Chapter 3

1. Chapter 3 Sexual Selection

2. Sexual Selection • Intrasexual • Within the same sex • Intersexual • Between the sexes • Female choice • Not mutually exclusive

3. 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

4. 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

5. 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

6. 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

7. Theories of Sexual Selection • Parental investment • Runaway selection • Handicap hypothesis • Parasite theory of honest signaling

8. 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

9. 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

10. 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

11. Extravagant Males http://en.wikipedia.org/wiki/Image:Peacock_courting_peahen.jpg

12. 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

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

14. 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

15. 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

16. Female Choice • Affects both future male and female traits • Male’s traits passed on to sons • Female’s preferences passed to daughters

17. 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

18. Reproduction Types • Sexual reproduction • Asexual reproduction very common • Parthenogenesis • Toggling between asexual and sexual forms

19. Costs of Sexual Reproduction • Meiosis (toss away half your genes) • Producing males • Courtship and mating

20. Cost of Meiosis Asexual Reproduction Full set of genes passed to each offspring Sexual Reproduction Half set of genes passed to each offspring

21. 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

22. Cost of Courtship and Mating • Very time and energy expensive • Foraging • Establishing/maintaining territory • Predator avoidance

23. Sexual Reproduction • Despite costs, most multicellular species utilize sexual reproduction • What benefit(s) outweigh the costs?

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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

30. 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

31. 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

32. 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

33. 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

34. 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

35. 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…