Sex and Evolution

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Sex and Evolution (Chapter 11) Youve seen, in
the last lecture, how sexual selection can lead
to sexual dimorphism. The example that begins
the text chapter identifies an example where
genetics plays an important role in the
dimorphism, as well as being a major factor in
the sex ratio and mating system of a fly species,
Cyrtodiopis whitei. This species has stalked
eyes males have varying stalk lengths. The
species also has a biased sex ratio. Only around
1/3 of a population are males. With fewer males
to service the females, any female who could
produce more male children would gain fitness.
How does a female select a male who will father
more sons?
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It turns out that male eye stalk length is an
indicator. Males with shorter eye stalks produce
mostly X bearing sperm, and father daughters,
due to defective sperm production. Males with
longer eye stalks produce normal sperm ratios,
i.e. 50 Y bearing and 50 X bearing. They,
therefore, father more sons. In sexual selection
within this species, females choose males with
longer eye stalks. All this (sexual dimorphism,
sexual selection, etc.) assumes that reproduction
is occurring sexually.
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• However, asexual reproduction occurs widely in
  nature.
• There are many ways to achieve asexual
  reproduction, and they have differing genetic
  consequences.
• It may occur by budding (as in corals) or by
  vegetative
• reproduction (e.g. in plants, by runners
  (strawberries) or
• horizontal rhizomes (in many goldenrods).
  Here every
• offspring is genetically identical to its
  parent. The offspring form a clone.
• 2) It may occur with partial meiosis. The first
  meiotic division, with crossing over and
  recombination, produces genetic variation. If
  there is no second division, the resulting cells
  are diploid and can develop into mature adults.

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3) There can be full meiosis, with fusion of a
pair of gametes to restore the diploid
number with genetic variation from both
meiosis and random gametes fusing. This is one
form of self-fertilization. This is
asexuality in which only sex (the mating of
different individuals) is lacking. Now lets go
back and think about sex. Fitness, in the
evolutionary sense, is measured by the number of
copies of an individuals genes, relative to
others parents, in the offspring generation.
What happens in sexual reproduction? Meiosis and
fertilization mean that each offspring carries ½
the genes of each parent.
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In asexual reproduction (whichever form) each
offspring carries the complete genome of its
parent. That is twice as much of a contribution
to parental fitness. That difference in genetic
contribution between sexual and asexual
reproduction is called the cost of meiosis. There
have been numerous attempts to study species that
can reproduce both asexually and sexually. The
object has been to show that females reproducing
sexually have twice as many surviving offspring
(or more), and (whatever advantages sex offers)
they can make up for the cost of meiosis. It has
been a failure. Sexual reproduction produces more
offspring, but not twice as many. So, why does
sex exist and persist?
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There is a single answer variation among
offspring.
This figure looks across generations (over
time). Recombination and crossing over generate
variation among offspring of a single set of
parents. Both within and across generation
variation are important.
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If the environment did not differ in time or
space, then the advantage of asexuality would
predominate, and sex would be rare among
species. The asexual parent was successful
(grew, survived, reproduced). The adaptations
that that the parent had would be equally
advantageous for the offspring if the
environment remained the same. However, the
environment varies over both time and space. Can
a parent predict the environmental conditions its
offspring will encounter? So, through sexual
reproduction (recombination, crossing over) a
variety of genotypes (and thus phenotypes) of
offspring are produced. At least some should
achieve success.
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There are two theoretical explanatory
constructs 1. An adaptation of the myth of
Sisyphus. Sisyphus was doomed to roll a
large boulder uphill. He could never reach
the top. The boulder would roll back to the
bottom, and hed have to try again. In
evolutionary terms, since the environment is
constantly changing, selection can never
achieve a perfect genotype, and only through
sex can evolution keep trying. 2. The Red
Queen hypothesis. Quoting Lewis Carroll
Here, you see, it takes all the running you can
do, to keep in the same place. Species, to
succeed, need to evolve relatively rapidly
to succeed in the biological realm, since
those they interact with are evolving new
defenses, attack strategies, or whatever is
important to their success.
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If sex is the means to reproduce, whats the best
way to do it? Think about all the problems of
finding mates and achieving a successful mating.
Wouldnt it be easier and more successful if
male and female functions could be combined in a
single individual? A number of species do things
that way. They are hermaphroditic. Some are
simultaneously male and female (typical in snails
and worms and many plants), and some are
sequential first one sex, then the other
(plants and some fish). Sometimes it is years
before sex change occurs. In many maples, they
are male first (its less energetically demanding
to be male), then, when theyve grown larger and
stronger they switch to being female. Under
harsh, energetically demanding conditions, they
may switch back.
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However, having both sexes can limit the energy
committed to and success of one sex or the
other. Evolution will inevitably select the sex
strategy that maximizes the sum of success from
being male and being female. When having both
sexes produces a greater total success than
being only one sex, hermaphroditism is
advantageous.
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If, on the other hand, if each sex interferes
with the success of the other one, the sum of
successes will be less than could be achieved by
having individuals function as only one
sex. Hermaphroditism is selected against.
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We can only surmise what factors in life history
or environment lead to different strategies, but
there are many examples of dioecious plants
(separate sexes), suggesting that there is some
interference in fitness contributions between the
sexes
Marijuana is dioecious, and female plants are
more valuable grown in the absence of males,
flowers of the plant are called sinsemilla
(meaning without seeds).
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Heres another example, with pictures of both
female and male plants. Its a cycad, a
relatively primitive gymnosperm.
Female Male
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Many plants have both sexes separately on the
same individual. They are monoecious the sexes
are separate (in different flowers) on the same
plant body. Heres one example, an oak
In some cases male and female flowers appear
simultaneously. In others (like the arctic dwarf
birch I study), flowers appear sequentially.
Sequencing flowers tend to prevent inbreeding.
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Finally, some plants produce perfect flowers,
in which both male and female parts occur
together. Even in these flowers the male and
female parts may function simultaneously or
sequentially. To ensure outcrossing, many
plants carry self-infertility genes that prevent
inbreeding. These loci must be heterozygous (the
alleles at these loci coming from different
individuals) for embryos to survive.
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Sex Ratios When the sexes are separate, there is
a ratio of the number of males to the number of
females. We typically think of that ratio as at
least approximately 11, but that isnt always
the case. When the ratio isnt 11, there is a
selective advantage to the minority sex. As a
result, the ratio tends to be driven back towards
11. This result is called frequency dependent
selection.
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As a result of differences in the survivorships
of the two sexes, the sex ratio may also change
with age in a population. Think of the human
population. There is a primary sex ratio. It is
the number of male and female embryos conceived
in a population. The secondary sex ratio is the
ratio of the number of male and female babies
born. We can assume that the primary sex ratio in
humans is 11. But wed be wrong. The primary sex
ratio is somewhere between 1.079 (1948 Carnagie
Institute data) and 1.2-1.61. Why? One favorite
explanation Y-bearing sperm are lighter (Y is
smaller than X) and more motile. The secondary
sex ratio is 1.061 106 male babies for every
100 female babies in the U.S. and Canada. Even
so, there is higher in-utero mortality in males
than females.
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As youll see in the demography lectures,
mortality through maturation (and even later in
life) is also higher in males than in
females. The tertiary sex ratio, the ratio at the
time of reproduction, is generally slightly
female-biased. By middle age and beyond the ratio
is even more female biased among older adults
there are more unmated females than males.