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Hello, and welcome back to Animal
Behavior.
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When sexually reproducing animals
produce
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their offspring, they have a choice.
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They can produce either a son or a
daughter in any given reproductive event.
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In this lecture, we'll look at the
question of whether in certain
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circumstances it might pay parents to
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preferentially produce either a son or
daughter.
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Now for a long time it was assumed
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that in species with genetic sex
determination - animals like
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birds or mammals - that sex determination
was pretty random -
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that it couldn't really be influenced by
the parents.
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But we now know that a huge variety of
organisms, including birds
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and mammals are capable of
manipulating the sex of their offspring.
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And manipulating the sex in ways that
increase their fitness.
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To understand interesting patterns of sex
allocation we
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first have to understand why from an
evolutionary perspective,
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we expect investment in the two sexes to
be equal at the population level.
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Now at first, this seems rather counter-
intuitive. After all, if the
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sperm of a single male can fertilize lots
of different females,
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why not produce a sex ratio of say, 1 male
for every 10 females.
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Now we can see why that wouldn't work by
doing a thought experiment.
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Suppose we had a population in which there
were ten times as many females as males.
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Such a female bias sex ratio can't be
evolutionarily stable. Why?
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Well, because any parent that produces a
rare male will gain about ten times as
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much reproductive success
as any individual that produces a female.
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That's because this male can potentially
mate with on average,
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10 females,
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whereas each of these females can only
mate with this one male.
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As a consequence, any gene that causes
parents to
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produce a son, will rapidly spread through
the population,
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and the proportion of sons will steadily
increase to more than one in ten.
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But the converse is equally unstable.
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If there are ten males all competing to
fertilize the eggs of a single female,
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then daughters are going to have an
enormous
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fitness advantage because only one of
these ten males.
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Can fertilize the offspring that's
produced by this female.
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So even though this female can only
produce one offspring, each of these males
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has only a one in ten chance of being the
father of that offspring.
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So again, there's a ten fold advantage in
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terms of reproduction to females in this
situation.
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Both sexes will only have the same
reproductive success
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when the population sex ratio is exactly
one to one.
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Even tiny deviations from that ratio are
going to provide fitness
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advantages that'll favor bias towards the
sex that returns better fitness.
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This insight, that persistent sex ratio
bias
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of the population can't be stable,
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came from a famous theoretician by the
name of Ronald Fisher.
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And the way that Fisher expressed it was
that "the sex
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ratio will so adjust itself, under the
influence of natural selection,
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that the total parental expenditure
incurred in
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respect of children of each sex will be
equal.
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A nice way to test Fisher's theory is to
experimentally
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push the sex ratio away from parity and
then try and
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observe whether it evolves back to that
point.
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And you can do that in species that have
unusual
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forms of sex determination by manipulating
the frequency of the sex alleles.
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And this little fish for example, which is
called a Southern Platyfish,
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sex is determined by a single
genetic locus that has three alleles.
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There are three female genotypes and there
are two male genotypes.
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Alexandra Basolo showed
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that if the relative frequency of those alleles
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is changed - by setting up populations
with biased sex ratio -
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selection will favor the rarer sex, and
within
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a few generations the ratio swings back to
parity.
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So in this case here, if we start off with
a population that is female biased, that
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has a relatively low proportion of males.
As we go through the generations,
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the sex ratio quickly swings back to
parity.
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The same is true if we start with a
population that is quite male biased.
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So here we have a high proportion of
males, again,
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as we move through just a couple of
generations of reproduction,
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we end up with a population that
ends up back at parity.
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We can refine Fisher's argument by
rephrasing it in terms of the resources
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that are invested.
So far, we've implicitly assumed
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that daughters and sons are equally costly to
produce.
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But what if we imagine that sons are twice
as
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costly as daughters to produce, because
they're twice as big,
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and eat twice as much food while they're
developing?
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Now, when the sex ratio is one to one, a
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son has the same number of children as a
daughter.
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But because the sons are twice as costly
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to make, they're a bad investment for a
parent.
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Because each of the grandchildren that's
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produced here, is twice as expensive as
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the grandchildren that are produced by a
daughter.
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It should therefore pay parents to produce
daughters instead.
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We expect the sex ratio to swing toward
the female bias.
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And as that happens, the expected
reproductive success of a son will go up.
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Until the average son produces twice the
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number of grand offspring as the average
daughter.
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At that point, sons and daughters will
give exactly the same return per unit of
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investment.
Because even though a son costs twice as
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much, he provides twice as many offspring
as a daughter does.
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What this means is that when the sexes
differ in their costs,
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the stable strategy for parents is not to
produce
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equal numbers of the two sexes, but to
invest equally.
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If males are twice as costly as females,
parents should only produce half as many.
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Now to test this prediction, all we need
to do is to
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find a study system where the costs of
producing sons and daughters differs.
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And a nice example of this comes from two
closely related wasp species.
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Polistes variatus is a species where males
and females are equal in size.
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A closely related species, Polistes
metricus, has females
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that are half the size of males.
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Now, in metricus female wasps produce
twice as many
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females as in males amd variatis equal
numbers are produced.
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So, exactly as predicted by Fisher, when
males cost
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twice as much as females, half as many are
produced.
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So, the take home message from this
lecture is that at the
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population level we expect sex ratios to
be pretty near parity.
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A parent should invest in sons and
daughters near
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equally, based on how many are produced
and what they cost to produce.
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However, we'll see in the next lecture
that there are some contexts that might
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favor a bias in production of sons and
daughters at the level of the individual. |
