Week 4 Flashcards
sexual dimorphism
difference between males and females.
Can be useful when males and females are adapted to eat different things so males do not monopolise resources
Divergent traits are adaptive when males and females play different roles in reproduction
sexual selection
differential reproductive success due to variation among individuals at getting mates.
what selection acts on each parent?
Parental investment is often larger for females than males
Recognising that sexual reproduction imposes different patterns of selection on males and females helps explain sexual dimorphism
When one parent invests more than the other in each offspring the reproductive success of that parent is often limited by resources and time. The reproductive success of the lightly investing parent is limited by number of mates
when sexual selection is strong for one and not for the other
Members of the sex subject to strong sexual selection will be competitive
Members of the sex subject to weak sexual selection will be choosy
intersexual selection
when the member from the other sex chooses a mate.
intrasexual selection
when the members of one sex fight for access to mates.
selection on males: competition
Combat over females eg. Stag beetles fight with mandibles
Alternative mating strategies
Sperm competition - if a female mates with two or more males the male whose sperm win the race to the eggs has the higher reproductive success
Infanticide - by killing other males’ cubs, male lions gain more opportunities to mate
selection on males: choosy females
Advertise for mates as they cannot monopolise females and the females choose
Why should females prefer one mate over another?
The traits which females choose their mates may be arbitrary
Choosy females may have preexisting sensory biases
Sometimes choosy females simply respond to courting mates as though the mates were prey
Choosy females may benefit directly through the acquisition of resources
Choosy females may get better genes for their offspring
sexual selection on females
Polyandry: multiple mating by females
True monogamy is rare
Females often benefit from mating with more than one mate
Species in which males invest more into each offspring and are thus a limiting resource for females provide a valuable opportunity to test the rules of sexual selection
sexual selection in plants
When male reproductive success is limited by access to pollinators but female reproductive success is not, male flowers may evolve showier displays
sexual dimorphism in humans
Hard to study because of cultural differences and ethical and practical reasons
It is unclear if sexual selection helps maintain the sexual dimorphism in body size in humans. Larger males do not necessarily win. Females are choosy but not all cultures prefer tall men
sex differences
Max number of offspring: Males can usually have more offspring than females
Minimum is equal (0)
Average number of offspring: equal between males and females
The maximum number of offspring is defining differences between sexes
Differences in size of gametes (anisogamy) and differences in parental investment
Differences in sexual activity - males have more time and energy for sex
Leads to competition between males as there are heaps and less mating females
Hence females can afford to be choosy
Bateman’s insights
Fruit flies
Males could have more offspring after one mate but females only need one male for all offspring to be fertilised
Abundant sperm vs limited eggs
Principle: variability in reproductive success is greater than males.
Males: reproductive success is limited by number of females mated with
Females: reproductive success is limited by number of eggs (expensive)
Males compete with each other
Females are choosy
Rough skinned newt
Mate in pond, females lay in forest
Females taken to lab to lay eggs
Genetic samples taken for paternity
Very large number of males had 0 offspring while all females had some reproductive success
Female number of mates and number of offspring is bell shaped
Male slope is a lot steeper than with females - Bateman supported!
For every extra mate a male has, he will produce about 75 extra offspring
For every extra mate a female has, she will have about 20 extra offspring
Average mating success is similar but higher variance in males
mating systems: sexual promiscuity
Chimpanzees
All mate with each other
Parental care - no care is provided in many species
polygamy
Polygyny - resource defence, harems eg. Red deer, leks - female parental care
Polyandry - females mate with multiple males causes switch in Bateman’s gradient - male parental care
monogamy
Genetic monogamy is much rarer than social monogamy - males and females parental care
mating systems and sexual dimorphism
Increase in levels of sexual dichromatism and sex differences when moving from monogamy to polygamy
Stronger Bateman’s gradient = greater sexual dichromatism
intrasexual selection: iguanas
Competition between males
Studied patterns of selection on body size across both sexes
Stabilising selection operates on both males and females - mid size is advantageous
But sexual selection produces smaller females and larger males than the mid advantageous size
All males that successfully mates were very large
Tradeoff between reproduction and survival
intersexual selection: Fisher’s Runaway Model
Some initial selective advantage eg. Horns that are under selection by female preference
Need to have some genetic correlation between male trait and female preference
Feedback look develops where male trait and female preference become more exaggerated
How far it will run depends on the strength of sexual selection vs strength of natural selection
case study of fisher’s runaway model
Case study: stalk eye flies
Prediction: experimental evolution for flies of longer (or shorter) eyestalk length will result in increase (or decrease) of female preference for males
Handicap Principle
What if the male trait carries information on the quality of the male?
Ornaments are not arbitrary; they have to impose a cost (handicap)
High quality individual can bear more cost
Positive correlation between quality and ornament expression
Females are looking for “good genes”
sensory bias
Posits female mating preferences are by-products of natural selection on sensory systems
