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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 requires time and energy and exposes organisms to predators Sex exposes individuals to infection with diseases and and parasites.
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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.
Adaptive significance of sex • Why not reproduce asexually? • Many organisms can reproduce both sexually and asexually. • E.g. plants, aphids.
Adaptive significance of sex • In populations that can reproduce both asexually and sexually will one mode of reproduction replace the other?
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.
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.)
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.
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
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.
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.
Adaptive significance of sex • Most likely advantage of sex is that it increases offspring’s prospects of survival.
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.
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.
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.
Dunbrack et al. (1995) experiment • Control results. • “Asexually” reproducing strain outcompeted the sexually reproducing strain.
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.
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.
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.
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
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.
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.
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.
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.
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.
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?
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.
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.
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.
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.
Do parasites favor sex in hosts? • Proportion of sexual vs asexual females varies from population to population. • Frequency of trematode infections varies also.
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.
White slice indicates frequency of males and thus sexual reproduction
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.
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.
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.
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.
The Fisher-Muller Hypothesis • What sexual reproduction is doing is breaking down linkage disequilibrium and creating new haplotypes
