The Power of Sex

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The Power of Sex
A good allele combination (1) long neck
alleles (2) strong heart alleles
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Consider 2 antelope, each w/ a rare allele
normal normal neck heart
normal normal neck heart
long normal neck heart
normal strong neck heart
- can reach high branches, but faints a lot
(heart cant get enough blood to head)
- cant reach high branches, but can run
forever and lives to be very old
What happens when they mate
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producing a super-adapted kid
normal normal neck heart
normal normal neck heart
long normal neck heart
normal strong neck heart
- can reach high branches, and doesnt
faint - will have superior fitness!
long normal neck heart
normal strong neck heart
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What happens when their kid mates?
long normal neck heart
normal normal neck heart
normal normal neck heart
normal strong neck heart
- since most females will be normal/normal for
both loci, the good allele combination will be
broken up in his offspring
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But with recombination
long normal neck heart
normal normal neck heart
normal strong neck heart
normal normal neck heart
long strong neck heart
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The power of recombination
Hypothesis recombination lets a new mutation
sample various genetic backgrounds - most
mutations will be unlucky and occur in
backgrounds of the living dead,
chromosomes carrying harmful alleles that
are doomed to go extinct (in asexual
populations) - recombination prevents
selection against the genetic background
(i.e., the other alleles on the chromosome)
from overpowering direct selection on the
mutation itself
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Experimental demonstration(Rice Chippindale,
2001)
Introduce a beneficial mutation, red eye color,
into 34 populations of white-eyed
Drosophila - red eyes allow color vision,
provide a fitness advantage Using
genetic tricks, half the populations could
recombine but the other half could not undergo
recombination
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Experimental demonstration
Initially, the red allele increased at the same
frequency in non- recombining and recombining
populations - Why? At the start, the red
allele was trapped in a few good backgrounds
(by chance) in non-recombining populations -
thus, it initially became more frequent in those
lines
Non-recombining
Recombining
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Experimental demonstration
After 8 generations, red steadily accumulated in
recombining lines but stopped accumulating in
non-recombining populations - Why? selection
stopped working on the red allele, which was
mostly trapped in bad backgrounds
Non-recombining
Recombining
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Experimental demonstration
Over time, the frequency of red arrested in
non-recombining lines - after 8 generations,
recombination increased the strength of
natural selection
Recombining
Non-recombining
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Advantages of sexual reproduction
(1) Genetic diversity create new combinations of
alleles - this creates the remarkable diversity
of life that natural selection goes to work
on, with the best combination of alleles
surviving in each generation (2) Avoid
background trapping new, helpful alleles can
be moved off of chromosomes carrying harmful
mutations (3) Flush bad mutations out of the
genome (escape Mullers ratchet, the
accumulation of harmful mutations over time)
(4) Eliminates other harmful things that build
up in an organism parasites, disease, cell
damage - like pushing the reset button gives
you a fresh start
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Conflict between the sexes
In order to have sexual reproduction, you need 2
sexes In many organisms, males and females are
in conflict over their investment of
reproductive effort - males invest in mating
opportunities - females invest directly in
offspring This conflict manifests in many traits
associated with sex - Males and females often
express very different traits to maximize
their respective success at reproduction
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Asymmetrical investment in offspring
In animals with internal fertilization
(pregnancies) and/or parental care for
offspring, females nearly always invest more
time and energy per offspring than
males Orangutans - males invest 15
minutes in copulation - females invest 8
months in pregnancy, 3 years in nursing,
and 8 years in protecting and nurturing her young
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Asymmetrical investment in offspring
Males need only contribute one haploid genome
(their genes) to their offspring Females
generally invest far more energy per
offspring Some animals release sperm and eggs
directly into the environment, and provide no
parental care - in these organisms, eggs are
larger and contain energetically expensive
yolk - females can make fewer gametes (egg
cells) than males
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Asymmetrical investment in offspring
Because of this asymmetry in reproductive
investment, males and females face different
challenges in maximizing their fitness (making
the most offspring) Once a females eggs have
been fertilized, she generally cannot mate
again until shes done caring for that batch of
offspring A male can mate as often as he can
find a receptive female
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Asymmetrical investment in offspring
Consider a population w/ one male fish and 10
females
one male can mate with, and fertilize, all 10
females
Male reproductive success is limited by access to
mates
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Asymmetrical investment in offspring
Now, consider a population w/ 10 male fish and
one female
Only one male can ultimately father her
offspring Female will (1) choose her mate, or
(2) mate with the male who can fend off all his
competitors
Female reproductive success is not limited by
access to mates
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Sexual dimorphism
In many species, males and females are extremely
different in their appearance or behavior
sexual dimorphism

male
Peacocks
Fiddler crabs
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Sexual dimorphism
Darwins sequel, The Descent of Man and
Selection in Relation to Sex, was dedicated to
this phenomenon How could traits involved
in male display or competition persist in
the face of natural selection? -
shouldnt natural selection tend to weed out
brightly colored individuals (by
predation), or those wasting energy on big
tails or hard-to-build body structures?
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Sexual selection
Darwins answer a distinct form of selective
pressure exists that tends to maximize
reproductive success sexual selection Whereas
natural selection makes a population more adapted
to its environment, sexual selection does not
- it makes one sex more successful in mating
with the other - increases fitness, by
increasing reproductive success
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Intrasexual selection
Males compete for territory, control of females,
or opportunities to fertilize females just go
with the winner (1) Combat is a common form of
pre-mating competition, resulting in
evolution of crazybig male features for fighting
- antlers of deer - body size in
iguanas elephant seals, big males larger than
the ideal size dont survive for long, but
farther many children due to holding better
larger territories
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Intrasexual selection
Males compete for territory, control of females,
or opportunities to fertilize females just go
with the winner (2) Sperm competition is a form
of post-mating competition - males can make
larger ejaculates when theres competition
for females - can be triggered by scent of
nearby competitor males - can be
modified depending on males assessment of
a females promiscuity - multiple
kinds of sperm sprinters, blockers and killers
- extreme case testicular hijacking in bugs
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Intersexual selection
When males cannot monopolize territory, females
may be able to choose among the males (or
are they?..) This introduces a novel form of
evolutionary pressure males versus females
in a continual struggle, each seeking to
maximize their different reproductive goals Many
aspects of this conflict are not a matter of
choice, but depend on a continuous
back-and-forth evolutionary tug of war
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Sexual selection and female choice
male, w/ large claw
Fiddler crabs
Peacocks
Males have traits important in mating displays
(color, tail, claws) whereas females lack such
ornamentation
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Sexual selection and female choice
Moller (1988) studied female choice in barn
swallows - males have long tails - hypothesis
females prefer males with longer tails - hence,
sexual selection has driven the evolution of
this male display trait
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Sexual selection and female choice
Data from Moller (1988) distribution of tail
length in males

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Sexual selection and female choice
Experiment manipulate tail length of 44 males,
divided into 4 groups of 11 ? Shortened tail
clip 2 cm out of tail feathers ? Control 1
clipped 2 cm off, glued back on (control for
gluing) ? Control 2 captured and banded birds,
didnt touch feathers ? Enlongated tail glued
the 2 cm cut off of shortened tails onto
the end of the tail of these birds

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pre-mating period
second clutches
of offspring
Result males with elongated tails (1) got
mates the fastest (2) frequently produced 2nd
clutches (3) had the most of offspring
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pre-mating period
second clutches
of offspring
Conclusion females choose males based on tail
length, which is presumed to indicate the
relative fitness of a male
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Shortened tail treatment clip 2 cm out of tail
feathers

What is a potential flaw in this experiment?
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Data from Andersson (1982) female choice for
tail length in the widowbird Experiment alter
tail length by 25 cm Result elongated males had
more nests on their territories shortened
males had fewer nests Conclusion male ornaments
are favored by female choice, and may have
evolved because of it
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Male display vs. female resistance
Female Choice theory males with big tails are
advertising their good genes/health, are
therefore attractive to females - may be
genetic (carry good alleles) - may be
phenotypic (conditional no parasites,
healthy) Run-away selection theory male alleles
super-stimulate females with dramatic mating
displays and exaggerated features ? females
counter-adapt by reducing their interest
in the exaggerated trait
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Run-away evolution of male display
Female attraction to a male trait that is
absent (pre-existing bias)
?
Mutation produces a rudimentary male display
trait (exploitation)
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Run-away evolution of male display
Female attraction to a male trait that is
absent (pre-existing bias)
?
Mutation produces a rudimentary male display
trait (exploitation)
?
Female fitness declines
Female resistance
Male attraction declines
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Run-away evolution of male display
Female fitness declines
Exaggeration of male display
Female resistance
Male attraction declines
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Male display vs. female resistance
Run-away evolution of male display features, to
overwhelm female disinterest (which increases
every generation) Eventually, males end up
stuck with huge display features that do
nothing, but are necessary to get any attention
at all - females appear to win this
evolutionary contest
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Receiver bias in female choice
Evidence for pre-existing sensory bias (receiver
bias) (1) foraging cues - female
stickleback fish prefer red objects when
foraging, also prefer males with red
throats bellies - female bowerbirds
are attracted to food objects that match
the color of objects males use in their
displays (2) female sailfin mollies prefer
males with more surface area (3) predator
avoidance female wax moths avoid bat sonar, but
are attracted to higher frequency
vibrations from male moths (4) general
attraction towards symmetry
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Female choice based on fitness cues
Presumption of many studies all exaggerated
traits somehow convey to females which males
are the most fit - may or may not some
traits overwhelm female sensory systems,
promoting bad mating decisions and not
indicating male fitness - other traits
may be honest indicators of male fitness, and
females that prefer these traits will produce
fitter offspring - combination flaws in big
display traits may indicate a males
lack of resistance to parasites, so may tell a
female something useful
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Female choice based on fitness cues 1
Female grey tree frogs prefer males who produce
longer, faster mating calls Do these males
have higher fitness? Experiment fertilize eggs
with sperm from long-calling vs.
short-calling males - fitness of offspring was
then judged by 5 criteria how fast offspring
grew and matured, their size, survival Results
longer-calling males fathered offspring that were
the same or more fit than kids
of short-calling males
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Fitness cues 2 male choice of females
Female baboons produce sexual swellings during
ovulation - also advertize her reproductive
value, as predicted by theory of honest
signaling - females with larger swellings
attained reproductive maturity at at younger
age, had more offspring, and more surviving
kids Result males expend more effort fighting
over, and spend more time
grooming, females with larger swellings
Domb Pagel 2001, Nature 410204-206
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Female choice can operate after mating
- female birds can be coerced into mating by
inferior (socially less dominant) males
- however, when this happens, they can lure
dominant competitors to displace sperm of their
rivals - can also selectively eject sperm
of inferior males - female ducks lay larger eggs
after mating with preferred males -
no effects of paternity on offspring condition