Sexual Selection Flashcards
What is the fundamental difference between males and females?
◦ Males and females are defined by the size of their gametes.
◦ Female – large, non motile
◦ Male – small, motile
Isogamy to anisogamy:
Survival of a zygote (two gametes fused) depends on its size -> Selects for larger gametes
Preponderance of larger gametes in population selects for production of more smaller gametes (although larger gametes would be selected to fuse with larger gametes):
◦ Large-small mating predominate – (small gametes are in excess)
◦ Little-little mating has higher cost than large-small
◦ Little selected to evolve traits that ensure mating with large gametes
Competition over mates:
- Sexual selection proposed by Darwin to explain male traits that didn’t fit natural selection
- There were two main types of traits:
◦ Male weaponry (large body size, antlers, horns)
◦ Exaggerated traits/ornaments (e.g. bright colours/long tails)
• Both types of male trait reduce survival:
◦ Weapons are expensive to produce and carry around
◦ Ornaments can attract predators
Two types of sexual selection:
Intrasexual selection:
• Competition between members of the same sex for access to mates
• Usually male-male competition
• This has resulted in evolution of large body size & weaponry
Intersexual selection:
• Choice of mate of opposite sex
• Usually females that choose
• This often explains evolution of exaggerated male traits
How did exaggerated male traits evolve?
Several mechanisms proposed to explain evolution of male ornaments and exaggerated traits
◦ Selection for ecological sex differences
◦ Unprofitable prey hypothesis
◦ Male-male competition
◦ Female mate choice
females don’t generally evolve ornaments or weapons…
E.g.
• Deer females do not have antlers for fighting
• Mandrill female doesn’t have colourful face
• Hercules beetle female doesn’t have horn for fighting
Why don’t females generally evolve ornaments or weapons?
Bateman’s Experiment measured reproductive success of Male and female Drosophila
◦ Male fitness increases with number of matings
◦ Female fitness does not.
Conclusions of baseman’s experiment:
- Males - usually higher potential reproductive success than females - egg production (large gametes) limits female reproductive rate
- Males often show greater variation in reproductive success than females (but contentious…)
- Traits that influence male reproductive success experience stronger selection
Sexual selection favours:
- Males that mate with many females
* Females that choose good quality males
Male-male competition:
The Intensity of male-male conflict varies between species - male-male conflict increases with…
- ratio of female:male parental investment
- More F investment more M-M conflict
• operational sex ratio (OSR)
◦ OSR is the ratio of males ready to mate in relation to females ready to mate – more male-biased OSR – more male conflict
In many species males assess the quality of rivals without fighting
e.g. male stalk-eyed flies compare eyespan
Courtship displays may serve to resolve male-male conflicts
e.g. Pacific Tree Frog, Hyla regilla - male calls attract females and repel other males (avoid conflict)
Age can influence outcome of competition
Older males more experienced and have less to lose
E.g. giraffes fight using their necks
In some species Females gain Direct benefits from mate choice:
- Avoidance of interspecific hybridisation
- Nuptial gifts
- Parental care
- Access to a territory
- Access to food
- Lower risk of predation or harassment
- Avoidance of inbreeding
But…in some species females only get genes from Male (no direct benefit)
Avoidance of interspecific hybridisation:
◦ E.g. In Central America several species of frogs breed in same ponds - Males make calls preferred by conspecific females
Nuptial gifts…
In many birds and insects males offer females nutrition during courtship or copulation = provide energy and nutrients for egg production
- E.g. Hangingfly - Males with larger prey gifts get to mate longer = Prey size affects female fecundity + more likely to fertilise egg
- E.g. Orthoptera - males pass seminal nutrients with the sperm (transmission of alkaloids, obtained from food plants, in spermatophore) = Eggs protected from predation - female’s choose males with high alkaloids:
‣ Males advertise their alkaloid content by pheromones
‣ Pheromone is derived from protective alkaloid
‣ Only males with alkaloid can signal
‣ Ensures ‘honesty’ - prevents cheating by males that dishonestly adverstise high alkaloids
Access to a territory…
◦ Important in birds; main limiting factor for breeding success is food supply
◦ Females prefer males with highest quality territories
◦ Also found in dragonflies, frogs and fish that guard sites where females oviposit
◦ E.g. North American Bullfrog, Rana catesbeiana - Females prefer males with territories containing warm water and little vegetation; these increase offspring survival
Lower risk of predation or harassment…
◦ Females are often wounded or killed in struggles between males attempting to mate with them.
◦ Female yellow dung flies prefer to mate with large males as less likely to be attacked by competitors
Avoidance of inbreeding…
An example is the chimpanzee
‣ Males usually remain in their natal community
‣ At puberty female chimps usually leave their natal group for another - & attracted to unfamiliar males
Fisher’s Runaway Process:
- If genetic variation in preference and the preferred trait, then females with strongest preference would mate with most extreme males - the two traits would become positively genetically correlated
- This explains how a preference allele can spread, but how does a preference originate?
- Fisher assumed that the male trait was initially favoured by natural selection. e.g. long tail length
- There is then selection in favour of females evolving a preference for long tailed males
- Once preference and positive genetic correlation are established then selective advantage to long tails is no longer required
- Fisher suggested that once preference and trait were genetically correlated the “runaway” process would occur
- He predicted that the runaway process would end when increased mortality associated with bearing the male trait offset any mating advantage◦ E.g. Female preference exhibits a genetic correlation with male trait in sticklebacks
Sexual selection for “good genes” - (indirect or genetic benefits)
- Male traits advertise male quality
- Females prefer males with the most elaborate traits as they are indicating their high fitness
- Signals must be “honest”
- Major problem with this theory since selection should remove variation in fitness – the “lek paradox”
◦ E.g. Female gray tree frogs prefer mating with long calling males, which father better quality young - Increased larval growth, larval survival, & postmetamorphic growth
Good genes -
Zahavi (1975) proposed “handicap hypothesis”
◦ Males advertise handicaps to show they have high-quality genes as - Only high-quality males can survive with handicaps
◦ Still problem with lek paradox
Hamilton and Zuk hypothesis = way around the “lek paradox”
- the “lek paradox” = selection should remove variation in fitness as all females go for best males
- Hamilton and Zuk (1982) proposed a way around the lek paradox - parasite mediated sexual selection
- Male traits could indicate parasite resistance
- Variation in parasite resistance is not lost as host-parasites are constantly coevolution (‘The red queen hypothesis)
Genetic compatibility? = way around the “lek paradox”
- females choose males they are compatible with
- Compatible genes are predicted to impart enhanced fitness to offspring when particular male and female haplotypes are combined
- Females will not necessarily share mate preferences, and selection will not be strongly directional (so no lek paradox)
- Genetic basis to compatibility is poorly understood
- How do females detect compatibility?
- Odour-based cues? (evidence - mice and humans)
- Studies on birds and fish support the idea
