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 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 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/i mages/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…