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“The sight of a feather in a peacock’s tail,
whenever I gaze at it, makes me sick.” --Charles Darwin
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In many sexually dimorphic species, males characteristically have
exaggerated, conspicuous traits Birds of Paradise, New Guinea
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Among closely related species,
males are much more divergent than females Petroica robins, Australia
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Sexual Selection Natural selection ---> viability or fecundity Sexual selection ---> mating success, fertilization success Only traits which affect courtship and mating Occurs when all individuals do not have equal probability of mating Usually stronger on males because: 1) greater female investment in young 2) single mating sufficient to fertilize female’s clutch
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Taricha salamanders
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Sexual Selection Natural selection ---> viability or fecundity Sexual selection ---> mating success, fertilization success only traits which affect courtship and mating Occurs when all individuals do not have equal probability of mating Usually stronger on males because: 1) greater female investment in young 2) single mating sufficient to fertilize female’s clutch Usually greater variance in male reproductive success, often related to age, size
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Red Deer, Cervus elaphus
Reproductive Success Hinds Stags Reproductive lifespan 27% 7% Fecundity/Mating success Offspring survival
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Northern elephant seal
#pups sired and weaned #pups weaned
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Selection asymmetries lead to
Competitive males Choosy females [Assumes: sperm are cheap; Females can determine the ‘best’ male] Two mechanisms: Male-male competition Female choice Sexual selection shapes variation in traits much like natural selection, although it may frequently conflict with natural selection Different types of selection may act at different stages in the life cycle
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Sexual selection can force a trait away from the natural selection optimum
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Male-male competition
Winners: bigger, more heavily armed, more vigorous Horns, antlers -- ungulates “Horns” -- beetles Song/display, territory defense -- birds Body size (males >>> females) -- mammals Read the example from your text: marine iguanas pp
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Males Females
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Male-male competition
winners: bigger, more heavily armed, more vigorous horns, antlers -- ungulates “horns” -- beetles song/display, territory defense -- birds body size (males >>> females) -- mammals Read the example from your text: marine iguanas pp , including box on alternate male strategies
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Male-male competition
Sperm competition Rapid divergent evolution of male genitalia in animals with internal fertilization Arnqvist Nature 393:784 “Lock-and-key” hypothesis -- selection against inferior hybrids Sexual selection hypothesis -- post-insemination % competition Compare related clades of insects that differ in the mating frequency of females (Single vs. Multiple) Morphometric distance ratio: (interspecific divergence in morphology within a polyandrous clade) / (interspecific divergence in morphology within a related single mating clade) H0: MR=1
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(Arnqvist 1998)
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Genitalia are significantly more divergent (MR = 2.19, p < 0.001)
than other traits (MR = 0.72, p > 0.38)
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Male-male competition
Sperm competition longer copulation duration transfer more sperm displace previous males’ sperm
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Fertilization success increase with copulation duration in Scatophaga stercoria
Simmons 2001
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Male-male competition
Sperm competition longer copulation duration transfer more sperm displace previous males’ sperm sperm morphology anucleate (apyrene) sperm in Lepidoptera
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Drosophila bifurca 58mm
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Sperm competition in Pieris napi Cook and Wedell 1999 Nature 397:486
mate virgin females to either 1) virgin males or 2) recently mated males allow females the opportunity to remate for 10 consecutive days pr(remating) affected by amount of apyrene sperm eupyrene apyrene
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Accessory gland proteins reduce female survival in Drosophila
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Reproductive Success Hinds Stags
Reproductive lifespan 27% 7% Fecundity/Mating success Offspring survival Males compete and this gives them higher reproductive success. Why should females be choosy? females choose and it gives them ‘better’ mates females choose and it increases survival of young females choose and they live longer?
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Female Choice Females prefer males with elaborate ornaments Moller 1994 female choice in barn swallows -- correlation between male phenotype and male mating success -- manipulation of male trait ---> effect on male reproductive success
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Females do choose and it makes a difference to males
What benefit do females get from being choosy? Long-tails are attractive--why Peacock Tails Fitness Male Trait
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2. Males with larger ‘tails’ have higher fitness
Male Trait optimum Trait ends here Trait starts here 1. Male Trait varies 2. Males with larger ‘tails’ have higher fitness 3. Females who choose those tails have better offspring 4. Females are choosy because it gives them an advantage 5. All females are choosy and continue to choose males with more elaborate traits 6. Advantage is then to possessing trait due to choosiness as well as due to fitness advantage 7. Only when choice advantage is outweighed by survival disadvantage will trait evolution stop.
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Fishers Runaway Process
Good Genes models Sensory Bias Fitness Male Trait
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Do males matter? Are males different Does the choice of a male make any difference to females? Sure, at least sometimes
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Female choice Direct benefits Increased viability +/or fecundity Territory quality -- Rana catesbiana
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Female choice Direct benefits Increased viability +/or fecundity territory quality -- Rana catesbiana “nuptial gift” -- insects
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Female choice Direct benefits Increased viability +/or fecundity territory quality -- Rana catesbiana “nuptial gift” -- insects parental care -- birds, insects Decreased probability of disease transmission
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What if Female does not directly benefit?
Fitness Male Trait
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Indirect benefits -- ‘good genes’ Higher quality offspring
Female choice Indirect benefits -- ‘good genes’ Higher quality offspring Male offspring with higher mating success Fitness Male Trait
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Condition-Dependent Traits
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Why don’t bad males make big tails?
Must be honest signals Why don’t bad males make big tails? They die? Trait must be costly to make or it will not be honest handicaps The only traits that females should pay attention to are those which reliably indicate quality
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Tail length is a costly trait
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Parus major Females prefer males with a large breaststripe Breaststripe size has a heritability of ~0.7
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breaststripe size is strongly positively correlated with offspring viability
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Viral infection clearance rate and ornament brightness in
male greenfinches (Carduelis chloris) Lindstrom & Lundstrom Behav Ecol Sociobiol 48:44-51 PC1: overall size, esp of wing and breast patches – size of yellow wing patch related to age PC2: tail patch - size of yellow patches unrelated to age
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r = 0.58, p = 0.001, n = 28 r = -0.46, p = 0.01, n =28
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Female choice Sensory bias -- females have an existing preference due to a pleiotropic sensory system Females prefer males that elicit the greatest amount of stimulation from the female’s sensory system example: Anolis lizards frequently, ‘sit-and-wait’ predators respond to small movements by prey male courtship displays– “pushups” expand colored throat sac
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grackle zebra finch
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Xiphophorus Priapella
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Preference should correlate with male trait
Fitness Male Trait optimum Trait starts here Trait ends here Preference should correlate with male trait Within populations Genetically Selection on one trait will cause changes in the other
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Sexual selection acts on traits which affect mating and
fertilization success Shapes phenotypic variation in traits much like natural selection but may frequently conflict with natural selection Operates via: male-male competition, including sperm competition mate choice by females Females may be choosy because of: - direct benefits - indirect benefits - sensory bias Correlation between female preference and male phenotype may cause the male trait to evolve to phenotypic extremes
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