Chapter 3

Sexual Selection

Sexual Selection

• Intrasexual– Within the same sex

• Intersexual– Between the sexes– Female choice

• Not mutually exclusive

Widowbirds

• 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

http://www.bees.unsw.edu.au/school/researchstaff/pryke/prykewidowbird.html

http://www.pbase.com/calliedewet/widowbirds

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)http://wonderclub.com/Wildlife/mammals/mandrill.html

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

http://en.wikipedia.org/wiki/Image:Peacock_courting_peahen.jpg

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

Handicap

Incredibly long tale of peacock rests on ground when not displayed, hampering movement http://en.wikipedia.org/wiki/Image:Peacock_DSC04082.jpg

Parasite Theory of Honest Signaling

• 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 parasites

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 conditions

• 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 ReproductionFull set of genes passed to each offspring

Sexual ReproductionHalf set of genes passed to each offspring

Cost of Producing Males

• Practically all females will reproduce

• Generally, minority of males in a population reproduce

• 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

• Foraging

• 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

variability

• Can evolve more rapidly in changing environment– 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

generations

• 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 population/group/species

• 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

competitors• 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

http://www.liv.ac.uk/researchintelligence/issue39/images/Red_Queen_with_red_background.jpg

• Hosts produce variation in offspring in response to parasite pressure

• At least some offspring will possess parasite resistance

• Parasites when then evolve to counter this resistance

• 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 monogamous

• 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 scarce

– 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…