Sex: causes and consequences
The adaptive significance of sexual reproduction
There are many risks and costs associated with sexual reproduction. For example, searching for and
courting a mate requires time and energy and it also exposes organisms to predators. Sex also exposes
individuals to infection with diseases and parasites. Once a mate is acquired that mate frequently also
requires investment (e.g. by the provision of food, territory and defense). Another potential
disadvantage of sex at the genetic level is that it can break up favorable combinations of genes.
Given all these costs of sex why do organisms not reproduce asexually?
We know that it’s possible for organisms to reproduce asexually and in fact many organisms (e.g. plants
and aphids) can reproduce both sexually and asexually.
In populations that can reproduce both asexually and sexually an interesting question is whether one
mode of reproduction will replace the other?
John Maynard Smith, a famous British evolutionary biologist, explored this question.
He considered a population in which some organisms reproduced asexually and the others sexually. He
made 2 assumptions.
1. Mode of reproduction (sexual or asexual) does not affect number of offspring she can produce.
2. Mode of reproduction (sexual or asexual) does not affect the probability that the offspring produced
will survive.
It’s important to realize that asexually reproducing organisms produce only females. Thus every
individual can lay eggs or give birth to babies. In contrast, in sexually species there are both males and
females and, of course, only females lay eggs or give birth.
Under Maynard Smith’s assumptions sexually reproducing females leave twice as many grandchildren as
sexually reproducing females. This is because each generation of sexually reproducing organisms
contains only 50% females (the rest of course are males).
Ultimately, if the rate of baby production is all that matters, asexual reproduction should take over.
However, in nature, clearly this is not the case. What we see in nature is that most organisms reproduce
sexually and both sexual and asexual modes of reproduction are used in many organisms
Therefore, sexual reproduction must confer benefits that overcome the mathematical reproductive
advantage of asexual reproduction. Consequently, one or both of Maynard Smith’s assumptions must be
incorrect.
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, this is not
likely to be a general explanation because in most species the male does not help to rear the offspring.
Thus, the most likely advantage of sex is that it increases offspring’s prospects of survival.
Sex in populations means genetic recombination
Sexual reproduction involves:
Meiosis with crossing over
Mating between with random individuals
Random meeting of sperm and eggs
The result of all these processes events is genetic recombination. New combinations of genes are
brought together each generation.
Why is sex beneficial?
There are two major benefits of sexual reproduction.
1. Sexual reproduction allows populations to escape the effects of Muller’s ratchet.
2. Selection imposed by changing environments makes sex beneficial
Muller’s ratchet
An asexually reproducing female passes all of her genes on to her offspring. Thus, all of her deleterious
alleles are passed to all her offspring, which are simply clones of her. Deleterious mutations occur at
random. Because the only way to fix a deleterious mutation is a “back mutation” that exactly reverses
the previous mutation, deleterious mutations will tend to accumulate over time In a small population
the effects of chance can help to reinforce the accumulation. This effect is called Muller’s ratchet.
In a small population if all but one individual has at least one mutation and by chance the individual with
one mutation either does not reproduce or has a mutation occur then the class of zero mutants will
disappear and the ratchet will click forward (see picture below).
The burden of increased number of deleterious mutations (genetic load) may eventually cause the
population to go extinct.
Sexual reproduction breaks Muller’s ratchet. Because it allows recombination to occur. E.g. two
individuals who each have 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.
They propagated multiple generations of a species of bacterium, but each generation was derived from
one individual (genetic drift). In all there were 444 cultures. At the end of the experiment (2 months)
1% of cultures had reduced fitness (lower than wild-type bacteria), none had increased fitness. These
results are consistent with Muller’s ratchet.
Selection favors sex in changing environments.
The effects of Muller’s ratchet are slow and take many generations to affect asexually reproducing
populations. However, the advantage of sex is apparent in only a few generations. This leads to the
question of what short-term benefit does sex provide?
In constant environments asexual reproduction is a good strategy (because if the mother is adapted to
environment, offspring will be too). However, if the environment changes, offspring may be poorly
adapted and all will be poorly adapted because they are identical.
Sexually reproducing females produce variable offspring so if the environment changes some may be
well adapted to the new environment.
Red Queen Hypothesis: suggests that an evolutionary arms race between hosts and parasites is what
drives sexual reproduction. Parasites are the environmental factor that changes from generation to
generation. (The Red Queen in the book “Through the Looking Glass” runs to stand still)
Parasites and hosts are in a perpetual struggle. Host evolve defenses, parasites evolve 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! This is clearly strong selection pressure.
Snail populations contain both obligate sexually and asexually reproducing females. Proportion of sexual
vs asexual females varies from population to population. Frequency of trematode infections varies also.
If evolutionary arms race favors sex, then sexually reproducing snails should be commoner in
populations with high rates of trematode infections.
Results match prediction.
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.
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.
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.
Figure 16.19 (Part 1) The Fisher–Muller hypothesis. A beneficial mutation, A, arises, and increases in
both large sexual and large asexual populations. If a second beneficial mutation, B, arises at a different
locus, AB individuals emerge more quickly in a sexual population than in an asexual population. The
same holds true for a third beneficial mutation C. This occurs because, in an asexual population, AB and
ABC individuals can only arise if the B mutation occurs in an individual with A, and the C mutation then
occurs in AB individuals (or if the C mutation occurs in an individual with A, and the B mutation then
occurs in AC individuals). But, in sexual populations, recombination brings together beneficial mutations
from separate lineages. Adapted from Crow and Kimura (1965) and Maynard Smith (1988), after Muller
(1932).
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.
Figure 16.19 (Part 2) The Fisher–Muller hypothesis. A beneficial mutation, A, arises, and increases in
both large sexual and large asexual populations. If a second beneficial mutation, B, arises at a different
locus, AB individuals emerge more quickly in a sexual population than in an asexual population. The
same holds true for a third beneficial mutation C. This occurs because, in an asexual population, AB and
ABC individuals can only arise if the B mutation occurs in an individual with A, and the C mutation then
occurs in AB individuals (or if the C mutation occurs in an individual with A, and the B mutation then
occurs in AC individuals). But, in sexual populations, recombination brings together beneficial mutations
from separate lineages. Adapted from Crow and Kimura (1965) and Maynard Smith (1988), after Muller
(1932).
What sexual reproduction is doing is breaking down linkage disequilibrium and creating new haplotypes