Sexual Selection

Elaborate traits, songs, dances, fights

Sexual selection• Recall:

– Special case of natural selection– Selection for enhanced ability to obtain

mates– Acts on phenotypes, so any genes

that help produce the beneficial phenotype will increase in frequency in the population

Bateman-Trivers Hypothesis

• Observation: Males have elaborate traits involved in courtship & mating, females do not

• Hypothesis– Pattern: Females choose males based on

traits; sexual selection acts more strongly on males than on females

– Mechanism: “eggs are expensive, sperm are cheap”** Females invest lots, males invest little

Fundamental Asymmetry of Sex

• 2 consequences*1. Because eggs are expensive,

females produce relatively few young over lifetime. Fitness limited by access to resources.

2. Sperm is inexpensive, so males can sire limitless offspring. Fitness limited by # of females he can mate with.*

Predictions?• Females invest lots, should choose mates

carefully– Bigger, stronger, healthier, etc…why?

• Males invest little, should compete with other males for females– Only limit to Reproductive Success (RS) are other

males• Sexual selection is stronger on males to

develop traits for attraction, courtship, or competition

• Male RS should be more variable than female RS

Support: Female choice• On what factors do female birds base

their choice of mates?– Pretty colors!

1. Males have bright plumage or beaks due to ingestion of carotenoids

• Carotenoids stimulate immune response & repair free radical damage

• Must obtain carotenoids from diet

Support: Female choice1. Males have bright plumage or beaks

due to ingestion of carotenoids• Suggests that the brightest beaks

= best fed (good foragers or competitors) & healthiest

– After stimulating immune system, enough carotenoids are “left over” to be used for producing color as well.

Is that a real trade-off?• Can a sickly,

unhealthy bird have a bright beak? Is color an honest signal?

• Ex: European blackbirds

• Carotenoids produce beak color

Is that a real trade-off?• Experimental group -

stimulate immune system by injecting RBC’s from sheep

• Control group - inject with saline solution…why?

• Results - experimentals dimmed, controls remained bright

• Conclusion: Experimentals diverted carotenoids to boost immune system

• beak color = honest signal of health

Do they prefer brighter beaks?

• Experimental - supplement with carotenoids

• Control - no supplement; groups are brothers. Why?– Brothers are genetically,

and therefore phenotypically, very similar

• Allow females to associate with whomever

Other traits females choose• Any display that suggests “I’m healthy

& well-fed. I’ll provide advantageous alleles for your offspring” – Songs– Dances– Other courtship displays

• Resource provisioning (nuptial gifts)• Protecting nest site

Proof: Male-Male Competition

• Male sexually selected traits are also those that enhance competitive ability– Giraffe necks? Bighorn sheep; wrestling

snakes• Used in battles for females, or to

defend territories with or without resources

Elephant seals• Females haul out in

large groups on small beaches

• Males battle for territories on beaches

• Males mate with females in their territories*– Actually fighting for

females; winners monopolize matings

Clicker Q• If males battle for territories, what

size of males do you expect to win more battles?

1. Large 2. Intermediate3. Small

Clicker Q• If winners monopolize matings with

more females, which males do you expect to have higher reproductive success?

1. Large 2. Intermediate3. Small

Proof: Asymmetry in R.S. • Few males leave

lots of offspring• Most males leave

none• What does this do

to Ne?1. Lowers Ne

2. Increase Ne

3. No effect

Consequences ofsexual selection

• In most animal species, ANY female that survives to adulthood will mate

• Many males get NO mates at all• Sexual selection is more intense in

males– Leads to sexual dimorphism– Any trait that differs between the sexes

Examples of dimorphism