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Sexual Selection and Reproductive Behaviour Sex vs. Reproduction Sex - process by which a genome is changed by rearrangement and combining of genes Reproduction - process by which individuals are added to a population Sex -changes in kinds of individuals in populations Reproduction -changes in the number of individuals in populations Reproductive Behaviour 1. What is ‘sex’? 2. What are some of the costs and benefits of sex? 3. Why are the sexes generally in a 1:1 ratio? Reproductive Behaviour Anisogamy Frequency of producers Gamete size Reproductive Behaviour Anisogamy - lots are produced by not enough mass for cell division - fewer produced but are competent - very few produced but are very competent Reproductive Behaviour Anisogamy + + + + - lots of contact but won’t divide properly - will divide but fewer contacts - will divide but very few contacts -greatest success -best combination of numbers and competence Reproductive Behaviour Costs of sex 1. Cost of meiosis -must combine genes with another organism -lose 50% every generation relative to asexuals -often referred to as cost of males - females can always do better asexually Reproductive Behaviour Costs of sex 2. Cost of recombination -take a functioning genome, split it and combine it with another one -chance of deleterious combinations Reproductive Behaviour Benefits of sex 1. Produce new combinations x Unique offspring can cope with different niches Reproductive Behaviour Benefits of sex 2. Muller’s ratchet - in asexual species, deleterious mutations accumulate Frequency of mutant gene Mutation A Mutation B Mutation C Time Reproductive Behaviour Benefits of sex 2. Muller’s ratchet - in sexual species, such mutations tend to be eliminated Frequency of mutant gene Mutation A Mutation B Mutation C Time Reproductive Behaviour Sex ratio -why 1:1? Excess males -some males won’t mate -females that produce more females are favoured 1:1 sex ratio -some females won’t mate -females that produce more males are favoured Excess females Time Darwin - Theory of sexual selection Natural selection - Observations and deductions Observation - all species can increase exponentially but don’t - more are produced than can survive Deduction - there is a struggle for existence Observation - there is variation in all organisms that is heritable Deduction - some are better equipped to survive than others - advantage is passed to offspring Differential survival = Natural selection Darwin - Theory of sexual selection Two parts 1. Intrasexual selection (Male-male competition) All those structures and behaviour patterns employed by males to fight other males for the chance to mate with females - claws, antlers etc. Darwin - Theory of sexual selection Two parts 2. Intersexual selection (Female choice) All those structures and behaviour patterns employed by males to attract females - plumage, song. Reproductive Behaviour Males and females have different reproductive interests What does each sex have to provide to produce offspring? Females Males 1. Manufacture eggs (expensive) 1. 2. Incubate eggs or fetus (expensive) 3. Post partum care (expensive) 4. Loss of mating opportunity when pregnant (expensive) Manufacture sperm (cheap) Males invest less in any offspring Reproductive Behaviour In more general terms Females Difference in reproductive investment higher investment Males lower investment lower reproductive rate lower levels of mating activity higher potential reproductive rate higher levels of mating activity Biased operational sex ratio Selection among potential mates Best mate = best fitness benefit Competition for mates Achieve greatest number of matings Reproductive Behaviour Operational vs Numerical Sex ratio Numerical Sex Ratio The ratio of the number of males to the number of females in the population Operational Sex Ratio The ratio of the number of males to the number of females in the population who are available for reproduction Reproductive Behaviour Operational vs Numerical Sex ratio Numerical Sex Ratio (close to 1:1) Males Females Reproductive Behaviour Operational Sex Ratio Males Remove males for: Females Remove females for: 1. Pre-puberty 2. Sterile ( Postmenopause) 3. Age 4. Pregnant 5. Lactating Operational sex ratio Reproductive Behaviour Operational vs Numerical Sex ratio Numerical Sex Ratio (close to 1:1) Males Females Operational Sex Ratio (very different from 1:1) Competition in more abundant sex Selectivity in less abundant sex Reproductive Behaviour Evidence?? Bateman (1948) -Drosophila melanogaster - nearly all females mated but not all males -variance in male reproductive success - higher Males Bateman gradient # of offspring Females # of mates Reproductive Behaviour Evidence?? - Lions Females % of breeders 10 0 # of surviving offspring Reproductive Behaviour Evidence?? - Lions Females Males % of breeders 10 0 # of surviving offspring Intersexual Selection Genetic Models for Mate Choice 1. Direct Benefits -females choose males that give them a concrete resource e.g. Nuptial gifts Intersexual Selection Size of prey is critical 20 min Duration of copulation # sperm transferred 16 mm Size of prey 8 25 Time in copulation Intersexual Selection 2. Good genes models Female should look for best genetic complement in male Traits should reflect genetic quality Intersexual Selection 2. Good genes models Fluctuating Asymmetry Horns in oribi - Ourebia ourebia Arese. 1994. Anim.Beh. 48:1485 Intersexual Selection 2. Good genes models Fluctuating Asymmetry 3 2 Harem size 1 0 Symmetric Asymmetric Damaged Intersexual Selection 2. Good genes models Good genes and parasites Number of matings Clean Infected No choice Intersexual Selection 3. Runaway Selection Assume 2 genes Gene A - Trait ‘X’ in males - more colourful feathers Gene B - In females - preference for trait ‘X’ A + B Expressed in males as the trait A + B Expressed in females as a preference for the trait Intersexual Selection 3. Runaway Selection Assume 2 genes Gene A - Trait ‘X’ in males - more colourful feathers Gene B - In females - preference for trait ‘X’ Frequency in males Frequency in females Intersexual Selection 3. Runaway Selection Example - stalk-eyed flies - Diopsidae Variation in males Female Intersexual Selection 3. Runaway Selection Example - stalk-eyed flies - Diopsidae Two experiments 1. Standard disruptive selection -every generation - allow shortest and longest stalked males to mate with randomly selected females Short stalked Long stalked 13 generations Stalk length Stalk length Intersexual Selection 3. Runaway Selection Example - stalk-eyed flies - Diopsidae Two experiments 2. Female choice Female offspring prefer short stalked males Female offspring prefer long stalked males Stalk length Intersexual Selection Other factors affecting mate choice Learning 1. Sexual imprinting -already discussed this 2. Mate choice copying - chances of a male being preferred as a mate at time 2 increases as a result of being preferred at time 1. Male 1 Model female Test female Male 2 Intersexual Selection Other factors affecting mate choice Learning 1. Sexual imprinting -already discussed this 2. Mate choice copying Frequency Near ‘chosen’ male Near ‘non-chosen’ male Time spent by test fish Intrasexual Selection - Male/Male Competition Competition before mating Fiddler Crab (Uca) 1. Direct Fighting Female digs burrow Male Male enters burrow and mates Female Male defends burrow and female Male leaves when female ovulates No evidence of female choice Intrasexual Selection - Male/Male Competition Competition before mating 2. Interference -male tries to interfere with copulating pair Intrasexual Selection - Male/Male Competition Competition before mating 2. Interference Elephant seals (Mirounga) -females incite males to fight via interference Males fight for dominance -8.3% of males mate Intrasexual Selection - Male/Male Competition Competition before mating 2. Interference Logic - if subordinate male tries to mate - female protests - dominant interferes -female gets ‘best’ mate % of matings protested by female Alpha Adult male Rank of male Subadult male Intrasexual Selection - Male/Male Competition Competition before mating 3. Cuckoldry Bluegill sunfish Males set up territories where females lay eggs Intrasexual Selection - Male/Male Competition Competition before mating 3. Cuckoldry Three kinds of males 1. Parental - larger, aggressive territory holders 2. Sneaker - smaller 3. Satellite - look like females spawning Male digs nest Female lays eggs Male fertilizes eggs Intrasexual Selection - Male/Male Competition Male digs nest Female lays eggs Parental - larger, aggressive territory holders Male fertilizes eggs Parental male Female Sneaker - smaller - rush in before parental male and fertilize eggs Satellite - look like females - spawn with pair Satellite Intrasexual Selection - Male/Male Competition Competition after mating 1. Sperm Competition Females of many species can 1) Store sperm 2) Mate with several males before fertilization Raises probability that sperm will compete for fertilizations Males can a) reduce the chances that a second male’s sperm is used (first male adaptations) b) reduce the chances that a first male’s sperm is used (second male adaptations) Intrasexual Selection - Male/Male Competition Competition after mating 1. Sperm Competition - ‘First male adaptations’ a. Postcopulatory mate guarding Intrasexual Selection - Male/Male Competition Competition after mating 1. Sperm Competition - ‘First male adaptations’ a. Postcopulatory mate guarding Stick insects Intrasexual Selection - Male/Male Competition Competition after mating 1. Sperm Competition - ‘First male adaptations’ a. b. c. Postcopulatory mate guarding Antiaphrodisiacs Mating plugs Intrasexual Selection - Male/Male Competition Competition after mating 1. Sperm Competition - ‘Second male adaptations’ a. Sperm removal Dunnock Damselfly Penes Intrasexual Selection - Male/Male Competition Competition after mating 2. Bruce effect - Mice - strange male (or male odour) -females abort fetuses and become receptive Intrasexual Selection - Male/Male Competition Competition after mating 3. Infanticide Summary - Sexual Selection Intersexual Selection 1. Direct Benefits 2. Good Genes 3. Runaway Selection Intrasexual Selection 1. Premating a. Male fighting b. Cuckoldry c. Interference 2. Postmating a. Sperm Competition b. Bruce Effect c. Infanticide