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Sexual Selection and Reproductive Behaviour

Sexual Selection and Reproductive Behaviour.

avram-caldwell
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Sexual Selection and Reproductive Behaviour

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  1. Sexual Selection and Reproductive Behaviour

  2. 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

  3. 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

  4. 1. What is ‘sex’? 2. What are some of the costs and benefits of sex? Reproductive Behaviour 3. Why are the sexes generally in a 1:1 ratio?

  5. What is the one feature that serves to separate male from female in all animals? Anisogamy Anisogamy (heterogamy) - a form of sexual reproduction involving the union or fusion of two dissimilar gametes Reproductive Behaviour

  6. Anisogamy – how did it come about? Reproductive Behaviour

  7. Anisogamy Reproductive Behaviour Frequency of producers Gamete size

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

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

  10. 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?” Reproductive Behaviour Carl Zimmer Evolution: The Triumph of an Idea

  11. Costs of sex 1. Cost of meiosis Reproductive Behaviour • 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

  12. Costs of sex 2. Cost of recombination Reproductive Behaviour • take a functioning genome, split it and combine it with another one -chance of deleterious combinations

  13. Benefits of sex 1. Produce new combinations Reproductive Behaviour Unique offspring can cope with different niches x

  14. Benefits of sex 2. Muller’s Ratchet Reproductive Behaviour - in asexual species, deleterious mutations accumulate A A+B Frequency of mutant gene Mutation A A+B+C Mutation B B+C Mutation C C Time

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

  16. Sex ratio -why 1:1? Excess males Reproductive Behaviour -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

  17. Contributing Elements to Darwin’s theory 1. Charles Lyell (1797 - 1875) Gradualism (Uniformitarianism) All change through time can be explained by processes at work today No need to invoke catastrophic events

  18. Contributing Elements to Darwin’s theory Charles Lyell (1797 - 1875) Thomas Malthus (1766 - 1834) Populations of organism will grow faster than their food supply Population Number Food supply Time

  19. Contributing Elements to Darwin’s theory Charles Lyell (1797 - 1875) Thomas Malthus (1766 - 1834) Plant and Animal Breeders • showed that the form of a species could be changed over time

  20. Natural selection - Observations and deductions Observation - all species can increase exponentially but don’t - more are produced than can survive Darwin - Theory of sexual selection 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

  21. Problem for Darwin ? ? ?

  22. 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. Charles Darwin, 1859. Origin of species

  23. Two parts 1. Intrasexual selection (Male-male competition) But in many cases, victory will depend not on general vigour, but on having special weapons, confined to the male sex. Darwin - Theory of sexual selection The war is, perhaps, severest between the males of polygamous animals, and these seem oftenest provided with special weapons. 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. Charles Darwin, 1859. Origin of species

  24. 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 Darwin - Theory of sexual selection - claws, antlers etc.

  25. Two parts 2. Intersexual selection (Female choice) All those structures and behaviour patterns employed by males to attract females Darwin - Theory of sexual selection - plumage, song.

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

  27. In more general terms Females Males Difference in reproductive investment Reproductive Behaviour higher investment lower investment lower reproductive rate higher potential reproductive rate lower levels of mating activity higher levels of mating activity Biased operational sex ratio Selection among potential mates Competition for mates Best mate = best fitness benefit Achieve greatest number of matings

  28. Operational vs Numerical Sex ratio Numerical Sex Ratio The ratio of the number of males to the number of females in the population Reproductive Behaviour Operational Sex Ratio The ratio of the number of males to the number of females in the population who are available for reproduction

  29. Operational vs Numerical Sex ratio Numerical Sex Ratio (close to 1:1) Males Females Reproductive Behaviour

  30. Operational Sex Ratio Males Females Reproductive Behaviour Remove females for: Remove males for: 1. Pre-puberty 2. Sterile ( Post-menopause) 3. Age 4. Pregnant 5. Lactating Operational sex ratio

  31. Operational vs Numerical Sex ratio Numerical Sex Ratio (close to 1:1) Males Females Reproductive Behaviour Operational Sex Ratio (very different from 1:1) Competition in more abundant sex Selectivity in less abundant sex

  32. Evidence?? Bateman (1948) -Drosophila melanogaster - nearly all females mated but not all males Reproductive Behaviour -variance in male reproductive success - higher Males Bateman gradient # of offspring Females # of mates

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

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

  35. Intersexual Selection Genetic Models for Mate Choice 1. Direct Benefits -females choose males that give them a concrete resource e.g. Nuptial gifts

  36. Intersexual Selection Size of prey is critical 20 min Duration of copulation # sperm transferred 8 25 16 mm Size of prey Time in copulation

  37. Intersexual Selection 2. Good genes models Female should look for best genetic complement in male Traits should reflect genetic quality

  38. Intersexual Selection Fluctuating Asymmetry 2. Good genes models Horns in oribi - Ourebia ourebia Arese. 1994. Anim.Beh. 48:1485

  39. Intersexual Selection 3 2 1 0 Fluctuating Asymmetry 2. Good genes models Harem size Symmetric Asymmetric Damaged

  40. Intersexual Selection Fluctuating Asymmetry 2. Good genes models Symmetry Ability to fight parasites (nematodes) Lagesen & Folstad, 1998. Beh.Ecol.Sociobiol.

  41. Intersexual Selection Fluctuating Asymmetry 2. Good genes models

  42. Intersexual Selection Good genes and parasites 2. Good genes models Number of matings Clean Infected No choice

  43. Intersexual Selection Assume 2 genes Gene A - Trait ‘X’ in males - more colourful feathers 3. Runaway Selection 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

  44. Intersexual Selection Assume 2 genes Gene A - Trait ‘X’ in males - more colourful feathers 3. Runaway Selection Gene B - In females - preference for trait ‘X’ Frequency in males Frequency in females

  45. Intersexual Selection Example - stalk-eyed flies - Diopsidae 3. Runaway Selection Variation in males Female

  46. Intersexual Selection Example - stalk-eyed flies - Diopsidae Two experiments 3. Runaway Selection 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

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

  48. Intersexual Selection Sexual imprinting -already discussed this Other factors affecting mate choice - Learning 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 Male 2 Model female Test female

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

  50. Intrasexual Selection - Male/Male Competition Competition before mating Fiddler Crab (Uca) 1. Direct Fighting Female Female digs burrow Male Male enters burrow and mates Male defends burrow and female No evidence of female choice Male leaves when female ovulates

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