22. Sexual selection

natural vs. sexual selection
1. Reproductive advantage through competition to survive
2. Reproductive advantage through competition to mate
– one way for natural selection is sexual selection

a sexually selected trait is…
– an exaggerated and costly character that appears to reduce the chances of survival

intra vs. inter-sexual selection examples
1. Male golden pheasant that has ornate tail feather that evolved through female choice?
– no! Tail used as support during fights between males
2. Male-ring necked pheasant – again ornate tail feathers? – no! females choose males by looking at their spurs (nokti)
– you might think that the sharp tarsal spurs are weapons in male-male competition

intrasexual selection
direct competition between members of the same sex for mates
– usually among males
intra: within species
intra: within

intersexual selection
– process of choosing a mate, mate choice
– stereotype of competing males and choosy females
– the ornament is not usually beneficial but is evidence of a better genetic make-up

bateman’s principle
– variability in reproductive success is greater in males than in females
– Limited reproductive capacity for females compared to
– The sex that invests most in offspring is a limited
– The other sex then competes for access to that sex
– Anisogamy: more investment in gamete by females than
– explains how sex becomes a limiting factor in reproductive success, females are a limiting factor over which males compete

conclusion of bateman’s principle
– males put their reproductive effort into mating effort and females into parental effort

outcomes of intrasexual selection
– ritualised fights and displays among rival males
– greater sexual dimorphism
– increased body size, weapons, armour, vocal capacity
– infanticide to return females to a fertile state (lions)
– more intense among males

Game theory models
– males invest in these traits only when they can monopolise many females
– value of a trait increases with age: large, old males are rare and good survivors: elephants; house sparrows and many other vertebrates

intrasexual selection – sperm competition
– competition between males to fertilise the same ovum
– females often mate with multiple males
– females may store good sperm or eject bad sperm
– even in man

Two ways to compete:
1. defensively: behaviourally
2. offensively: superior ejaculate/sperm

example of adaptation for sperm competition
Bed bugs Cimex lectularius have traumatic insemination. Males punch through the female’s body wall to place sperm close to eggs. Females no longer have a functional vagina and have a small immunity ‘sac’ at the site of injection.
– various penis scoops (grebe) i scraper styrga za da izwadi spermata
– investment in sperm size or size number increases with sperm competition

intersexual selection conflict
– female coyness is a bluff to assess male quality
– but females incur heavy costs when mating and when resisting mating attempts (death)
– interests of males and females rarely coincide
– leads to sexual conflict

Fisher’s runaway effect
Male trait varies in population – e.g.: tail length
• Females prefer to mate with long-tailed males
• Why? More noticeable?
• Offspring inherit long tails (males) and preference for long tails (females)
• Covariation between male trait and female preference will push population towards longer and longer tails (runaway effect)
• Tails will get longer until they are detrimental to survival and natural selection regulates maximum tail length
– trait would eventually get too big

female choice criteria
1. Non-genetic benefits
• Resources – food, good territory, nesting sites
• Parental care – provisioning, incubation, defence
2. Genetic benefits
• Viability indicators – competitive ability, disease resistance, survivorship, longevity

nuptial gifts in hangingflies
– females hunt less if fed by male, less risk of being caught in spider webs
– females only accept prey items above a certain size
– males that bring larger prey may be higher quality

Zahavi’s handicap principle
• ‘Handicap principle’ is a metaphor for the
costs imposed upon males by sexually
selected traits:
• Nutritional (e.g.: deer antlers)
• Survival (e.g.: long tail may impede mobility)
• Handicaps must be heritable
• Handicaps must be costly to males to
ensure honesty of signals

Good genes model
Other explanations for the long tail:
• Males that have a long tail in good condition have the genetic quality to maintain long, parasite-free feathers
• Parasites and disease serve as promotors of honest genetic quality
• Females choose males with long tails because they will obtain high quality genes for their offspring

Immunocompetence handicap theory
• Elaborate secondary sexual traits sustainable only at cost to immune function
• Testosterone promotes growth of traits but also suppresses immune function
• Good quality male = secondary sexual characters despite high levels of immunodepressive hormones
• Sexually selected traits are therefore honest signals of male quality
– when immunity is down parasites load increases

genetic benefits
costs (handicaps) of sexually selected traits:
– survival
– nutritional
– developmental stability: evidence an individual genotype can resist environmental insults; measure fluctuating assymetry – departures from bilateral symmetry
– if you are able to withstand an environmental change – symmetry

genetic benefits genetic compatibility
females choose mate with compatible genes to her own, different males good for different females
– relatedness (interbreeding) has advantages and disadvantages

MHC and sexual selection
– major histocompatibility complex
– Cluster of genes that produce proteins that present peptides to immune system
• Variation in MHC is associated with resistance to disease but also contributes to natural body odour
• Potentially enables kin discrimination

Mice mate choice and MHC
Mice can discriminate kin (similar MHC type) and non -kin (dissimilar MHC) by scent when choosing mates, and prefer to mate with dissimilar MHC types
The male mouse scent is his peacock’s tail…

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