sexual selection i - university of oxfordusers.ox.ac.uk/~grafen/lectpres/ss1.pdf · • modern...
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Sexual Selection I
A broad overview
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Charles Darwin with his sonWilliam Erasmus in 1842
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Emma Darwinin 1840
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A section ofDarwin’s “noteson marriage”,1838.
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Lecture Outline
• Darwin and his addition to NaturalSelection– Difficulties with Natural Selection– Definition and idea– Humans
• Modern Study of Sexual Selection– Types of study– Focus on advantages to mate choice
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Lecture Outline
• Darwin and his addition to NaturalSelection– Difficulties with Natural Selection– Definition and idea– Humans
• Modern Study of Sexual Selection– Types of study– Focus on advantages to mate choice
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Examples of problematic phenomenafor the theory of Natural Selection
• Peacocks’ tails• Blackbird song• Antlers in deer• Sexual dimorphism in many species• Mating display of Great Crested Grebe
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Examples of problematic phenomenafor the theory of Natural Selection
• Peacocks’ tails (technically trains)• Blackbird song• Antlers in deer• Sexual dimorphism in many species• Mating display of Great Crested Grebe
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Lecture Outline
• Darwin and his addition to NaturalSelection– Difficulties with Natural Selection– Definition and idea– Humans
• Modern Study of Sexual Selection– Types of study– Focus on advantages to mate choice
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Lecture Outline
• Darwin and his addition to NaturalSelection– Difficulties with Natural Selection– Definition and idea– Humans
• Modern Study of Sexual Selection– Types of study– Focus on advantages to mate choice
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Darwin (1871, p256):
“We are, however, hereconcerned only with that kindof selection, which I havecalled sexual selection. Thisdepends on the advantagewhich certain individuals haveover other individuals of thesame sex and species, inexclusive relation toreproduction.”
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‘In relation to reproduction...’
• Being more noticed by, more attractive to,or more persuasive towards the oppositesex, and so gaining a mating advantage– Inter-sexual selection, ‘female’ choice
• Out-competing other members of the samesex in contests whose outcome determinesmating success– Intra-sexual selection, ‘male’ competition
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Darwin’s criteria
• Age: juvenile vs adult• Sex: male vs female• Season: breeding season vs year-round• Use: specially displayed during mating?
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Why does advantage in reproductionlead to exaggeration?
• ‘Disproportionate gains’– chooser makes an all or nothing choice
however small the difference is– all choosers may make the same choice
• Our similarity to targets of propaganda– any preference rule can be exploited– sensory systems have limitations
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Natural Selection vs Sexual Selection
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Natural Selection vs Sexual Selection
• Utilitarian, functional • Showy, elaborate
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Natural Selection vs Sexual Selection
• Utilitarian, functional• Solves a problem
• Showy, elaborate• Impresses an audience
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Natural Selection vs Sexual Selection
• Utilitarian, functional• Solves a problem• Sensible
• Showy, elaborate• Impresses an audience• Whimsical
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Natural Selection vs Sexual Selection
• Utilitarian, functional• Solves a problem• Sensible• Economical
• Showy, elaborate• Impresses an audience• Whimsical• Wasteful
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Natural Selection vs Sexual Selection
• Utilitarian, functional• Solves a problem• Sensible• Economical• Fixed
• Showy, elaborate• Impresses an audience• Whimsical• Wasteful• Changeable
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Natural Selection vs Sexual Selection
• Utilitarian, functional• Solves a problem• Sensible• Economical• Fixed• Constructive
• Showy, elaborate• Impresses an audience• Whimsical• Wasteful• Changeable• Destructive
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Natural Selection vs Sexual Selection
• Utilitarian, functional• Solves a problem• Sensible• Economical• Fixed• Constructive• Dull
• Showy, elaborate• Impresses an audience• Whimsical• Wasteful• Changeable• Destructive• Exciting
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Darwin (1871, p256):
“We are, however, hereconcerned only with that kindof selection, which I havecalled sexual selection. Thisdepends on the advantagewhich certain individuals haveover other individuals of thesame sex and species, inexclusive relation toreproduction.”
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Human traits possibly related to sexual selection
• Racial differences– skin colour– hair colour and texture
• Gender differences– physical– psychological
• Analogy to fashion in clothes andpersonal styles
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Lecture Outline
• Darwin and his addition to NaturalSelection– Difficulties with Natural Selection– Definition and idea– Humans
• Modern Study of Sexual Selection– Types of study– Focus on advantages to mate choice
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Lecture Outline
• Darwin and his addition to NaturalSelection– Difficulties with Natural Selection– Definition and idea– Humans
• Modern Study of Sexual Selection– Types of study– Focus on advantages to mate choice
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Lecture Outline
• Darwin and his addition to NaturalSelection– Difficulties with Natural Selection– Definition and idea– Humans
• Modern Study of Sexual Selection– Types of study– Focus on advantages to mate choice
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Kinds of modern work (with one example)
• Field empirical– Møller on barn swallows
• Experimental– Gerhardt on tree frogs
• Comparative method– Eberhard on reproductive anatomy
• Concepts / Mathematical modelling– Lande on runaway processes
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Modern topics
• Fireflies• Bird coloration• Anatomy of reproductive organs• Sperm competition• Pollen competition• Disease resistance
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Modern topics
• Fireflies• Bird coloration• Anatomy of reproductive organs• Sperm competition• Pollen competition• Disease resistance• … and more, discussed by Marian
Dawkins later in the course
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Modern topics
• Fireflies• Bird coloration• Anatomy of reproductive organs• Sperm competition• Pollen competition• Disease resistance• … and more, discussed by Marian
Dawkins later in the course
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Eberhard (1985)
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Population level effects...
• include polygamy, polyandry, leks andruts
• are discussed further later in the course
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Lecture Outline
• Darwin and his addition to NaturalSelection– Difficulties with Natural Selection– Definition and idea– Humans
• Modern Study of Sexual Selection– Types of study– Focus on advantages to mate choice
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Lecture Outline
• Darwin and his addition to NaturalSelection– Difficulties with Natural Selection– Definition and idea– Humans
• Modern Study of Sexual Selection– Types of study– Focus on advantages to mate choice
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Modern study of sexual selection
• focusses mainly on preferences,• treats preferences as adaptive,• looks for a selective advantage to
preference,• and there are several possibilities.
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Females might choose
• non-adaptively: because of sensory bias,species recognition.
• adaptively directly: for reasons of fertilisationability, fecundity, nutrition, parental ability,territory quality, avoiding venereal disease,social status of offspring
• adaptively through good genes: because ofinherited attractiveness of sons, or heritabledisease resistance, or more generallyheritable ‘quality’.
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Some formal ideas
• Fisher’s runaway model• Lande’s model• Measuring Sexual Selection:
Lande/Wade/Arnold• Hamilton/Zuk• Handicaps & Signalling
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Andersson (1994)
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Literature
• C. Darwin (1871) The Descent of Man andSelection in Relation to Sex. Republishedin 1981 by Princeton University Press.
• Extracts in M.Ridley (1987) The EssentialDarwin. Unwin Hyman.
• M. Andersson (1994) Sexual Selection.Princeton University Press.
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Lecture Outline
• Darwin and his addition to NaturalSelection– Difficulties with Natural Selection– Definition and idea– Humans
• Modern Study of Sexual Selection– Types of study– Focus on advantages to mate choice
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Lecture Outline
• Darwin and his addition to NaturalSelection– Difficulties with Natural Selection– Definition and idea– Humans
• Modern Study of Sexual Selection– Types of study– Focus on advantages to mate choice
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In summary,
• Darwin invented sexual selection toaccommodate certain kinds of factswithin his scheme, partly as a defenceagainst non-biological theories
• He established there was female choicebut said little about why
• We now assume selection is at work,and want to know why there is choice