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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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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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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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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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Female choice Indirect benefits -- ‘good genes’ Higher quality offspring Male offspring with higher mating success
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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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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” inflate colored throat sac
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grackle zebra finch
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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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