BIOE 109
Summer 2009
Lecture 9- Part I
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
Sexual selection in rough-skinned newts
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
Sexual selection in pipefish
A baby pipefish emerging from Dad’s brood
pouch
Sexual selection in pipefish
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
http://video.google.com/videoplay?docid=7730341199662689386&ei=OR2DSoK9KY-SqAOPmbijBA&q=male-male+competition+in+animals&hl=en&client=firefox-a
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 female’s
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:
dancing…...
http://www.youtube.com/watch?v=bEhAbwCYc1c&NR=1
Intersexual selection
singing……
• http://www.youtube.com/watch?v=VjE0Kd
fos4Y&NR=1&feature=fvwp
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
http://www.youtube.com/watch?v=0QhHB6_8SQU
1. Direct Benefits- Good genes
Gerhardt et al. 1996
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
-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
Example: stalk-eyed flies
P
Preference
for long
eye-stalks
T
Eye-stalk
length
Example: stalk-eyed flies
P
Preference
for long
eye-stalks
T
Eye-stalk
length

mutation increasing
♂ eye-stalk length…
Example: stalk-eyed flies
P
Preference
for long
eye-stalks

…will be quickly driven
to fixation by ♀ choice
T
Eye-stalk
length

mutation increasing
♂ eye-stalk length…
Example: stalk-eyed flies
P
Preference
for long
eye-stalks

…will be quickly driven
to fixation by ♀ choice
T
Eye-stalk
length

mutation increasing
♂ eye-stalk length…
… resulting in continued exaggeration of ♂ trait
Runaway selection

♀ preference for
long eye-stalks
Elongation of
♂ eye-stalks

LIMITS to 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
Interesting videos
• http://www.youtube.com/watch?v=GPbWJPsBPdA&NR=1
For nest decoration of a Australian Bowerbird