Chapter 16 evolution of sex

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Chapter 16 evolution of sex
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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.

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Adaptive significance of sex

• Why not reproduce asexually?
• Many organisms can reproduce both sexually and
  asexually.
• E.g. plants, aphids.

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Adaptive significance of sex

• In populations that can reproduce both asexually
  and sexually will one mode of reproduction
  replace the other?

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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.

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Maynard Smiths 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.)

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Adaptive significance of sex

• Asexually reproducing females under Maynard
  Smiths 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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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

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Adaptive significance of sex

• Sex must confer benefits that overcome the
  mathematical reproductive advantage of asexual
  reproduction.
• One or both of Maynard Smiths assumptions must
  be incorrect.

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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.

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Adaptive significance of sex

• Most likely advantage of sex is that it increases
  offsprings prospects of survival.

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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.

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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.

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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.

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Dunbrack et al. (1995) experiment

• Control results.
• Asexually reproducing strain outcompeted the
  sexually reproducing strain.

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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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Dunbrack et al. (1995) experiment

• Conclusion Assumption 2 of Maynard Smiths null
  model is incorrect.
• Descendants produced by sexual reproduction
  achieve higher fitness than those produced
  asexually.

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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.

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Why is sex beneficial?

• 1. Genetic drift plus mutation make sex
  beneficial. Escapes Mullers ratchet.
• 2. Selection imposed by changing environments
  makes sex beneficial

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Genetic drift plus mutation Mullers ratchet

• An asexually reproducing female will pass a
  deleterious mutation to all her offspring.
• Back mutation only way to eliminate it.
• Mullers ratchet accumulation of deleterious
  alleles in asexually reproducing populations.

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Mullers ratchet

• Small, asexually reproducing population.
• Deleterious mutations occur occasionally.
• Mutations selected against.
• Population contains groups of individuals with
  zero, one, two, etc. mutations.

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Mullers 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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Mullers 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.

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Anderson and Hughes (1996) test of Mullers
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 Mullers ratchet.

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Selection favors sex in changing environments.

• Effects of Mullers 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?

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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.

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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.

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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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Do parasites favor sex in hosts?

• Lively (1992) studied New Zealand freshwater
  snail. Host to parasitic trematodes.
• Trematodes eat hosts gonads and castrate it!
  Strong selection pressure.
• Snail populations contain both obligate sexually
  and asexually reproducing females.

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Do parasites favor sex in hosts?

• Proportion of sexual vs asexual females varies
  from population to population.
• Frequency of trematode infections varies also.

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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.

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White slice indicates frequency of males and
thus sexual reproduction
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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.

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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.

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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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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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The Fisher-Muller Hypothesis

• What sexual reproduction is doing is breaking
  down linkage disequilibrium and creating new
  haplotypes