Sexual selection Flashcards
What is sexual selection?
Intense competition over reproductive opportunity among males leads to a strong, open ended, directional process of selection leading to exageration.
Sexual selection depends on the variation on reproductive success determined by competitive access to reproductive opportunity.
The higher the variation, the more intense sexual selection is.
Sexual selection pulls phenotpes away from the naturally selected optima.
Why is sexual selection stronger on males?
Males show more exageration than females as they have experienced more sexual selection.
There is more variation on reproductive success in males reproductive succcess is a function of how many males they can mate with. Female reproductive success is limited to the number of ova they can produce.
Male intent to mate with as many females as possible leads to male male competition and female choice.
Hisotry of sexual selection
Intially there was a paradox as exagerated display and courtship behavior seemed incosistent with natural selection.
Sexual selection explains the evolution of these ecxagerated traits.
Darwin’s, the decent of man, was first account.
The evolution of sex roles
Males tend to ‘eagerly persure females’ and females tend to ‘coy’.
This is has evolved due to anisogamy: differential investment in gamete size. (sperm and egg)
Anisogamny evolved in externally fertilising sessile organisms that broadcasted spawner. Initially gametes were equal sizes and fertilisation success was proportional to gamete size. However, there was a trade off between size and number. This strategy was invaded by small, numerous gametes. Fertilisation between a small and large gemete had intermediate frequency and success so was selected for.
This has lead to males that produce many cheap sperm and females that produce few expensive eggs.
This leads to sex specific selection on mating success: Batesman experiment
Batesman experiment
Batesman allowed fruit flies to mate with different numbers of partners and he observed the number of offspring produced by males and females.
Males: No. of offspring increased linearly with no. of mates.
Females: No. of offspring did not increase linerly with number of mates due to limited number of ova (female fecundity which depends on resource extraction).
Batesman gradient is the gradient of this line and it measure the sexual selection on mating success.
- Steaper for males than females due to greater variance in total reporductive success and number of mates.
Limitations: mating success was measured through offspring produced rather than mating behavior missing out mating that fails to result in fertilisation.
Evidence: meta analyses of many species shows those with greater variation in mating success and no. of mates have greater sexual selection.
Where does sexual selection occurs
Pre-copulations
- Male competition
- Female choice
Post copulation
- female crypsis
- Sperm competition
Pre copulation sexual selection
Male - male competition: Males must compete for the females in contests, scrambles and endurance rivalries leaiding to the selection for large body sizes, armaments and social competitive ability.
Example: Intrasexual favouring large male bull frogs.
Example: pronghorn antelope compete for haraems of females.
Female mate choice: The female maximises offspring viability through the quality of the male reproductive competition (quanitty but not quality is limited for females). Females select for males that are better at conveying higher offspring viability by having differential responses to males.
Example: Females use the length of tails in Long tailed widow birds to select for mates.
Example: Females use Indian peahens prefer peacock with large and colourful tails
Mate choice threshold
- Study found on Red jungle fowl found that there was a threshold for mate choice
- Above which female chose males with the largest come length and below which females mated reluctantly with males at random. -> threshold around 78mm
- In natural condition the fowl may have refused to mate with lower quality individuals.
Male competition and female mate choice example:
- Chickens females bias male mate choice by manipulating male male competition.
- When they are being mated they let out a stress call to attract other potentiallly more dominant males who will fight for the feamles.
What is the advatage of female preference for males?
Non adaptive
- Naturally selected sensory bias leads to discrimination and the male are able to tap into this to attract females.
Adaptive
Females benefits
- Reproductive resouces and protections
offspring benefits
Non genetic
- parental care/ resources
Genetic
- The offspring benefits from the fathers genes
-> Fisherman runaway
-> Good genes
Fisherian runaway
Due to female assortative mating and preference there is linkage disequilibrium between genes controlling female preference for ornaments and male ornaments.
This leads to the evolutinog of more extreme ornaments and more extreme preferences.
By choosing a male with a particular trait value, the females gain the indirect benefit of producing offspring that will be more sexually attractive to females that carry the preference -> trait may be arbitary
Example: Sticklebacks
- Redder males are related to females with higher preference to red males.
- Escalating selection
Good genes
Handicap principle and condition dependence
- Sexually selected traits are costly to produce for males and large ornaments can only be produced by males that can affor them. (they are condition dependent)
- Sexual ornamnents therefore show genetic quality.
- There is a trade off between investment in ornaments and fitness enhancing traits (e.g. immune system) limiting who can afford ornaments.
Example: stalk eyes flies
- Wider eyes = More attractive
- Study showed that diet had a strong effect on eye width (condition dependent)
Example: testosterone and the immune system
- Testosterone determines the expression of male sexual traits but depresses the imune system
- This means males producing large ornaments are able to withstand depression of the immune system.
But good gene selection can also occur without ornaments
Example: Pronghorn
- Females switch between hareems before chosing a mate. -> relentless mate sampling process to test male testing running speed, endurance, agility, and tactical spatial sense -> has too much emphasis been put on ornaments.
- Offspring of attractive males were more likely to survive to weaning and to age classes as late as 5 years. -> faster growth rates
- Fitness for random mating was 3.53 and fitness for sampling and active choice was 5.15
- Females switch between hareems before chosing a mate.
polyandry And Post copulatory selection
Females can mate with multiple males meaning there sperm overlaps in time and space. (polyandry)
Why does this evolve?
maladaptive: Male driven, correlated evolution
Female adaptive: convevienve due to cost of resistance, aqcuisition of more resources, sperm replensihment
Offsrping adaptive: fertilization of genetically superior males, protect against inbreeding
This leads to variance in paternity share in males due to
- Sperm competition
- Cryptic female choice
Example: Drosophila
- They can store sperm and chose
- Ejaculate what is not needed
Example: mussel
- Chemoattractants in the fluid associated with the eggs differentially mediate the migration of sperm of individual males by changing sperm swimming behavior
Cryptic mate choice can be hijacked by males: Dunnok
- males peck female cloaca leading to previous sperm ejactuation
This leads to post copulatory sexual selection.
- selection on individuals that produce more/ competitive sperm
This can be hijacked by males: Dunnok
- males peck female cloaca leading to previous sperm ejactuation
Alternative mating
There is a trade off between male investment pre and post copulation leading to different strategies/ different solutions.
Example: blue gill Sunfish
- Two strategies
-> Grow large and then reproduce to maximise pre-coppulation success
-> Reproduce when small, with lots of sperm to maximise post copulation success.
