Sexual Selection

• NS: who SURVIVES

• SS: who REPRODUCES

• SS can explain some deleterious/costly characteristics that can’t be explained by NS

Ornaments/Courtship Displays/Fights

Sexual Dimorphism

differences among indivs in mating opportunity

Types of Sexual Selection

Competition (usually M)• Sperm is cheap RS with # of matings

Choice (usually F)• Eggs are expensive• No Δ in RS with # of

matings

# offspring

# mates

e.g. Newts

Sexual Selection depends on Investment

• Higher investment = limited resource Usually Indiscriminate M & Choosy F

But, Sex role reversal/ biparental care/nuptial gifts

Male-male competition

1) COMBAT

• Competition for access to F or territory

• Correlation b/w dominance rank & # of matings

• Size often important in competition

(Sexual Size Dimorphism ~ Degree of Polygyny)

e.g. 2: Male marine iguanas larger than optimal for survival

e.g. 1: Elephant Seals

2) Sperm competitionFertilization more important

than mating

1) Large ejaculate/lots of sperm

2) Mate guarding

3) Copulatory plugs

4) Sperm removal

Comparative method: relative testis size often correlates to mating system

e.g. Primates 200 kg: 30 g

45 kg: 110g 70 kg:40g

3) Infanticide

• Males increase repro opportunities

• Not good for female• Often female has no

choice

• e.g. Lions

Female Choice – Benefits?On Basis of Resources vs. In Absence of Resources

e.g. Scorpionflies

“making males pay”

Nuptial gift:

arthropod –

salivary –

forced copulation

Polygyny Threshold

e.g. Lark Bunting

• choose on the

basis of M territory

• Fitness benefits

1 1 2 32 1 23 1 2 34 1 25 1 26 1 27 1 28 1 29 1

10 111 112 113 114 115 116 117 118 119 120

1 2 3 4 5 6 7 8

mal

e te

rrito

ry q

ualit

y ra

nked

acc

ordi

ng to

sha

de

day of female arrival

Direct Benefit

F may choose on the basis of :

gift, paternal care, territory

If no M investment, how do F choose?

Non-resource based Female Choice

• Mutant F with no preference for showy M

• non-showy M offspring survive better

• M offspring not preferred as mates

survival benefit must > repro benefit

Runaway Sexual Selection

• Innate female preference for showy M

The ‘Runaway’ Process

• SS trait associated with higher survival• Mutant F (preference) = RS b/c high quality sons• 2 advantage: M with trait preferred by high

proportion of pop’n as mutation spreads• Reinforcement• Offspring carry genes for trait & preference (linkage

disequilibrium)Advantage:

Survival Survival + Mating Mating

Honest Advertisement

• F preference for traits which

demonstrate good quality

• Zahavi – Handicap Principle– Characters that reduce survival– “I must be good to pull this off” (good genes)– Indicator must be costly so M can’t cheat– Often cost of trait is less for high quality males

Runaway vs. Good Genes

• Open-ended • Heritable variation in

male character• SS b/c sons have

mating advantage

• Open-ended• Heritable variation in

male fitness• SS b/c sons have

survival advantage

Indicators of Health• e.g. guppies – orange: diet quality

• Special case of “good genes”:– Hamilton & Zuk : parasite load & plumage

brightness– Species (not individuals) with brighter plumage

gen’lly higher parasite load

able to use colour as indicator of

risk of parasitism