CHAPTER 11: SEX AND EVOLUTION

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CHAPTER 11 SEX AND EVOLUTION

• Robert E. Ricklefs
• The Economy of Nature, Fifth Edition

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Stalk-eyed flies
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Background

• Among the most fascinating attributes of
  organisms are those related to sexual function,
  such as
• gender differences
• sex ratios
• physical characteristics and behaviors that
  ensure the success of an individuals gametes

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Sexual reproduction mixes genetic material of
individuals.

• In most plants and animals reproduction is
  accomplished by production of male and female
  haploid gametes (sperm and eggs)
• gametes are formed in the gonads by meiosis
• Gametes join in the act of fertilization to
  produce a diploid zygote, which develops into a
  new individual.

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Asexual Reproduction

• Progeny produced by asexual reproduction are
  usually identical to one another and to their
  single parent
• asexual reproduction is common in plants
  (individuals so produced are clones)
• many simple animals (hydras, corals, etc.) can
  produce asexual buds, which
• may remain attached to form a colony
• may separate to form new individuals

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Other Variants on Reproduction

• Asexual reproduction
• production of diploid eggs (genetically
  identical) without meiosis (common in fishes,
  lizards and some insects)
• production of diploid eggs (genetically
  different) by meiosis, with suppression of second
  meiotic division
• self-fertilization through fusion of female
  gametes
• Sexual reproduction
• self-fertilization through fusion of male and
  female gametes (common in plants)

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Sexual reproduction is costly.

• Asexual reproduction is
• common in plants
• found in all groups of animals, except birds and
  mammals
• Sexual reproduction is costly
• gonads are expensive organs to produce and
  maintain
• mating is risky and costly, often involving
  elaborate structures and behaviors
• So why does sexual reproduction exist at all?

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Cost of Meiosis 1

• Sex has a hidden cost for organisms in which
  sexes are separate
• only half of the genetic material in each
  offspring comes from each parent
• each sexually reproduced offspring contributes
  only 50 as much to the fitness of either parent,
  compared to asexually produced offspring
• this 50 fitness reduction is called the cost of
  meiosis
• for females, asexually produced offspring carry
  twice as many copies of her genes as sexually
  produced offspring
• thus, mating is undesirable

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Cost of Meiosis 2

• The cost of meiosis does not apply
• when individuals have both male and female
  function (are hermaphroditic)
• when males contribute (through parental care) as
  much as females to the number of offspring
  produced
• if male parental investment doubles the number of
  offspring a female can produce, this offsets the
  cost of meiosis

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Advantages of Sex

• One advantage to sexual reproduction is the
  production of genetically varied offspring
• this may be advantageous when environments also
  vary in time and space
• Is this advantage sufficient to offset the cost
  of meiosis?

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Whos asexual?

• If asexual reproduction is advantageous, then it
  should be common and widely distributed among
  many lineages
• most asexual species (e.g., some fish, such as
  Poeciliopsis) belong to genera that are sexual
• asexual species do not have a long evolutionary
  history
• suggests that long-term evolutionary potential of
  asexual reproduction is low
• because of reduced genetic variability, asexual
  lines simply die out over time

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Why have sex?

• By the late 1980s, in the contest to explain sex,
  only two hypotheses remained in contention.
• One the deleterious mutation hypothesis
• sex exists to purge a species of damaging genetic
  mutations Alexey Kondrashov (at the National
  Center for Biotechnology Information) argues that
  in an asexual population, every time a creature
  dies because of a mutation, that mutation dies
  with it. In a sexual population, some of the
  creatures born have lots of mutations and some
  have few. If the ones with lots of mutations die,
  then sex purges the species of mutations. Since
  most mutations are harmful, this gives sex a
  great advantage.
• But But why eliminate mutations in this way,
  rather than correcting more of them by better
  proofreading?
• Kondrashov It may be cheaper to allow some
  mistakes through and remove them later. The cost
  of perfecting proofreading mechanisms escalates
  as you near perfection.

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But

• According to Kondrashov's calculations, the rate
  of deleterious mutations must exceed one per
  individual per generation if sex is to earn its
  keep eliminating them if less than one, then his
  idea is in trouble.
• The evidence so far is that the deleterious
  mutation rate teeters on the edge it is about
  one per individual per generation in most
  creatures.
• But even if the rate is high enough, all that
  proves is that sex can perhaps play a role in
  purging mutations. It does not explain why sex
  persists.
• The main defect in Kondrashov's hypothesis is
  that it works too slowly. Pitted against a clone
  of asexual individuals, a sexual population must
  inevitably be driven extinct by the clone's
  greater productivity, unless the clone's genetic
  drawbacks can appear in time. Currently, a great
  deal of effort is going into the testing of this
  model by measuring the deleterious mutation rate,
  in a range of organisms from yeast to mouse. But
  the answer is still not entirely clear.

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So why have sex?
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Sex and Pathogens

• The evolution of virulence by parasites that
  cause disease (pathogens) is rapid
• populations of pathogens are large
• their generation times are short
• The possibility exists that rapid evolution of
  virulence by pathogens could drive a host species
  to extinction.

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The Red Queen Hypothesis

• Genetic variation represents an opportunity for
  hosts to produce offspring to which pathogens are
  not adapted.
• Sex and genetic recombination provide a moving
  target for the evolution by pathogens of
  virulence.
• Hosts continually change to stay one step ahead
  of their pathogens, likened to the Red Queen of
  Lewis Carrolls Through the Looking Glass and
  What Alice Found There.
• it takes all the running you can do, to keep in
  the same place.

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Sex vs Asex

• One of the main proponents of the Red Queen
  hypothesis was the late W. D. Hamilton.
• In the late 1970s, with the help of two
  colleagues from the University of Michigan,
  Hamilton built a computer model of sex and
  disease, a slice of artificial life. It began
  with an imaginary population of 200 creatures,
  some sexual and some asexual. Death was random.
  Who won?
• As expected, the sexual race quickly died out. In
  a game between sex and "asex," asex always wins
  -- other things being equal. That's because
  asexual reproduction is easier, and it's
  guaranteed to pass genes on to one's offspring.

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Now add parasites

• Next they introduced 200 species of parasites,
  whose power depended on "virulence genes" matched
  by "resistance genes" in the hosts.
• The least resistant hosts and the least virulent
  parasites were killed in each generation.
• Now the asexual population no longer had an
  automatic advantage -- sex often won the game. It
  won most often if there were lots of genes that
  determined resistance and virulence in each
  creature.
• In the model, as resistance genes that worked
  would become more common, then so too would the
  virulence genes. Then those resistance genes
  would grow rare again, followed by the virulence
  genes. As Hamilton put it, "antiparasite
  adaptations are in constant obsolescence." But in
  contrast to asexual species, the sexual species
  retain unfavored genes for future use. "The
  essence of sex in our theory," wrote Hamilton,
  "is that it stores genes that are currently bad
  but have promise for reuse. It continually tries
  them in combination, waiting for the time when
  the focus of disadvantage has moved elsewhere."

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Real-world evidence

• asexuality is more common in species that are
  little troubled by disease boom-and-bust
  microscopic creatures, arctic or high-altitude
  plants and insects.
• The best test of the Red Queen hypothesis,
  though, was a study of a little fish in Mexico
  called the topminnow. The topminnow, which
  sometimes crossbreeds with another similar fish
  to produce an asexual hybrid, is under constant
  attack by a worm that causes "black-spot
  disease." The asexually reproducing topminnows
  harbored many more black-spot worms than did
  those producing sexually.
• That fit the Red Queen hypothesis The sexual
  topminnows could devise new defenses faster by
  recombination than the asexually producing ones.

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More on sex and evolution

• a 2005 study shows that sex leads to faster
  evolution.
• To demonstrate this, a team of scientists created
  a mutant strain of yeast that, unlike normal
  yeast, was unable to divide into the sexual
  spores that allow yeast to engage in sexual
  reproduction. Yeast can reproduce either sexually
  or asexually.
• When testing this mutant strain in stress-free
  conditions, the scientists found that it
  performed as well as normal yeast. In more
  extreme conditions, however, the normal yeast
  grew faster than the asexual mutants.
• This shows "unequivocally that sex allows for
  more rapid evolution," said Matthew Goddard of
  the School of Biological Sciences at the
  University of Auckland in New Zealand.

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Perhaps

• It could well be that the deleterious mutation
  hypothesis and the Red Queen hypothesis are both
  true, and that sex serves both functions.
• Or that the deleterious mutation hypothesis may
  be true for long-lived things like mammals and
  trees, but not for short-lived things like
  insects, in which case there might well be need
  for both models to explain the whole pattern.
• Perpetually transient, life is a treadmill, not a
  ladder.

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Individuals may have female function, male
function, or both.

• The common model of two sexes, male and female,
  in separate individuals, has many exceptions
• hermaphrodites have both sexual functions in the
  same individual
• these functions may be simultaneous (plants, many
  snails and most worms) or
• sequential (mollusks, echinoderms, plants, fishes)

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Sexual Functions in Plants

• Plants with separate sexual functions in separate
  individuals are dioecious
• this condition is relatively uncommon in plants
• Most plants have both sexual functions in the
  same individual (hermaphroditism)
• monoecious plants have separate male and female
  flowers
• plants with both sexual functions in the same
  flower are perfect (72 of plant species)
• most populations of hermaphrodites are fully
  outcrossing (fertilization takes place between
  gametes of different individuals)
• Many other possibilities exist in the plant world!

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Separate Sexes versus Hermaphroditism

• When does adding a second sexual function
  (becoming hermaphroditic) make sense?
• gains from adding a second sexual function must
  not bring about even greater losses in the
  original sexual function
• this seems to be the case in plants, where basic
  floral structures are in place
• for many animals, adding a second sexual function
  entails a net loss in overall sexual function

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Sex ratio of offspring is modified by evolution.

• When sexes are separate, sex ratio may be defined
  for progeny of an individual or for the
  population as a whole.
• Sex ratio number of males relative to the number
  of females
• Humans have 11 malefemale sex ratios, but there
  are many deviations from this in the natural
  world.
• Despite deviations, 11 sex ratios are common.
  Why?
• Every product of sexual reproduction has one
  father and one mother
• if the sex ratio is not 11, individuals
  belonging to the rarer sex will experience
  greater reproductive success
• such individuals compete for matings with fewer
  individuals of the same sex
• such individuals, on average, have greater
  fitness (contribute to more offspring) than
  individuals of the other sex

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11 Sex Ratios An Explanation

• Consider a population with an unequal sex
  ratio...
• individuals of the rare sex have greater fitness
• mutations that result in production of more
  offspring of the rare sex will increase in the
  population
• when sex ratio approaches 11, selective
  advantage of producing more offspring of one sex
  or another disappears, stabilizing the sex ratio
  at 11
• this process is under the control of
  frequency-dependent selection

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Why do sex ratios deviate from 11?

• One scenario involves inbreeding
• inbreeding may occur when individuals do not
  disperse far from their place of birth
• a high proportion of sib matings leads to local
  mate competition among males

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Sex ratio and pollution

• Recent study Lower oxygen levels in polluted
  waters could lead to a higher ratio of male fish
  that may threaten certain species with
  extinction
• hypoxia (O2 depletion) can affect sex
  development, sex differentiation and the sex
  ratio in fish species. hypoxia can inhibit the
  activities of certain genes that control the
  production of sex hormones and sexual
  differentiation in embryonic zebra fish.
• In his study, Wu found that 61 of zebra fish -
  a universal freshwater fish widely used in
  scientific and pollution research - spawned into
  males under regular oxygen conditions. Under
  hypoxia conditions, the ratio of males increased
  to 75 .
• Hypoxia can be a naturally occurring phenomenon,
  particularly in areas where salt and fresh waters
  meet in estuaries such as the Pearl River Delta.
  It can also be caused by pollution.

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Human sex ratio and pollution PCBs

• PCBs were banned in the 1970s, they are linked
  to problems with the brain, nervous and hormone
  systems, and although average levels in the human
  body have dropped, human exposure continues. Why?
  PCBs are persistent contaminants, which means
  they build up in the environment and in us.
• Evidence continues to build that PCBs also affect
  birth sex. A recent study of blood serum from
  women who were pregnant in San Francisco in the
  '60s found that those with higher PCB levels were
  more likely to give birth to boys than those with
  low PBC levels.

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Is it PCBs?

• Dr. Pete Myers brings up an important point in
  his summary of the report The exposure levels
  observed in the study are high compared to today.
  Thus if these results are indicative of a causal
  relationship (never possible to confirm with
  epidemiological studies) then the simplest
  prediction would be that the chances of having a
  boy baby should be increasing because PCBs have
  been decreasing. That is not the case, at least
  as of the most recent analysis from Canada and
  the US.
• Evidence from a large-scale study of four
  industrialized nations indicates that the sex
  ratio is skewed, and fewer boys are being born
  But PCB levels have dropped

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So? What do we know?

• in-utero exposure to pollutants can affect a
  child's sex.
• There are more than 80,000 chemicals in
  production today, many of which are known to be
  persistent or to disrupt hormone systems, and
  most of which haven't really tested for their
  impact on human health.
• A 2007 study from the University of Pittsburgh
  found that during the past thirty years, the
  number of male births has steadily decreased in
  the U.S. and Japan. The study found a decline of
  17 males per 10,000 births in the U.S. and a
  decline of 37 males per 10,000 births in Japan.

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Human sex ratio and pollution

• The steepest sex ratio declines observed in the
  world have occurred on the 3,000-acre Aamjiwnaang
  (pronounced AH-jih-nahng) First Nation
  reservation in Canada.
• The ratio of boys to girls there began dropping
  in the early 1990s. Between 1999 and 2003,
  researchers found, only 46 boys were born out of
  132 recorded births. (35)
• Dozens of petrochemical, polymer and chemical
  plants border the reservation on three sides.
  Mercury and PCBs contaminate the creek that runs
  through the land, and air-quality studies show
  the highest toxic releases in Canada, said Jim
  Brophy, executive director of Occupational Health
  Clinics for Ontario Workers, based in Sarnia, the
  nearest city.
• Boys made up only 42 of the 171 babies born
  from 2001 to 2005 to Aamjiwnaang living on the
  reserve or nearby.

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Mating Systems Rules for Pairing

• There is a basic asymmetry in sexually
  reproducing organisms
• a females reproductive success depends on her
  ability to make eggs
• large female gametes require considerable
  resources
• the females ability to gather resources
  determines her fecundity
• a males reproductive success depends on the
  number of eggs he can fertilize
• small male gametes require few resources
• the males ability to mate with many females
  determines his fecundity

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Promiscuity is a mating system for which the
following are true

• males mate with as many females as they can
  locate and induce to mate
• males provide their offspring with no more than a
  set of genes
• no lasting pair bond is formed
• it is by far the most common mating system in
  animals

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Promiscuity 2

• it is universal among outcrossing plants
• there is a high degree of variation in mating
  success among males as compared to females
• especially true where mating success depends on
  body size and quality of courtship displays
• less true when sperm and eggs are shed into water
  or pollen into wind currents

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Polygamy

• Polygamy occurs when a single individual of one
  sex forms long-term bonds with more than one
  individual of opposite sex
• a common situation involves one male that mates
  with multiple females, called polygyny (eg
  elephant seals)
• polygyny may arise when one male controls mating
  access to many females in a harem
• polygyny may also arise when one male controls
  resources (territory) to which multiple females
  are attracted

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Monogamy

• Monogamy involves the formation of a lasting pair
  bond between one male and one female
• the pair bond persists through period required to
  rear offspring
• the pair bond may last until one of the pair dies
• monogamy is favored when males can contribute
  substantially to care of young
• monogamy is uncommon in mammals (why?),
  relatively common among birds (but recent studies
  provide evidence for extra-pair copulations in as
  many as a 1/3 of the broods leading to
  mate-guarding)

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The Polygyny Threshold

• When should polygyny replace monogamy?
• For territorial animals
• a female increases her fecundity by choosing a
  territory with abundant resources
• polygyny arises when a female has greater
  reproductive success on a males territory shared
  with other females than on a territory in which
  she is the sole female
• the polygyny threshold occurs when females are
  equally successful in monogamous and polygynous
  territories
• polygyny should only arise when the quality of
  male territories varies considerably

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Sexual Selection

• In promiscuous and polygynous mating systems,
  females choose among potential mates
• if differences among males that influence female
  choice are under genetic control, the stage is
  set for sexual selection
• there is strong competition among males for mates
• result is evolution of male attributes evolved
  for use in combat with other males or in
  attracting females

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Consequences of Sexual Selection

• The typical result is sexual dimorphism, a
  difference in the outward appearances of males
  and females of the same species.
• Charles Darwin first proposed in 1871 that sexual
  dimorphism could be explained by sexual selection
• Traits which distinguish sex above primary sexual
  organs are called secondary sexual
  characteristics.

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Pathways to Sexual Dimorphism

• Sexual dimorphism may arise from
• (1) life history considerations and ecological
  relationships
• females of certain species (e.g., spiders) are
  larger than males because the number of offspring
  produced varies with size
• (2) combats among males
• weapons of combat (horns or antlers) and larger
  size may confer advantages to males in
  competition for mates
• (3) direct effects of female choice
• elaborate male plumage and/or courtship displays
  may result

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Female Choice

• Evolution of secondary sexual characteristics in
  males may be under selection by female choice
• in the sparrow-sized male widowbird, the tail is
  a half-meter long males with artificially
  elongated tails experienced more breeding success
  than males with normal or shortened tails

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Runaway Sexual Selection

• When a secondary sexual trait confers greater
  fitness, the stage is set for runaway sexual
  selection
• regardless of the original reason for female
  preference, female choice exaggerates fitness
  differences among males
• leads to evolution of spectacular plumage (e.g.,
  peacock) and other seemingly outlandish plumage
  and/or displays

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The Handicap Principle

• Can elaborate male secondary sexual
  characteristics actually signal male quality to
  females?
• Zahavis handicap principle suggests that
  secondary characteristics act as handicaps --
  only superior males could survive with such
  burdens
• Hamilton and Zuk have also proposed that showy
  plumage (in good condition) signals genetic
  factors conferring resistance to parasites or
  diseases

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