Week 11.1 Sexual Selection in Evolution

Sexual Selection

Evolution caused by differential mating success.

Sexual Dimorphism

Morphological or behavioral differences between sexes.

Darwin's Insight

Traits may evolve to improve mating success even if they lower survival.

Bateman & Trivers' Predictions

Eggs = expensive | Sperm = cheap.

Female reproductive success

Limited by number of eggs/offspring.

Male reproductive success

Limited by number of mates.

Intrasexual Selection

Direct competition (usually males).

Example: Marine Iguanas

Larger males win breeding territories.

Trade-off in mating

Big = better at mating, but lower survival.

Alternative Strategies in Iguanas

Small iguanas sneak-mate by pre-ejaculating into cloacal pouch.

Sperm Competition

When females mate with multiple males: More sperm = advantage.

Intersexual Selection

One sex chooses mates from the other based on traits.

Example of intersexual selection

Widowbirds: Long-tailed males attract more mates despite survival cost.

Runaway Selection

Females prefer a trait, causing exaggerated traits to evolve over generations.

Pre-existing Sensory Bias

Females evolved to respond to certain cues before sexual selection.

Resource Acquisition Hypothesis

Females choose mates who provide resources (food, care, protection).

Example of resource acquisition

Hanging flies: Males offer prey as gifts for mating.

Good Genes Hypothesis

Costly traits signal high genetic quality. Choosy females get better offspring.

Case Study: Rough-skinned Newts

Males: Yes. More mates = more offspring. Females: No benefit from more mates.

Why are males more affected by sexual selection?

Males produce cheap sperm and compete; females invest in eggs and are choosy.

What did Bateman & Trivers predict?

Sexual selection is stronger in males due to cheaper gametes.

What is sperm competition?

Males compete via sperm quantity/quality when multiple males mate with a female.

What is sexual selection?

Evolution from differences in mating success.

What is sexual dimorphism?

Physical or behavioral differences between sexes.