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Senior Freshman Evolution
Course
Darwin had two worries:
1)  Altruism: Why animals are
so often nice to each other
when their direct fitness
would be higher if they were
selfish. (last 2 of my lectures)
Lecture 18 : Why Sex?
2)  Sexual Selection and how
such mal-adaptively showy
males could evolve.
Actually there are two problems with sex:
The two-fold cost of sex (John Maynard Smith 1978)
Which grows faster, a
population of aphids
(asexual) or a population
of ladybirds (sexual)?
1)  How sexual reproduction
could evolve in the first
place
Sexual reproduction less
efficient for female as:
1)  males don’t produce
offspring directly
2)  How such mal-adaptively
showy males could
evolve.
(This picture is the right way up!)
Sexual
Asexual
2)  Only half the female’s
genes are present in
each offspring .
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Also, courtship and mating takes
time, energy and is dangerous.
And why do aphids and
water fleas, who have
asexual reproduction
most of the year, have
sexual reproduction just
before winter?
Finding and identifying a mate of the
right species, age, sexual state etc.
is not always easy.
Courtship is showy so attracts
predators, and takes lots of time.
Over-winter as eggs, so
next offspring will not
emerge for several
months.
Letting another individual that close
carries dangers of parasites and
diseases as well as physical injury.
Sexual reproductive
phase before
encountering an uncertain
future environment.
So with all these costs, why are so many
species sexual?
Advantages of Sex
At the population level:
1)  More variation in offspring
2)  Muller’s Ratchet
3)  Speed of adaptation
4)  Sexual species survive longer
At the individual level:
1)  Sib-competition is reduced
2)  Better protection against parasites
Variable offspring a good
bet when future uncertain.
1)  Variation in offspring
Every individual from sexual
reproduction is different.
But is this an advantage?
Only one songbird in 10 gets to
breed, so Mum’s genotype must
be pretty good if she’s breeding.
Changing genotype to an untested
combination may not be an
improvement.
Asexually produced offspring will do
well as long as the environment
is stable. Use sexual
reproduction when the
environment may change.
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2) Muller’s Ratchet (Muller 1964)
Asexual Reproduction
Asexual reproduction can’t get rid of
Deleterious
deleterious alleles. Offspring have
whatever the parent had, plus any new allele
mutations.
Parent 1
Parent 2
Over generations these build up until they kill the
germ line.
Sexual Reproduction
Mum
To be well adapted this frog
must have several traits in one
body, e.g. colour, shape,
background matching
behaviour (so it sits on the right
background).
Mum
Dad
Sexual reproduction allows half the
offspring to lose the deleterious allele.
Those from both parents can be shed by
occasional offspring.
}
}
Escapes from
detrimental
alleles
The “ratchet” is the gradual accumulation
of bad mutations in asexual reproduction,
from which there is no return.
R. A. Fisher (1930) recognised the importance of this for the
evolution of complex adaptations.
Frog needs A) colour, B) shape,
C) behaviour
If frog asexual it must wait for each mutation to arise in the
germ line in which the previous mutation arose.
In the meantime frogs with each single adaptation compete.
If frog sexual then wherever
each mutation arises,
recombination will bring them
together into one very well
adapted body.
Dad
Sexual reproduction allows mutations
originating in different bodies to get into
the same body.
But note cost – some offspring don’t get
either mutation.
}
Gets both
detrimental
alleles
3) Speed of adaptation
Gets both
adaptation alleles
4) Sexual species survive longer
Fossil record suggests asexual
species go extinct more often than
sexual.
Assumed that’s because can’t adapt
when environment changes.
However this asexual rotifer in 2007 was found to have
produced multiple new species, suggesting evolution into new
niches possible for asexuals.
So we’re unsure why asexuals go extinct more, but they do.
BUT All these benefits of sex are benefits to the species, not
to the individual, so cannot be the cause of the evolution of
sex.
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2) Better protection against parasites
Benefits to the individual:
1)  Sib-competition is reduced
G.C. Williams 1975 suggested the “sib-competition model”
Diverse offspring from sexual
reproduction means
a)  One parent’s offspring can
survive in many different
habitats
b)  Offspring compete with their
siblings less because fewer
well adapted to any one
patch. So well-adapted
individual grows unimpeded.
Benefit of sex to individual parent
as offspring do better.
Red Queen Hypothesis (Hamilton and Zuk 1982)
Evolutionary arms race between parasites and
their hosts.
Red Queen in “Alice in Wonderland” had to run
as fast as she could just to stay in the same
place.
Hosts constantly evolve to repel parasites; parasites constantly evolve to
get round those defenses and exploit hosts more effectively.
Parasites are smaller, breed faster, and win this race.
Variable offspring from sexual reproduction will share fewer parasites with
their mother than asexually produced offspring, who are exactly like their
mother.
So sexual reproduction my gain relief from parasites for your offspring, at
least briefly.
“Parasites” here includes diseases.
Darwin’s other problem with sex:
How can mal-adaptively showy
males evolve?
Why do females choose these
males?
Sexual Selection
2 types:
Female Choice = inter-sexual selection
- female selects her mate on some criterion she
prefers
Male-Male Competition = intra-sexual
selection
- Males compete for access to females
-  leads to weapons such as antlers and tusks
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To impress females, males display with
long showy feathers….
Large colorful
areas of exposed
skin…
Delicate
symmetrical
plumes…
Bodily contortions and dances….
Bower birds build huge
structures which are not nests,
which they decorate with one or
two colours of ornament.
They then coax the female
through the “bower”, and she
will mate if she thinks his bower
is impressive enough.
Huge amounts of time and
resources spent by the male on
these bowers.
Why is the female impressed?
Her sons will, after all, do the
same.
Ardent males and reluctant females
Two theories to explain extreme males:
The female wants to choose the best male
as she can only raise a few offspring per
year. But sperm is so cheap the males can
mate as often as they like, so they aren’t
choosy who they mate with.
1) Fisher’s Run-away Process (Fisher 1930)
Honest Signaling
So the males do best to woo every female. The
females must choose between them using a
feature which the males can’t lie about.
Bright feathers, delicate plumes, structures
which take ages to build, are all features which
a sick male could not maintain.
But why doesn’t the female stop at that – why
select mal-adaptively extreme males?
Initially females choose a trait which is good in a
male, like long tails for better flying ability.
These females do well as offspring are fitter, so the trait for
choosing long tails spreads, until all females show it.
The male with the longest tail will now get most matings, so
selection will drive the population to ever longer tails.
Even when the tail is so long it makes flying difficult, selection
will keep making tails longer because only those with the
longest tails succeed in breeding.
This will continue until the detrimental effect of
the tail is so strong that the extra chance of
breeding is not enough to make up for it.
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Why don’t females stop preferring extreme males?
Females can’t call a halt to this runaway process
because of the Sexy Sons Hypothesis.
Mutant female loses preference for long tails, mates
with a shorter tailed (but fitter) male.
Has lots of offspring, but the sons have short tails and
the daughters inherit her preference for short tials.
Sons will then not get mates, as only she and her
daughters like males with short tails, so no
grandchildren via sons.
Daughters mate with short-tailed mates, and so
produce un-sexy sons too.
2) Zahavi’s Handicap Principle
(Zahavi 1975)
Male is displaying uncheatable signal of
good genes because he has survived
despite terrible handicap of excessive tail.
Like fighting with one hand tied behind your back. If you win,
you really are good!
Modellers showed this could work, but only if size of trait varies
with real health (= conditional handicap).
Fluffy feathers and exposed skin
would change to dull if the male was
ill, so it could work.
Trait for sensible choice dies out as out-competed by
trait for producing offspring with the more commonly
preferred trait.
Marian Petrie (1991) tested the
handicap principle using peacocks.
She showed that
1) Females choose males with
longer tails
2) Longer tailed males survived
better
3) Offspring of longer tailed males
grew faster and survived better
So despite the handicap of their huge tail, male peacocks with
big tails have such good genes that it’s worth the female
choosing them…. And her sons will be sexy too!
Carrying a handicap may be a way to signal your good genes,
which is uncheatable as males with worse genes can’t do it.
Evolution does not produce
what is best for the species.
Selection favours whoever breeds
best, and this leads to some maladaptively extreme results.
As long as the costs are
outweighed by the advantages to
their breeding potential, both
Fisher’s runaway and Zahavi’s
handicap principles will drive males
to ever more extreme displays
Sexual selection is not an exception to natural selection (as
Darwin thought), it’s a rather extreme case of how natural
selection always works.
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Required Reading; at least 2 of:
Freeman and Herron “Evolutionary Analysis” 3rd Edition
Chapters 7 and 10
Skelton “Evolution: a biological and palaeontological
approach” Chap. 5 p 189-210
Ridley "Evolution" p 269-300
Barnard “Animal Behaviour” 2004 Chapter 10
Extra reading if you are interested:
New Scientist 14 Sept 2002 pp42-45 “Boy or girl?” – Sex ratio
skewing in humans.
New Scientist 11 May 2002 pp 36-39 “Natural Born Killers” –
Evolutionary psychology tries to explain suicide bombers.
New Scientist 25 May 2002 pp 26-30 “X-rated Brains” –
controversial theory about female choice in humans for big
brains.
Alcock “Animal Behaviour” 6th Edition Chapter 12
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