Sexual selection

Sexual dimorphism is very common in nature

What is sexual selection?

What is sexual selection? Natural Selection: Differential reproductive success due to variation among individuals in survival and reproduction.

What is sexual selection? Darwin (1871) defined sexual selection as: Differential reproductive success due to variation among individuals in success at getting mates.

What is sexual selection? Darwin (1871) defined sexual selection as: Differential reproductive success due to variation among individuals in success at getting mates. sexual selection refers to one component of fitness: mating success. Fitness = Viability +Fecundity +Longevity +Mating success

What is sexual selection? Fitness = Viability +Fecundity +Longevity +Mating success

What is sexual selection? Fitness = Viability +Fecundity +Longevity +Mating success trade-offs between fitness components may occur!

What is sexual selection? Fitness = Viability +Fecundity +Longevity +Mating success trade-offs between fitness components may occur! a trait improving mating success may increase total fitness yet reduce viability!

If sexual selection is indeed the explanation for sexual dimorphism, then it will have to act on sexes differently….. And it does! Sexual reproduction creates different selection pressures for males and females. How?

If sexual selection is indeed the explanation for sexual dimorphism, then it will have to act on sexes differently….. And it does! Sexual reproduction creates different selection pressures for males and females. Through asymmetric parental investment!

What is parental investment? Energy and time expended in either or both: -constructing an offspring -caring for it.

What is parental investment? Energy and time expended in either or both: -constructing an offspring -caring for it. Producing eggs (or pregnancies) are more expensive than ejaculates.

What is parental investment? Energy and time expended in either or both: -constructing an offspring -caring for it. Producing eggs (or pregnancies) are more expensive than ejaculates. In more than 90% of mammal species, females provide substantial parental care and males provide little to none.

What acts to limit the lifetime reproductive success of males and females? females: limited by no. of eggs and/or pregnancies

What acts to limit the lifetime reproductive success of males and females? females: limited by no. of eggs and/or pregnancies males: limited by no. of females mated

What acts to limit the lifetime reproductive success of males and females? females: limited by no. of eggs and/or pregnancies males: limited by no. of females mated (access to mates) This sets up a conflict = sexual selection. To understand sexual selection we must quantify the relationship between number of mates and reproductive success for both males and females

Sexual selection in rough-skinned newts

Access to females increases reproductive success in males

What does this mean? Heritable traits associated with mating success will become common in males

What does this mean? Heritable traits associated with mating success will become common in males tall crests that appear during breeding season

What does this mean? Heritable traits associated with mating success will become common in males Heritable traits that are not associated with mating success will tend to disappear from the population

Predictions: Based on Parental Investment: females: limited by no. of eggs and/or pregnancies males: limited by no. of mates Members of the sex subject to strong sexual selection will be competitive. Members of the sex subject to weaker sexual selection will be choosy.

Predictions: 1. Males should be competitive

Predictions: 1. Males should be competitive they should compete among themselves for access to females.

Predictions: 1. Males should be competitive they should compete among themselves for access to females. this is male-male competition or intrasexual selection.

Predictions: 1. Males should be competitive they should compete among themselves for access to females. this is male-male competition or intrasexual selection. 2. Females should be choosy

Predictions: 1. Males should be competitive they should compete among themselves for access to females. this is male-male competition or intrasexual selection. 2. Females should be choosy since her investment is larger, she has more to lose by making a bad decision.

Predictions: 1. Males should be competitive they should compete among themselves for access to females. this is male-male competition or intrasexual selection. 2. Females should be choosy since her investment is larger, she has more to lose by making a bad decision. this is female choice or intersexual selection.

Intrasexual selection (male-male competition)

Intrasexual selection occurs when individual males can monopolize access to females. -Fight for control of mates and/or control of resources vital to mates

Intrasexual selection occurs when individual males can monopolize access to females. -Fight for control of mates and/or control of resources vital to mates Types of intra-sexual selection: 1.Combat 2.Sperm competition 3.Infanticide 4.Sneaky strategy

Intrasexual selection occurs when individual males can monopolize access to females. 1. Combat leads to sexual dimorphism in size

Intrasexual selection occurs when individual males can monopolize access to females. 1. Combat leads to sexual dimorphism in size Example: northern elephant seal. ~ 4,000 lbs ~1500 lbs

Intrasexual selection 1. Combat leads to sexual dimorphism in size Example: northern elephant seal. can also lead to the evolution of weaponry

Intrasexual selection 1. Combat leads to sexual dimorphism in size Example: northern elephant seal. can also lead to the evolution of weaponry Example: horns in ungulates and beetles.

Intrasexual selection 2. Sperm competition

Intrasexual selection 2. Sperm competition If a female mates with two or more males, the male whose sperm win the race to the eggs has higher reproductive success.

Intrasexual selection 2. Sperm competition If a female mates with two or more males, the male whose sperm win the race to the eggs has higher reproductive success. Ex. of traits needed to be successful Large ejaculates More and longer mating occurrences

Intrasexual selection 2. Sperm competition If a female mates with two or more males, the male whose sperm win the race to the eggs has higher reproductive success. Ex. of traits needed to be successful Large ejaculates More and longer mating occurrences Other examples include Sperm plugs, scooping out sperm, prolonged copulation, guarding of mate, applying hormones that reduce females attractiveness to other males.

Sperm competition in damselflies barbed horns on penis

Intrasexual selection 3. Infanticide Example: the African lion

Intrasexual selection 4. Alternative male reproductive strategies. Example: Sneaky strategy in Pacific salmon. Hooknoses (male- 18mon) Female Jacks (male-6mon)

Intersexual selection (female choice)

Intersexual selection occurs when males advertise for mates and females choose among different males. Elaborate courtship displays: singing, dancing, or showing off bright color leads to sexual dimorphism in ornate features

Intersexual selection occurs when males advertise for mates and females choose among different males. Elaborate courtship displays: singing, dancing, or showing off bright color leads to sexual dimorphism in ornate features Types of intersexual selection: 1. Direct Benefits -Acquisition of resources -Good genes 2. Pre-sensory Bias 3. Runaway selection

Intersexual selection 1. Direct benefits- Acquisition of resources females directly benefit from choosing certain males. Example: the common tern

Intersexual selection 1. Direct benefits- Acquisition of resources females directly benefit from choosing certain males. Example: the common tern females show a strong preference for males that bring a lot of food.

Intersexual selection 1. Direct benefits- Acquisition of resources females directly benefit from choosing certain males. Example: the common tern females show a strong preference for males that bring a lot of food. the amount of food a male brings during courtship is strongly correlated with the amount he brings to feed young.

1. Direct Benefits- Good genes

1. Direct Benefits- Good genes Example: call length of male gray tree frog

1. Direct Benefits- Good genes

1. Direct Benefits- Good genes females choose males with certain traits because they are honest indicators of overall genetic quality.

1. Direct Benefits- Good genes females choose males with certain traits because they are honest indicators of overall genetic quality. -calling in frogs -calling in birds -plumage color

2. Sensory bias

2. Sensory bias predicts that female preference for certain male traits evolves prior to the appearance of the male trait.

2. Sensory bias predicts that female preference for certain male traits evolves prior to the appearance of the male trait. females have a pre-existing sensory bias for the trait and males exploit this preference.

2. Sensory bias predicts that female preference for certain male traits evolves prior to the appearance of the male trait. females have a pre-existing sensory bias for the trait and males exploit this preference. Example: Water mite ( Neumania papillator )

Water mite (Neumania papillator)

3. Runaway selection

3. Runaway selection results from a genetic correlation between a male trait and female preference for that trait.

3. Runaway selection results from a genetic correlation between a male trait and female preference for that trait. Components needed: -Assortative mating ( individuals with similar genotypes and/ or phenotypes mate with one another more frequently than would be expected under a random mating pattern. ) -Heritable trait

3. Runaway selection results from a genetic correlation between a male trait and female preference for that trait. the simplest model assumes two genetic loci in linkage disequilibrium:

3. Runaway selection results from a genetic correlation between a male trait and female preference for that trait. the simplest model assumes two genetic loci in linkage disequilibrium: 1.a female preference gene (P locus) for the male trait.

3. Runaway selection results from a genetic correlation between a male trait and female preference for that trait. the simplest model assumes two genetic loci in linkage disequilibrium: 1. a female preference gene (P locus) for the male trait. 2. a gene for the male trait (T locus).

3. Runaway selection results from a genetic correlation between a male trait and female preference for that trait. the simplest model assumes two genetic loci in linkage disequilibrium: 1. a female preference gene (P locus) for the male trait. 2. a gene for the male trait (T locus). P T

Example: stalk-eyed flies

P T Example: stalk-eyed flies Preference Eye-stalk for long length eye-stalks

P T Example: stalk-eyed flies Preference Eye-stalk for long length eye-stalks mutation increasing eye-stalk length…

P T Example: stalk-eyed flies Preference Eye-stalk for long length eye-stalks mutation increasing eye-stalk length… …will be quickly driven to fixation by choice

P T Example: stalk-eyed flies Preference Eye-stalk for long length eye-stalks mutation increasing eye-stalk length… …will be quickly driven to fixation by choice … resulting in continued exaggeration of trait

preference for long eye-stalks Elongation of eye-stalks Runaway selection

Recap Sexual selection explains sexual dimorphism in nature Asymmetry in parental investment leads to differential selection pressures in males and females (sexual selection) Sex that invests less is more competitive, sex that invests more is choosy Sexual selection is more potent force of evolution in males than in females (with few exceptions like pipe-fishes) Intersexual selection (male-male competition): leads to combat, sperm Competition, infanticides, alternative male strategies Intrasexual selection (female choice): due to direct benefits like resources, good genes etc., female sensory bias, runaway selection