Chapter 16 evolution of sex

Adaptive significance of sexMany 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 sexWhy not reproduce asexually?Many organisms can reproduce both sexually

and asexually.E.g. plants, aphids.

Adaptive significance of sexIn populations that can reproduce both

asexually and sexually will one mode of reproduction replace the other?

Adaptive significance of sexJohn Maynard Smith explored the question.Considered population in which some

organisms reproduce asexually and the others sexually.

Made 2 assumptions.

Maynard Smith’s assumptions1. 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 sexAsexually 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 sexUltimately, 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 sexSex 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 sexAssumption 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 sexMost likely advantage of sex is that it

increases offspring’s prospects of survival.

Dunbrack et al. (1995) experimentLab populations of flour beetlesMixed populations of red and black strains.Strains designated as “sexual” or “asexual” in

experimental replicates.

Dunbrack et al. (1995) experimentAsexual 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) experimentTwo 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) experimentControl results.“Asexually” reproducing strain outcompeted

the sexually reproducing strain.

Dunbrack et al. (1995) experimentExperimental 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) experimentConclusion: 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 recombinationSex involves:

Meiosis with crossing overMatings with random individualsRandom meeting of sperm and eggsConsequence 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 ratchetAn 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 ratchetSmall, asexually reproducing population.Deleterious mutations occur occasionally.Mutations selected against.Population contains groups of individuals

with zero, one, two, etc. mutations.

Muller’s ratchetFew 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 ratchetBurden 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 HypothesisAnother 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 HypothesisConsider 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 HypothesisIn 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 HypothesisHowever, 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 HypothesisWhat sexual reproduction is doing is

breaking down linkage disequilibrium and creating new haplotypes