Chapter 11 Sex: causes and
consequences
 Assigned Reading: Chapter 11
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.
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 wildtype 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