Sexual Selection Flashcards
sexual dimorphism
a difference between the phenotypes of females vs males within a population
natural selection
- a difference between the survival & reproduction of individuals with different phenotypes
- usually refers to components of fitness other than mating success
sexual selection
- a difference, among members of the same sex, between the average mating success of individuals with different phenotypes
- a special kind of natural selection
what explains the differences between the sexes?
sexual selection, it must act differently on the sexes
why is sexual selection different between the sexes?
sperm are cheaper than eggs
sperm are cheaper than eggs
- human eggs are 4000x larger than a sperm
- women ovulate ~300 to 400 eggs in their lifetime
- men make 10’s of millions of sperm everyday
reproductive success
the number of viable, fertile offspring produced
the sexes differ in reproductive investment
- male reproduction is (often) limited by access to females eggs
- females are (often) limited by the number of eggs she can nurture & rear to maturity
Bateman’s Principle
- when one sex invests more resources into producing offspring, this sex will be a limiting resource over which the other sex will compete
- access to mates will usually limit male reproductive success, but fecundity will usually limit female reproductive success
fecundity
number of pregnancies
Bateman’s Gradient
the slop of the best fit line relating reproductive output to mating opportunities
the slope of Bateman’s gradient
- strongly positive for males & horizontal or weakly positive for females
- predictions flow from differences in relative investment, not from gender
reproductive variance mating success & reproductive success
- most males don’t mate & produce no offspring
- most females do mate & produce many offspring
what is the cause of sexual selection?
Bateman’s Gradient
sexual selection reversed
when males invest more than females
sexual selection reversed: seahorses
female seahorses transfer eggs to males & males give birth
sexual selection reversed: broad-nose pipefish
- after eggs are fertilized, male provides all parental care by carrying eggs in brood pouch
- females need less time & energy to produce a clutch of eggs than it takes a male to brood them
what does reversed sexual selection mean for Bateman’s Gradient?
reproductive variance is reversed
reproductive variance reversed mating success & reproductive success
- most males mate & produce several offspring
- many females do not mate & produce zero offspring
when sexual selection is strong for one sex & weak for the other, we predict that…
- members of the sex subject to strong sexual selection will compete for mates
- members of the sex subject to weak sexual selection will be choosy, because it is more important to pick the right mate that is to mate frequently
forms of selection when sexual selection is strong for one sex but weak for the other sex
- intrasexual selection
- intersexual selection
intrasexual selection
typically male-male competition
example of intrasexual selection
elephant seal males fighting
male-male competition
most extreme when males are territorial & can control a large number of females
types of male-male competition
- combat
- defenses
- indicators of strength
types of male-male competition: combat
- horns
- antlers
- pincers
- large size
types of male-male competition: defenses
- armor
- large body size
types of male-male competition: indicators of strength
large body size
other types of male-male competition
- sperm competition
- infanticide
Galapagos Marine Iguanas
- females lay one clutch of 1-6 eggs per year
- males only contribute sperm
- females mate once per year, males fight hard for that opportunity
alternative male mating tactics: yellow morph
- sneaker
- mimics female behavior
- mimics female coloration
- not territorial
alternative male mating tactics: orange morph
- territory usurper
- large size
- high testosterone
- large territories
alternative male mating tactics: blue morph
- mate guarder
- cooperates with other blues
- small territories
sperm competition
- increase ejaculate
- mate guard
- sperm plugs
- inject hormones that influence the females reproductive behavior
infanticide
- 25% of all pups deaths
- females return to breeding condition on average 8 months sooner
intrasexual selection
female choice
intrasexual selection: male red-collared widowbird
- males in better condition have longer tails
- males with longer tails attract more females to their territory due to female choice
- harder to survive with longer tail
intrasexual selection: female choice in the grey treefrog
- females prefer long calls vs short calls
- females will pass by short calls to approach long calls
4 explanations of female choice
- good genes
- direct benefits
- sensory bias
- chase-away sexual selection
good genes
- certain traits indicate the bearer has good genes
- females that prefer males with good indicator traits will have offspring with better genes
good genes: prediction
- offspring of males with good gene indictor traits will have increased survival of reproductive success
- these are indirect genetic benefits to female fitnesses
good genes: examples
- the offspring of peacocks with more elaborate trains enjoy improved growth & survival
- human women prefer men that have immune system genes that differ from their own
the tadpoles of long-calling animals
- grew faster
- metamorphosed sooner
- greater mass at metamorphosis
- survived better
- grew faster at metamorphosis
direct benefits
females prefer males with resources or other attributes that help them now
direct benefits: prediction
- females provided with male resources should have higher fitness
- this is a direct benefit to the female
direct benefits with shrikes
- male shrikes display their foraging success
- females choose males with evidence of fewer parasites, diseases, etc
- territory quality important for offspring success
sensory bias
taking advantage of the female sensory system
example of sensory bias
- Tungara frog males call with a whine & a chuck. Females prefer the chuck but so do bats
- detailed studies of the inner ear have shoqn that females are naturally sensitive to the chuck
chase-away sexual selection
- males evolve mechanisms that increase their reproductive success
so females evolve counter adaptations, since what is good for the gander is not always good for the goose - then males evolve in response to the females
- the result is continual selection & evolution, like an arms race
chase away sexual selection in fruit flies
- females prevented from evolving
- some males selected for success in mating competitions
- some males not sleected for success in mating competitions
result of chase away sexual selection in fruit flies
slected males evolve seminal fluids that can kill females