefer long, continuous calls; this increases female fitness by producing offspring with better growth and survival rates.

Arbitrary Choice (Null Model)

The hypothesis that female preference evolves randomly via mutation/drift with no actual survival benefit to the offspring.

Genetic Correlation

Over time, genes for a male display and genes for a female preference become linked; daughters prefer what brothers display.

Zebrafish "GloFish"

Example of arbitrary preference where wild females prefer genetically engineered red males despite no evolutionary history.

Stalk-Eyed Fly Study

Wilkinson/Reillo proved that selecting for short stalks in males also caused females to evolve a preference for short stalks (traits are correlated).

Self-Perpetuating Selection

The process where a trait moves away from the "survival optimal" until the cost of death finally balances the benefit of mating.

Preexisting Sensory Bias

Hypothesis that female preference evolves first for non-mating reasons (food/predators) and males later exploit that bias.

Water Mite Predatory Bias

Males vibrate legs to mimic copepod prey; females "clutch" thinking it's food, allowing the male to release spermatophores.

Proctor (1991) Result

Proved male mites use "sensory exploitation" to fool females into mating by mimicking the vibrations of a swimming meal.

Resource Acquisition

Hypothesis where females choose mates based on immediate benefits, like the Hangingfly providing a "nuptial gift" of food.

Hangingfly Mating

Females mate only while eating the insect provided by the male; "preference" is actually for the meal, not necessarily the male's genes.

Better Genes Hypothesis

The idea that male displays are honest signals of high-quality "fitness genes" that will be passed to offspring.

Welch et al. (1998)

Experiment proving long-call frog offspring grew faster and survived better than short-call offspring, even on restricted diets.

Genetic Linkage Decay

Mathematical reality that drift and recombination usually break the link between "display genes" and "fitness genes" over time.

Sensory Predisposition

A response selected for benefits unrelated to reproduction (like finding food) that is later hijacked by the opposite sex for mating.

Sexual Selection Conclusion

Mating displays are occasionally about "better genes," but often result from arbitrary choice, sensory bias, or resource gain.