“The sight of a feather in a
peacock's tail, whenever I
gaze at it, makes me sick!”
Sexual selection
Types of selection
Natural:
fecundity,
survival
guppies
Sexual:
number
of mates
Gametic investment
Sex differences in
gamete investment,
production
Bateman‟s principle
pike cichlid
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
Generalities from Bateman’s principle
Angus Bateman
Consequences of Bateman’s principle
Males compete for access to females
Females choose among males
Higher variance in mating success in males
satin bowerbird
1 Sexually selected traits
Sexually selected traits
Gamete production (primary) +
gamete delivery (secondary)
Primary sexual traits =
necessary for reproduction
Secondary sexual traits =
increase the probability of
mating or the number
of mates obtained
Sceloporus undulatus
Sexually selected traits
black
grouse
Expression of
secondary sexual
traits often mediated
by androgens
Mechanisms of sexual selection
Intrasexual selection
(„male-male competition‟)
Mechanisms of sexual selection
Intrasexual selection
(„male-male competition‟)
Selects for traits that aid in
combat, territory defense
Intersexual selection
(„female mate choice‟)
Intersexual selection
(„female mate choice‟)
Selects for traits that
increase attractiveness
2 Mechanisms of sexual selection
Consequences of sexual selection
Intrasexual selection
(„male-male competition‟)
More likely concordant with
natural selection
Marine iguanas on
the Galápagos
Intersexual selection
(„female mate choice‟)
More likely to oppose
natural selection
Intrasexual selection: precopulatory
Males larger than optimal range
Intrasexual selection: precopulatory
Contest competition = direct interaction
Scramble competition = indirect interaction
sage grouse
Hercules beetle
Intrasexual selection: precopulatory
Intrasexual selection: precopulatory
Males guard females when most receptive
Seychelles warbler
Seychelles warbler
3 Alternative mating strategies
• Males may adopt alternative strategies
Alternative mating strategies
Three tactics in marine isopods
– Satellite or sneaker behavior
– Female mimicry
• Strategies usually conditional, occasionally genetic
One locus, three alleles
horseshoe crabs
Alternative mating strategies
Alternative mating strategies
“Resident” (85%) and “satellite” (15%) morphs in ruffs
Reproductive
success
If genetically controlled, what would
we predict about the fitness of each
genotype?
Depends on
density of females
alpha males
Pedigree analysis suggests one gene, two alleles
gamma males
alpha
beta
gamma
Recently discovered “faeder” morph has enlarged testes
Intrasexual competition: postcopulatory
Intrasexual competition: postcopulatory
Copulatory plugs: (i) sperm storage, (ii) chastity
enforcement or (iii) prevention of sperm loss?
Male garter snakes avoid mating
with females for 48 h – based on plug odor?
Thamnophis radix
palm civet
no sperm in plugs
Females often mated
2x per night,
2nd male dislodged
plug left by first
Suggests sperm
loss function
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
4 Sperm competition
Sex role reversals
Modified penis can remove
>90% of previous males‟ sperm
black damselfy
female
male
dunnock
Fertility window results in sex role reversal in topi
Birds lack morphological
sophistication of insects
Sex-role reversals in birds
Wilson’s phalarope
black coucal
Sex-role reversals and sexual selection
Even in clade with apparently
reversed sex roles, does sexual
selection act more strongly on females?
• Males rarely sing
• Males provide all care for young
• Females larger
Frequency
Parameter
Length
Mass
Color index
Unmated females
33.80 mm
0.29 g
3.3
Gulf pipefish
males
females
0
Intensity of sexual selection
Mated females
38.10 mm
0.4 g
5
1
2
3
Number of mates
4
Intensity of sexual selection
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?
Drosophila melanogaster
Reproductive
success
broad nosed pipefish
Sexual selection
or “Bateman‟s”
gradient
Mating success
5
Primary and Secondary Sexual Traits Part1
of 5
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