Download 7. The Evolution of Sex

7. The Adaptive Significance of
of Sex
Why is sexual reproduction so
common in multicellular organisms?
Sex is costly and dangerous
• Energetic costs: mate finding, courtship, male male competition
• Increased predation risk
• Disease: STDs
• Genetic cost: sexual reproduction means that a
parent passes on only 1/2 of its genes to offspring
• Demographic cost: all other things being equal, an
asexual clone will replace sexual individuals in a
“mixed” population, because asexual females will
produce twice as many daughters as sexual
females (John Maynard Smith)
Maynard Smith’s model for the demographic
advantage of parthenogenetic reproduction
Parthenogenesis is reproduction via diploid eggs without
fertilization (aphids, Daphnia, rotifers, some lizards, etc.).
Parthenogenetic populations are all-female (or produce
males only when switching to sexual reproduction)
• Assumptions of the model:
– A female’s reproductive mode does not affect
the number of offspring that she can make
– A female’s reproductive mode does not affect
the probability that her offspring will survive
Asexual reproduction has a 2-fold demographic advantage
compared to sexual reproduction (Fig. 7.17)
The prevalence of sexual reproduction is
a paradox
• Despite the apparent advantages of asexual
reproduction, the vast majority of multicellular
species reproduce sexually (many exclusively)
• This suggests that sexual reproduction must in
general confer higher fitness than asexual
reproduction; that is, one or both of the
assumptions of Maynard Smith’s model are
incorrect
– Assumption #1 - equal numbers of offspring - might be
violated if males provide parental care
– Assumption #2 would be violated if offspring of sexual
females have higher survivorship than offspring of
asexual females
Experiments with the flour beetle, Tribolium
• Red beetles and black beetles compete in the presence of
insecticide (malathion)
• One color of beetle, say red, is designated as the sexual
strain. It must survive in the presence of insecticide by
evolving resistance without “outside” help from the
experimenters
• The other color of beetle, say black, is designated as the
“asexual” strain. Every generation, all the black adults are
removed and replaced with three times as many black
adults from a culture that is not exposed to insecticide.
The black beetles have a strong demographic advantage
but cannot evolve resistance to the insecticide
Competition
between
sexual and
“asexual”
flour beetles
in the
presence of
insecticide – 1
(Fig. 7.18)
0
10
20
Generation
30
Does sexual reproduction allow populations to
adapt more quickly to changing
environments?
• These experiments suggest that the advantage of
sexual reproduction is that it increases the chance
that a population can adapt to a changing
environment. The 3-fold demographic advantage
of the black beetles was not enough to keep them
from going extinct when faced with an
evolutionary challenge (competition with red
beetles and insecticide).
• This argument is supported by the fact that the red
beetles “won” more quickly at higher
concentrations of insecticide (= stronger
selection).
Competition
between
sexual and
“asexual”
flour beetles
in the
presence of
insecticide – 2
(Fig. 7.18)
The outcome of the
experiment does
not depend upon
which color of
beetle is “asexual”
0
10
20
Generation
30
Why does sexual reproduction enhance
evolutionary adaptation?
• Sex = genetic recombination
– Crossing-over during meiosis
– Mixing of genes from 2 parents
• Sex “reshuffles” genes to create new
multilocus genotypes in every generation
Artificial selection on other traits often
results in increased recombination (Fig. 7.19)
R.A. Fisher:
Sex increases the rate of evolution – 1
• Suppose 2 favorable mutations, A´ and B´, occur
in a population – most likely they will occur in
separate individuals
• In a sexual population, these two favorable
mutations can be combined in the same individual
by mating between carriers of A´ and B´
• In an asexual population, the only way that both
mutations can be in the same individual is if the B´
mutation occurs in an individual that is already A´
(or vice versa)
Objections to Fisher’s model
• Fisher’s argument requires a relatively high rate of
favorable mutations. Suppose A´ occurs first. Selection
will tend to fix it in the population (either sexual or
asexual). If B´ occurs after A´ is fixed in the population,
then it will necessarily occur in an individual that is
already A´, in which case sex has no advantage. For sex to
have an advantage, B´ must occur before A´ rises to high
frequency.
• Fisher’s model is generally considered to be a group
selection argument: sex is good for the “group”; sex is
common because species that reproduce sexually are less
likely to go extinct – most evolutionary biologists prefer
arguments that posit an advantage to individuals
Muller’s Ratchet:
Deleterious mutations will accumulate in asexual lineages
• Most individuals (clones) will carry one or more harmful
mutations
• A small number of individuals might have zero harmful
mutations. They might have a slight fitness advantage,
compared to individuals with 1 or 2 mutations. But they are
also likely to be few in number and may be lost from a
population by drift.
• If the zero-mutation class is lost from a population then the
most fit class will be those individuals with 1 harmful
mutation (the ratchet has clicked once).
• If those individuals with 1 mutation are lost from the
population, then the most fit class will be those with 2
mutations (the ratchet has clicked again).
Muller’s
ratchet
(Fig. 7.20)
Muller’s Ratchet:
Sexual recombination can produce individuals with fewer
deleterious mutations
• Suppose a sexual male and female both carry a harmful
mutation, C´. If they are both heterozygous, then we expect
1/4 of their offspring to not have C´
Objections to Muller’s ratchet
• It’s “groupy”: groups (species or populations) that
reproduce asexually accumulate genetic load and
are more likely to go extinct than groups that
reproduce sexually
• Although there is both theoretical and
experimental support for Muller’s ratchet, it works
best when population size is small (< 1,000) and
drift is important. It does not appear to be a
general explanation for the prevalence of sexual
reproduction.
Sex is good in a changing environment
• The Tribolium experiments suggest that sex may
increase individual fitness when selection is
strong, or when environments change on a timescale similar to the generation time of a species.
• If the environment experienced by offspring is
different from that experienced by parents, then it
may pay to reshuffle genes to produce genetically
variable offspring, at least one of which may have
a genotype that is well-suited to the new
environment
Sex is like buying lottery tickets with different
numbers
• The environment in the next generation is like a
lottery
• 10 tickets, each with a different number, will give
you a better chance of winning (= variable
offspring produced by sex)
• 10 tickets, all with the same number (= identical
offspring produced asexually), is a bad strategy
Host – parasite coevolution and sex:
the Red Queen Hypothesis
• One important component of the environment for many
species is parasites
• Hosts and parasites are involved in a coevolutionary “arms
race” in which the host evolves defenses against the
parasite, and the parasite, in turn, evolves to overcome host
defenses — both sides must constantly evolve just to
maintain the status quo
• Evolution by the parasite represents a changing
environment for the host, and sexual reproduction allows
the host to produce offspring that are more likely to be
resistant to prevalent parasite genotypes.
The Red Queen’s race in Alice in Wonderland
• The Red Queen's race is an incident that appears in
Lewis Carroll's Through the Looking-Glass and
involves the Red Queen and Alice constantly running
but remaining in the same spot.
• "Well, in our country," said Alice, still panting a little,
"you'd generally get to somewhere else — if you run
very fast for a long time, as we've been doing.”
• "A slow sort of country!" said the Queen. "Now, here,
you see, it takes all the running you can do, to keep
in the same place. If you want to get somewhere
else, you must run at least twice as fast as that!"
A host-parasite arms race can make sex
beneficial - 1 (Fig. 7.22)
A host-parasite arms race can make sex
beneficial - 2 (Fig. 7.22)
Sex and parasitism in a freshwater snail
(Potamopyrgus antipodarum) (Lively 1992)
• Parasitized by several species of trematodes
(flukes) that eat the gonads
• Snail populations consist of:
– males
– obligately sexual females (which produce male and
female offspring)
– obligately asexual females
• Populations with higher incidence of parasitism
had higher proportion of males (= higher
proportion of sexual females)
Frequency of sexual individuals in snail populations
with differing degrees of parasitism (Fig. 7.23)
a. White “slice” indicates the frequency of males
b. Fequency of males versus proportion of snails with trematode parasites