Chapter 3
Sexual Selection
Sexual Selection
• Intrasexual
– Within the same sex
• Intersexual
– Between the sexes
– Female choice
• Not mutually exclusive
• Both intrasexual and
intersexual competition
• Territorial competition is fierce
amongst males; only ~30%
successfully hold territory
• Only males with territory can
find mates
• Colour intensity determines
male status
• Females prefer to mate with
males with longer tail feathers
Competition in Males
• Many forms of intrasexual competition
• One-on-one, alliances (short-term, long-term)
• Gaining/maintaining territory, gathering resources,
direct physical conflict (mock/staged or fatal
• Ultimate point is gaining access to fertile females
• Not all males will be successful
• Form of competition will vary greatly across
species and environment
Competition in Females
• Female intrasexual competition as well
• Generally exclusionary tactics
– E.g., dominant female Scottish red deer force smaller
does from best grazing; increases her milk production;
benefits her offspring
• Limit other females’ access to preferred male(s)
• May be limits on number of females a choice male
can mate or support
Sexual Dimorphisms
• Differences in size, shape, attributes
between the sexes in a species
Mandrills: male (L) and females with infant (R)
Theories of Sexual Selection
Parental investment
Runaway selection
Handicap hypothesis
Parasite theory of honest signaling
Parental Investment
• Robert Trivers (1972)
• Female choice and male-male competition
• Females invest more in producing and
rearing offspring than males
– Asymmetry of parental investment
– Females have finite number of offspring they
can produce; theoretically, males only limited
by number of matings secured
• Cost disparity makes females more selective
Runaway Selection
• Ronald Fisher (1930)
• Ancestral females attracted to males with
some trait showing superior survival value
• To get noticed, males need more extreme
forms of this trait
• Sons receive fathers’ trait -- become “sexy”
• Daughters receive mothers’ preferences
• Preference for, and form of, the male trait
continually “ramps up”
• Male trait eventually “runs away” from its
original survival function
• Runaway process eventually stops when
reproductive benefits outweighed by
survival costs
Extravagant Males
Handicap Hypothesis
• Amotz Zahavi (1975)
• Males evolve costly, self-damaging
characteristics to demonstrate their fitness
to females
• If still alive despite costly handicap, must
be good mate choice
• Handicapping traits are phenotypic
demonstration of genotype fitness
Incredibly long tale of peacock rests on ground when not displayed, hampering movement
Parasite Theory of Honest
William Hamilton & Marlene Zuk (1982)
Most mortality due to parasitism
Elaborate male traits energetically costly
Maintaining such traits despite parasitism
implies good genes
• Healthy appearance of trait serves as
“honest signal” of genetic superiority
• Females’ offspring would benefit
• Similar to Handicap Hypothesis, except in
HH it is the elaborate traits that are the
handicap that the male is overcoming
• In PTHS, elaborate trait demonstrates
male’s ability to withstand dangerous
Female Choice
• Affects both future male and female traits
• Male’s traits passed on to sons
• Female’s preferences passed to daughters
Male Ornamentation
• What is male ornamentation for?
• Fisherian argument
– Makes males attractive to females
• Good-genes argument
– Serves as signal of genetic fitness
• Some support for both camps
• Could be both systems in operation in different
species and/or under different environmental
• Also, ongoing interaction between natural and
sexual selection re: development, maintenance, and
possible loss of evolved traits
Reproduction Types
• Sexual reproduction
• Asexual reproduction very common
– Parthenogenesis
– Toggling between asexual and sexual forms
Costs of Sexual Reproduction
• Meiosis (toss away half your genes)
• Producing males
• Courtship and mating
Cost of Meiosis
Asexual Reproduction
Sexual Reproduction
Full set of genes passed to each offspring Half set of genes passed to each offspring
Cost of Producing Males
• Practically all females will reproduce
• Generally, minority of males in a population
• Most male offspring will not produce
offspring, so waste of resources vis-à-vis
differential reproductive success for parents
Cost of Courtship and Mating
Very time and energy expensive
Establishing/maintaining territory
Predator avoidance
Sexual Reproduction
• Despite costs, most multicellular species
utilize sexual reproduction
• What benefit(s) outweigh the costs?
Fisher (1958)
• Sexually reproducing species will have
more variability
– Half genes from each parent --> high offspring
• Can evolve more rapidly in changing
– Asexual species have to rely on mutation
effects to put variability into population; slower
Muller’s (1964) Ratchet
• Harmful mutations evolving in asexual
individual will be passed to all offspring
– Harmful mutations will accumulate across
• Only half offspring get deleterious mutation
in sexual species
– Individuals with mutation less likely to
reproduce, so mutation selected against
• Sexual reproduction reduces/limits spread
of deleterious mutations in gene pool
Group vs. Individual
• Both Fisher and Muller approach sexual
reproduction as benefit to
• But, more recent evolutionary theory argues
selection operates at the individual level
Raffle Analogy
• Williams (1975)
• Each offspring like a raffle ticket in a draw
– Sexual species: each ticket/offspring different
– Asexual species: each ticket/offspring the same
• Environmental stability the issue here
• If environment changes, in sexual species at
least some offspring might be successful; in
asexual species, all offspring could lose out
Tangled Bank
• Counter to Raffle Theory prediction, Bell
(1982) found more sexual species in stable
environments (e.g., oceans, low altitudes)
• Environment should include both physical
(abiotic) and living (biotic) factors
Stable Abiotic Environment
• Predictable, less severe
• Benefits to individual, but also to
• Greatest competition from members of own
species (need same resources)
• Biotic environment
• Small variations might give individual an
“edge” over competitors
Red Queen Hypothesis
• “It takes all the running you can
do, just to keep in the same place.”
The Red Queen in Lewis Carroll’s
Through the Looking Glass
• Van Valen (1973)
• Parasites and hosts are in a
continual “arms race”
• Parasites’ short lifespans and huge
numbers lets them evolve more
rapidly than their hosts
• Hosts produce variation in offspring in
response to parasite pressure
• At least some offspring will possess parasite
• Parasites when then evolve to counter this
• The back-and-forth may result in ancestral
forms re-evolving
Lively (1987)
• Test of the theories
• New Zealand water snails
– Stable lakes, unstable streams
– Can use sexual or asexual reproduction
• Red Queen: sexual reproduction in lakes, asexual
in streams
• Stable lakes result in high numbers of parasites
• High sexual reproduction in lake snails; supports
Red Queen
Ancestral changes
• From about 4-5 million years ago:
• Upright posture, bipedal locomotion, reduced body
hair, increased sweat glands
– Good for distance traveling
• Brain size increasing
– Adding 150 million brain cells per 100,000 years
• Increased dietary need for oils, fats, proteins
• Increase in size
– Nariokotome boy of 1.6 mya would have been almost 6
feet tall
• Females increasing in size relative to males
Driven by Sexual Selection
• Promiscuity in chimpanzees, harem polygamy in gorillas
• Reduced size sexual dimorphism shifts towards something more
• Pressure on each sex to be choosy when picking a mate; long pair-bonds
necessitates careful choice
– Males look for younger females, females for provider males
• Sexual division of labour: hunting (male) and gathering (female)
– Females gain access to high protein meat without having to abandon fairly
helpless young to gain it; males have access to plant foods when animal game
– Both sexes benefit; reciprocal altruism
Feedback Cascade
• Big brains need meat
• Food sharing allowed meaty diet
• Food sharing demands big brains (better cognition
reduces being cheated by freeloaders, etc.)
• Larger brains allows formalizing of division of
labour agreements
• Sexual division of labour promotes monogamy (a
pair-bond is now a useful “economic unit”)
• Monogamy led to neotenous sexual selection
• And so on…
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