Sexual Selection and Reproductive Behaviour

Sex is the queen of problems in evolutionary biology. Perhaps no other natural phenomenon has aroused so much interest; certainly none has sowed as much confusion. The insights of Darwin and Mendel, which have illuminated so many mysteries, have so far failed to shed more than a dim and wavering light on the central mystery of sexuality, emphasizing its obscurity by its very isolation. Graham Bell The Masterpiece of Nature: The Evolution of Genetics and Sexuality

Sex vs. Reproduction Reproduction - process by which individuals are added to a population Sex - process by which a genome is changed by rearrangement and combining of genes 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 Anisogamy (heterogamy) - a form of sexual reproduction involving the union or fusion of two dissimilar gametes What is the one feature that serves to separate male from female in all animals?

Reproductive Behaviour Anisogamy – how did it come about?

Reproductive Behaviour Gamete size Frequency of producers Anisogamy

Reproductive Behaviour Anisogamy - lots are produced by not enough mass for cell division - very few produced but are very competent - fewer produced but are competent

Reproductive Behaviour Anisogamy - lots of contact but won’t divide properly - will divide but very few contacts - will divide but fewer contacts greatest success -best combination of numbers and competence

Reproductive Behaviour Costs of sex “Sex is not only unnecessary, but it ought to be a recipe for evolutionary disaster. For one thing, it is an inefficient way to reproduce…And sex carries other costs as well…By all rights, any group of animals that evolves sexual reproduction should be promptly outcompeted by nonsexual ones. And yet sex reigns… Why is sex a success, despite all its disadvantages?” Carl Zimmer Evolution: The Triumph of an Idea

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 Time Mutation A Mutation B Mutation C A A+BA+B A+B+CA+B+C B+CB+C C

Reproductive Behaviour Benefits of sex 2. Muller’s ratchet - in sexual species, such mutations tend to be eliminated Frequency of mutant gene Time Mutation A Mutation B Mutation C

Reproductive Behaviour Sex ratio -why 1:1? Time 1:1 sex ratio Excess females Excess males -some males won’t mate -females that produce more females are favoured -some females won’t mate -females that produce more males are favoured

Contributing Elements to Darwin’s theory 1. Charles Lyell ( ) Gradualism (Uniformitarianism) All change through time can be explained by processes at work today No need to invoke catastrophic events

Contributing Elements to Darwin’s theory 1.Charles Lyell ( ) 2.Thomas Malthus ( ) Populations of organism will grow faster than their food supply Population Food supply Number Time

Contributing Elements to Darwin’s theory 1.Charles Lyell ( ) 2.Thomas Malthus ( ) 3.Plant and Animal Breeders -showed that the form of a species could be changed over 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

Problem for Darwin ? ? ?

Charles Darwin, Origin of species And this leads me to say a few words on what I call Sexual Selection. This depends, not on a struggle for existence, but on a struggle between the males for possession of the females; the result is not death to the unsuccessful competitor, but few or no offspring. Sexual selection is, therefore, less rigorous than natural selection. Generally, the most vigorous males, those which are best fitted for their places in nature, will leave most progeny.

Darwin - Theory of sexual selection Two parts 1. Intrasexual selection (Male-male competition) 2. Intersexual selection (Female choice) …but if man can in a short time give elegant carriage and beauty to his bantams, according to his standard of beauty, I can see no good reason to doubt that female birds, by selecting, during thousands of generations, the most melodious or beautiful males, according to their standard of beauty, might produce a marked effect. But in many cases, victory will depend not on general vigour, but on having special weapons, confined to the male sex. Charles Darwin, Origin of species The war is, perhaps, severest between the males of polygamous animals, and these seem oftenest provided with special weapons.

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 Females 1.Manufacture eggs (expensive) 2. Incubate eggs or fetus (expensive) 3. Post partum care (expensive) Males 1.Manufacture sperm (cheap) What does each sex have to provide to produce offspring? Males invest less in any offspring 4. Loss of mating opportunity when pregnant (expensive)

Reproductive Behaviour In more general terms Difference in reproductive investment FemalesMales higher investmentlower investment higher potential reproductive ratelower reproductive rate lower levels of mating activityhigher levels of mating activity Biased operational sex ratio Selection among potential matesCompetition for mates Best mate = best fitness benefitAchieve 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 Males Females Operational Sex Ratio Remove males for: 1. Pre-puberty 2. Sterile 3. Age Remove females for: 4. Pregnant ( Post- menopause) 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 # of mates # of offspring Females Males Bateman gradient

Reproductive Behaviour Evidence?? - Lions # of surviving offspring % of breeders Females 0 10

Reproductive Behaviour Evidence?? - Lions # of surviving offspring % of breeders Females 0 10 Males

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 Size of prey Duration of copulation 20 min 16 mm Time in copulation # sperm transferred 25 8

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 Anim.Beh. 48:1485

Intersexual Selection 2. Good genes models Fluctuating Asymmetry SymmetricAsymmetricDamaged Harem size

Intersexual Selection 2. Good genes models Fluctuating Asymmetry Lagesen & Folstad, Beh.Ecol.Sociobiol. Symmetry Ability to fight parasites (nematodes)

Intersexual Selection 2. Good genes models Fluctuating Asymmetry

Intersexual Selection 2. Good genes models Good genes and parasites Number of matings CleanInfectedNo 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+BA+B A+BA+B Expressed in males as the trait 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 females Frequency in males

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 stalkedLong stalked 13 generations Stalk length

Intersexual Selection 3. Runaway Selection Example - stalk-eyed flies - Diopsidae Two experiments 2. Female choice Stalk length Female offspring prefer short stalked males Female offspring prefer long stalked males

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 1Male 2 Model female Test female

Intersexual Selection Other factors affecting mate choice - Learning 1.Sexual imprinting -already discussed this 2.Mate choice copying Frequency Near previously ‘chosen’male Near previously ‘non-chosen’male Time spent by test fish

Intrasexual Selection - Male/Male Competition 1. Direct Fighting Competition before mating Fiddler Crab (Uca) Male Female Female digs burrow Male enters burrow and mates 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 Rank of male AlphaAdult maleSubadult male

Bluegill sunfish Lepomis macrochirus Males set up territories where females lay eggs Intrasexual Selection - Male/Male Competition Competition before mating 3. Cuckoldry

Sunfish Territory

Three kinds of males 1. Parental - larger, aggressive territory holders 2. Sneaker - smaller 3. Satellite - look like females spawning Male digs nestFemale lays eggsMale fertilizes eggs Intrasexual Selection - Male/Male Competition Competition before mating 3. Cuckoldry

Parental - larger, aggressive territory holders Male digs nestFemale lays eggsMale fertilizes eggs Sneaker - smaller - rush in before parental male and fertilize eggs Satellite - look like females - spawn with pair Parental male Female 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.Postcopulatory mate guarding b.Antiaphrodisiacs c.Mating plugs

Intrasexual Selection - Male/Male Competition Competition after mating 1. Sperm Competition - ‘Second male adaptations’ a. Sperm removal Dunnock Damselfly (Argia) 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 2. Postmating b. Bruce Effect a. Sperm Competition b. Cuckoldry c. Interference c. Infanticide