Primary and Secondary Sexual Traits2520281%25… · Sexual: number of mates guppies pike cichlid...
Transcript of Primary and Secondary Sexual Traits2520281%25… · Sexual: number of mates guppies pike cichlid...
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Sexual selection
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“The sight of a feather in a
peacock's tail, whenever I
gaze at it, makes me sick!”
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Sexual:
number
of mates
guppies
pike cichlid
Natural:
fecundity,
survival
Sexual selection depends on the success of certain individuals over others
of the same sex, in relation to the propagation of the species; while natural
selection depends on the success of both sexes, at all ages, in relation to
the general conditions of life. -- Charles Darwin
Types of selection
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Sex differences in
gamete investment,
production
Bateman‟s principle
Gametic investment
Angus Bateman
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Males compete for access to females
Females choose among males
Generalities from Bateman’s principle
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Higher variance in mating success in males
satin bowerbird
Consequences of Bateman’s principle
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Primary sexual traits =
necessary for reproduction
Secondary sexual traits =
increase the probability of
mating or the number
of mates obtained
Sexually selected traits
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Gamete production (primary) +
gamete delivery (secondary)
Sexually selected traits
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Expression of
secondary sexual
traits often mediated
by androgens
Sceloporus undulatus
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black
grouse
Sexually selected traits
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Intrasexual selection
(„male-male competition‟)
Intersexual selection
(„female mate choice‟)
Mechanisms of sexual selection
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Selects for traits that aid in
combat, territory defense
Selects for traits that
increase attractiveness
Intrasexual selection
(„male-male competition‟)
Intersexual selection
(„female mate choice‟)
Mechanisms of sexual selection
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More likely concordant with
natural selection
More likely to oppose
natural selection
Intrasexual selection
(„male-male competition‟)
Intersexual selection
(„female mate choice‟)
Mechanisms of sexual selection
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Consequences of sexual selection
Males larger than optimal range
Marine iguanas on
the Galápagos
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Scramble competition = indirect interaction
Intrasexual selection: precopulatory
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Contest competition = direct interaction
Intrasexual selection: precopulatory
Hercules beetle
sage grouse
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Males guard females when most receptive
Seychelles warbler
Intrasexual selection: precopulatory
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Seychelles warbler
Intrasexual selection: precopulatory
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horseshoe crabs
• Males may adopt alternative strategies
– Satellite or sneaker behavior
– Female mimicry
• Strategies usually conditional, occasionally genetic
Alternative mating strategies
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Three tactics in marine isopods
One locus, three alleles
Alternative mating strategies
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alpha beta gamma
Repro
ductive
success
If genetically controlled, what would
we predict about the fitness of each
genotype?
Depends on
density of females
alpha males
gamma males
Alternative mating strategies
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“Resident” (85%) and “satellite” (15%) morphs in ruffs
Pedigree analysis suggests one gene, two alleles
Alternative mating strategies
Recently discovered “faeder” morph has enlarged testes
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Copulatory plugs: (i) sperm storage, (ii) chastity
enforcement or (iii) prevention of sperm loss?
palm civet
no sperm in plugs Females often mated
2x per night,
2nd male dislodged
plug left by first Suggests sperm
loss function
Intrasexual competition: postcopulatory
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Thamnophis radix
Male garter snakes avoid mating
with females for 48 h – based on plug odor?
Observed Mating Status and Courting
Activities of Female Garter Snake
Number
Courted
Number
Not Courted
T. sirtalis without plug 21 6
T. sirtalis with plug 0 19
Intrasexual competition: postcopulatory
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black damselfy
Modified penis can remove
>90% of previous males‟ sperm
Sperm competition
dunnock
Birds lack morphological
sophistication of insects
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Sex role reversals
male female
Fertility window results in sex role reversal in topi
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• Males rarely sing
• Males provide all care for young
• Females larger
Sex-role reversals in birds
Wilson’s phalarope black coucal
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Even in clade with apparently
reversed sex roles, does sexual
selection act more strongly on females?
Parameter Mated females Unmated females
Length 38.10 mm 33.80 mm
Mass 0.4 g 0.29 g
Color index 5 3.3
Fre
qu
en
cy
Number of mates
0 1 2 3 4
males
females
Gulf pipefish
Sex-role reversals and sexual selection
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Mating success = number of matings
Reproductive success = number of offspring
What did Bateman predict
about the relationship between
mating success and reproductive
success for males and females?
Repro
ductive
success
Mating success
Sexual selection
or “Bateman‟s”
gradient
Intensity of sexual selection
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broad nosed pipefish
Drosophila melanogaster
Intensity of sexual selection