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Chapter 16 evolution of sex. Adaptive significance of sex Many risks and costs associated with sexual reproduction. Searching for and courting a mate.

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Presentation on theme: "Chapter 16 evolution of sex. Adaptive significance of sex Many risks and costs associated with sexual reproduction. Searching for and courting a mate."— Presentation transcript:

1 Chapter 16 evolution of sex

2 Adaptive significance of sex Many risks and costs associated with sexual reproduction. Searching for and courting a mate requires time and energy and exposes organisms to predators Sex exposes individuals to infection with diseases and and parasites. Mate may require investment (food, territory, defense). Sex can break up favorable combinations of genes.

3 Adaptive significance of sex Why not reproduce asexually? Many organisms can reproduce both sexually and asexually. E.g. plants, aphids.

4 Adaptive significance of sex In populations that can reproduce both asexually and sexually will one mode of reproduction replace the other?

5 Adaptive significance of sex John Maynard Smith explored the question. Considered population in which some organisms reproduce asexually and the others sexually. Made 2 assumptions.

6 Maynard Smith’s assumptions 1. Mode of reproduction does not affect number of offspring she can produce. 2. Mode of reproduction does not affect probability offspring will survive. (asexually reproducing organisms produce only females, sexually reproducing produce both males and females.)

7 Adaptive significance of sex Asexually reproducing females under Maynard Smith’s assumptions leave twice as many grandchildren as sexually reproducing females. This is because each generation of sexually reproducing organisms contains only 50% females.

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9 Adaptive significance of sex Ultimately, asexual reproduction should take over. However, in nature this is not the case. Most organisms reproduce sexually and both sexual and asexual modes of reproduction are used in many organisms

10 Adaptive significance of sex Sex must confer benefits that overcome the mathematical reproductive advantage of asexual reproduction. One or both of Maynard Smith’s assumptions must be incorrect.

11 Adaptive significance of sex Assumption 1 (mode of reproduction does not affect number of offspring she can produce) is violated in species where males helps females (humans, birds, many mammals, some fish). However, not likely a general explanation because in most species male does not help.

12 Adaptive significance of sex Most likely advantage of sex is that it increases offspring’s prospects of survival.

13 Dunbrack et al. (1995) experiment Lab populations of flour beetles Mixed populations of red and black strains. Strains designated as “sexual” or “asexual” in experimental replicates.

14 Dunbrack et al. (1995) experiment Asexual strain in culture. Every generation each adult replaced by 3 new individuals from reservoir population of sexual strain. This simulates a 3X reproductive advantage, but there is no evolution in response to the environment. Sexual strain allowed to breed and remain in culture. Could evolve.

15 Dunbrack et al. (1995) experiment Two strains prevented from breeding with each other. Populations tracked for 30 generations. 8 replicates in experiment. Four different concentrations of malathion (insecticide). Controls: No evolution, but one strain had 3x reproductive advantage.

16 Dunbrack et al. (1995) experiment Control results. “Asexually” reproducing strain outcompeted the sexually reproducing strain.

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19 Dunbrack et al. (1995) experiment Experimental cultures: Initially asexual strain increased in frequency, but eventually sexual strain took over. Rate at which sexual strain took over was proportional to malathion concentration.

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22 Dunbrack et al. (1995) experiment Conclusion: Assumption 2 of Maynard Smith’s null model is incorrect. Descendants produced by sexual reproduction achieve higher fitness than those produced asexually.

23 Sex in populations means genetic recombination Sex involves: Meiosis with crossing over Matings with random individuals Random meeting of sperm and eggs Consequence is genetic recombination. New combinations of genes brought together each generation.

24 Why is sex beneficial? 1. Genetic drift plus mutation make sex beneficial. Escapes Muller’s ratchet. 2. Selection imposed by changing environments makes sex beneficial

25 Genetic drift plus mutation: Muller’s ratchet An asexually reproducing female will pass a deleterious mutation to all her offspring. Back mutation only way to eliminate it. Muller’s ratchet: accumulation of deleterious alleles in asexually reproducing populations.

26 Muller’s ratchet Small, asexually reproducing population. Deleterious mutations occur occasionally. Mutations selected against. Population contains groups of individuals with zero, one, two, etc. mutations.

27 Muller’s ratchet Few individuals in each group. If by chance no individual with zero mutations reproduces in a generation, then the zero mutation group is lost. Rate of loss of groups by drift will be higher than rate of back mutation so population will over time accumulate deleterious mutations in a ratchet fashion.

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30 Muller’s ratchet Burden of increased number of deleterious mutations (genetic load) may eventually cause population to go extinct. Sexual reproduction breaks ratchet. E.g. two individuals each with one copy of a deleterious mutation will produce 25% of offspring that are mutation free.

31 Anderson and Hughes (1996) test of Muller’s ratchet in bacteria. Propagated multiple generations of bacterium, but each generation was derived from one individual (genetic drift). 444 cultures. At end of experiment (2 months) 1% of cultures had reduced fitness (lower than wild- type bacteria), none had increased fitness. Results consistent with Muller’s ratchet.

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33 Selection favors sex in changing environments. Effects of Muller’s ratchet are slow and take many generations to affect asexually reproducing populations. However, advantage of sex is apparent in only a few generations. What short-term benefit does sex provide?

34 Selection favors sex in changing environments. In constant environments asexual reproduction is a good strategy (if mother is adapted to environment, offspring will be too). However, if environment changes, offspring may be poorly adapted and all will be poorly adapted because they are identical.

35 Selection favors sex in changing environments. Sexually reproducing females produce variable offspring so if the environment changes some may be well adapted to the new environment.

36 Selection favors sex in changing environments. Red Queen Hypothesis: evolutionary arms race between hosts and parasites. (Red Queen runs to stand still) Parasites and hosts are in a perpetual struggle. Host evolving defenses, parasite evolving ways to evade them. Different multilocus host genotypes are favored each generation. Sex creates the genotypes.

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39 Do parasites favor sex in hosts? Lively (1992) studied New Zealand freshwater snail. Host to parasitic trematodes. Trematodes eat host’s gonads and castrate it! Strong selection pressure. Snail populations contain both obligate sexually and asexually reproducing females.

40 Do parasites favor sex in hosts? Proportion of sexual vs asexual females varies from population to population. Frequency of trematode infections varies also.

41 Do parasites favor sex in hosts? If evolutionary arms race favors sex, then sexually reproducing snails should be commoner in populations with high rates of trematode infections. Results match prediction.

42 White slice indicates frequency of males and thus sexual reproduction

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44 The Fisher-Muller Hypothesis Another advantage of sex is that recombination allows natural selection to operate at a faster rate than in asexual populations. Sex does this by bringing together combinations of beneficial alleles. Sexual reproduction can produce them faster than asexual reproduction can.

45 The Fisher-Muller Hypothesis Consider two populations one that reproduces sexually and the other asexually. Imagine that a beneficial mutation A arises in each population and increases in frequency. Then imagine another beneficial mutation B occurs in each population.

46 The Fisher-Muller Hypothesis In an asexually reproducing population the only way to produce an individual with the AB genotype is for a B mutation to occur in an individual who already possesses the A mutation.

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48 The Fisher-Muller Hypothesis However, an individual with the genotype AB can easily be produced through sexual reproduction between an individual with the A mutation and one who possesses the B mutation.

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50 The Fisher-Muller Hypothesis What sexual reproduction is doing is breaking down linkage disequilibrium and creating new haplotypes


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