7. The Adaptive Significance of of Sex

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7. The Adaptive Significance of of Sex

• Why is sexual reproduction so common in
  multicellular organisms?

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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)

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Maynard Smiths 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 females reproductive mode does not affect the
  number of offspring that she can make
• A females reproductive mode does not affect the
  probability that her offspring will survive

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Asexual reproduction has a 2-fold demographic
advantage compared to sexual reproduction (Fig.
7.17)
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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 Smiths 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

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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

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Competition between sexual and asexual flour
beetles in the presence of insecticide 1 (Fig.
7.18)
Generation
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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).

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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
Generation
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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

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Artificial selection on other traits often
results in increased recombination (Fig. 7.19)
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R.A. FisherSex 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)

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Objections to Fishers model

• Fishers 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.
• Fishers 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

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Mullers 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).

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Mullers ratchet (Fig. 7.20)
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Mullers 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

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Objections to Mullers ratchet

• Its 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 Mullers ratchet, it
  works best when population size is small (lt
  1,000) and drift is important. It does not appear
  to be a general explanation for the prevalence of
  sexual reproduction.

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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
  time-scale 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

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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

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Host parasite coevolution and sexthe 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.

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The Red Queens 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!"

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A host-parasite arms race can make sex beneficial
- 1 (Fig. 7.22)
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A host-parasite arms race can make sex beneficial
- 2 (Fig. 7.22)
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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)

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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