Prof: Okay.
 
Tomorrow is Darwin's birthday.
 
It's 200 years since he was
born, on the same day as Abraham
Lincoln.
 
Astrologers have made a lot of
the fact that both men were
deeply opposed to slavery.
 
But today we're going to talk
about sexual selection.
So this is a Valentine's Day
lecture.
>
 
Sexual selection is actually a
component of natural selection.
When Darwin looked at the
extravagant plumage of the birds
that you just saw,
he thought there has got to be
something special going on here.
 
>
 
When he saw all of these
extravagant behaviors,
he thought there must be
something else going on,
besides natural selection,
and that's because Darwin
thought that natural selection,
in some sense,
was the survival of the
fittest.
We now know that natural
selection really is the survival
of those that reproduce the
best,
and that sexual selection is a
component of natural selection
that is associated with mating
success.
So basically sexual selection
is a case in which mating
success is trading off with
survival.
And to bring this home to you,
as you sit here in your 18 to
45-year-old state--
there are various ages in the
audience--
I'd like you to consider the
fact that the ratio of male to
female mortality,
in the United States,
starts to diverge early in
life,
and by the time one has hit
late teenage and early twenties,
the ratio of male to female
mortality is climbing rather
strikingly.
This is from all causes;
this is from external causes;
this is from internal causes.
 
And actually this divergence
here is enough to account for
the different life spans of
human males and females,
which differ by about four or
five years.
Now that is sexual dimorphism,
and sexual dimorphism in
mortality rates,
and it appears to be associated
with blockheaded risk-taking
behavior.
It appears that males behave
differently at those ages than
females do.
 
Now we don't know whether
that's evolved and genetic,
or whether it's culturally
influenced,
but I invite you to consider
those alternative possibilities
and think how one might test
them.
From animal studies,
we know that the more
polygynous the species,
the greater the difference in
male and female lifespan.
 
There isn't any difference in
monogamous species.
So if you look at swans or
other monogamous birds,
the males and the females live
the same amount of time,
but the more polygamous a
species is,
the shorter the male's lifespan
relative to the female's.
So how does sexual selection
work?
It might account for that
pattern.
How does it work?
 
Well if a change in a trait is
going to increase lifetime
reproductive success,
by improving the ability of an
individual to attract or to
control a mate,
or to achieve fertilization,
it can be favored by selection,
even if it lowers survival
probability.
This is a Woody Allen film
about sex and death.
Now sexual selection will
change traits that influence
mating success until the
improvement in mating success is
balanced by costs in other
fitness components;
then the response will stop.
 
This will not go on to the
point where there's wholesale
slaughter and suicide.
 
Although in some few cases
there is sexual cannibalism
during copulation--
in the Australian Redback
Spider-- and the male does
appear to insist on committing
suicide.
 
So that would be probably the
most extreme case.
But that's not normal,
that's really an extreme case.
The normal case is that if
there is sexual selection going
on,
then the sexes will diverge in
their behavior and their
morphology,
and their traits and their
behavior will be modified in
ways so that they are getting
better mating success,
but they are incurring costs,
in terms of survival.
Now this line of thought
explains why organisms will take
risks to mate--
and often they will take
extreme risks to mate--
and it explains why juveniles
develop secondary sexual
characters only on maturation.
That's because the secondary
sexual characters,
which are the things that we
think of as causing the two
sexes to look different in any
species,
bear with them costs,
and those costs can be imposed
from a variety of sources.
 
The costs may be that if you
look like an adult,
you're going to elicit
competitive behavior from other
adults--
you might get beaten up--or it
may that it simply makes it much
more difficult for you to escape
a predator.
 
So the costs can come in from
all sorts of places.
The main questions about sexual
selection are how did it
originate?
 
And we are now pretty well
satisfied that before there was
anisogamy, there could not be
any sexual selection.
There had to be gametes of
different sizes,
and there had to be individuals
specialized on producing small
ones and big ones before you
could start getting things that
functioned as males and females,
and then evolve to begin to
look like and behave like males
and females.
The mechanisms of sexual
selection are basically two:
competing for mates and
choosing mates.
So there's force and subtlety
involved in this process.
There's a lot of evidence for
it.
This is, as you might imagine,
since we are among the more
sexual primates--
we're not quite as sexual as
bonobos,
but we are a relatively highly
sexed primate--
it's not unusual that this is
an area of biology that has
attracted an awful lot of
attention.
 
You can find thousands of
papers on sexual selection.
The strength of sexual
selection is actually on the day
of mating determined by the
operational sex ratio;
that is, the local ratio of
males that are ready to mate
with females who are ready to
mate.
Okay?
 
So that's just a brief outline
of some of the main points.
Now about competing and
choosing.
Competing and choosing have
different consequences.
It is the limiting sex that can
be choosey--okay?--and normally
individuals of the other sex
compete.
Now it is quite possible that
if one sex is limiting,
and the other sex is competing
for the attentions of that sex,
that the individuals among the
limited sex can actually compete
with each other for the
attentions of the suitors.
There's nothing ruling that out.
 
It's just that I'm describing
here the processes and forces
that are stronger in the two
sexes.
That doesn't mean that other
things aren't going on;
there's arguably a lot going on.
 
Generally speaking,
mate competition will be
stronger in the sex that has the
greater reproductive potential,
and it's the one that should be
competing for the sex with the
lesser reproductive potential.
 
And usually males have greater
reproductive potential,
females have lesser
reproductive potential;
so males compete and females
choose, generally speaking.
You'll see that there are
interesting exceptions.
Now, what should a female
choose?
I'll already tell you this is a
big topic in evolutionary
psychology.
 
David Buss, at the University
of Texas, wrote a big review
paper on this a few years ago.
 
It's highly controversial.
 
It's fun to read this stuff.
 
It's difficult to come to a
hard science conclusion on it.
Basically what Buss says is
that males choose young females
that look healthy and have high
reproductive potential,
and females choose males that
have access to resources and are
likely to contribute to child
rearing.
Okay?
 
And he says this is
cross-cultural;
all cultures,
all times, that's what people
do.
 
Obviously that could be
controversial,
and there are a lot of
refinements on that picture,
and there's a lot of evidence
that could be better.
But that's what Buss claims.
 
That's for humans.
 
What about animals,
where you can actually do
experiments and do manipulations
and ask them to tell you why
they are choosing mates?
 
Well here are some hypotheses.
 
A female could look at a male
and say, "Does he control
important resources?
 
Will that one be a good parent?
 
Will it supply food
efficiently?"
Or, looking at it,
"Is that potential mate
healthy?
 
Is it free of parasites,
and is it advertising its
ability to resist parasites and
pathogens with a costly signal
that gives me an indication that
in fact that thing is honest and
is not trying to deceive
me?"
Or, "Does that potential
mate have traits that are
attractive to sexual partners?
 
Will I have a sexy son?"
 
Now this sets off a very
interesting co-evolutionary
process between the preference
genes that are sitting there
being expressed in the female's
brain and the expression of that
in the form of some kind of sexy
morphological trait in the
male's morphology.
 
And these genes for preference
and genes for attraction come
together in the offspring,
and that has very interesting
consequences.
 
Well what do we know about this?
 
André
Dhondt and his students,
who at that time were in
Belgium--and André
is now at Cornell--
did an interesting study on
Blue Tits.
 
Now the point about Blue Tits,
that makes them useful for
this, is that they frequently
have extra-pair copulations.
That means that Blue Tits
indulge in adultery a lot.
And because of genetic
fingerprinting,
you can go out and you can
determine the paternity of
offspring in this brood,
where the male is having his
primary residence,
and in neighboring broods,
and when the male flies out,
perhaps to have an extra-pair
copulation with a female in a
neighboring territory,
another male might fly in and
have an extra-pair copulation
with his female.
 
And you can trace the
consequences of this.
Okay?
 
We define as unattractive guys
that don't get much action,
and we define as attractive
guys that get a lot of action.
Okay?
 
So that's an operational
definition of attractive.
And if the unattractive males
turned out to be smaller and
they died younger than
attractive males--
so female Blue Tits are able to
pick up on something about the
physical health of the males
that they're looking at--
the offspring of attractive
males, in their own home brood,
in their own home nest,
lived longer than the offspring
of unattractive females.
 
Now you could explain that by
direct benefit rather than by
females looking at some signal
in the male's morphology or
behavior that he had especially
good genes,
if the attractive males had
better territories and were
better parents.
 
That we don't know a lot about.
 
If the extra-pair offspring of
the attractive males--
so the babies that were laid in
other nests,
in other territories and were
reared by other people;
so they were foster-children in
other territories--
if they also had better
survival, that would be hard to
explain in terms of direct
phenotypic reward,
and would then indicate good
genes.
So this is an example to show
you how you can apply natural
experiments,
in the field,
to try to settle the question
of which hypothesis for sexual
selection is correct--
is it good genes,
is it direct benefits?--
and show you that some of the
evidence had been collected by I
think--
2002, I think,
is about when this came out.
There may be more out there now
and you can go find out.
At any rate,
this is something that can be
done now with DNA fingerprinting
and with small birds.
There is a sexual selection
maxim,
which is kind of a default
condition,
and that is eggs are expensive
and sperm are cheap,
so females are limiting and are
going to be choosey,
and males have potentially
higher reproductive success over
a lifetime than a female,
but probably also higher
variance in reproductive
success.
And the consequence of this is
that the lifetime reproductive
success of the females is
basically limited by the number
of offspring they can produce;
the lifetime reproductive
success of males,
by the number of females they
can fertilize.
 
That's really asymmetrical.
 
That means that in this default
condition the females become a
limiting resource,
and that sets off competition
among males for mates.
 
It allows females to choose
partners.
And so usually females are
choosier than males,
and males are more promiscuous
than females.
But there are a lot of
exceptions.
Right at the end I'll show you
a couple of very beautiful
polyandrous birds where females
hold harems of males,
and their morphology has been
changed so that in those species
of birds they are the bright,
dominant, colorful ones,
that look like males.
 
So this is a general principle,
and you should note that
sometimes sperm are actually
more expensive than eggs;
it's not always true that sperm
are cheap.
Here's a case.
 
This is a case in which the sex
that is choosey changes in a
plastic fashion as a function of
how much food they have,
and that's because in Katydids
males contribute nourishment to
the females in their
spermatophores.
So a female is not only getting
sperm from a male,
she's getting food from the
male.
Okay?
 
Now if food is scarce,
then female Katydids are
actually reproductively limited
by how many male spermatophores
they can get.
 
And at that point males back
off, they don't court so much,
they get to be kind of coy,
and the females fight over the
males and the males become
choosey.
However, when the food is
abundant, the males are
reproductively limited by the
availability of females;
females are getting a lot of
food from sources other than
spermatophores;
males court and females are
choosey.
 
And you can take these and you
can just flip them back and
forth, from one mode to the
other, just by how much you feed
them.
 
So that's actually a nice test
case,
because here you're taking the
same individual organisms and
changing them with an
experimental manipulation
between two different courtship
modes;
and it gets directly at this
idea.
Now, what about competition for
mates?
There is a lot of armament out
there in Nature.
You've seen some of it.
 
You've seen the antlers on deer.
 
You've seen the tusks on
narwhales.
You've seen the tusks on
elephants.
You've seen that a male
silverback gorilla weighs 450
pounds, and a female gorilla
weighs about 120.
There are major differences in
body size of males and females
in many species.
 
So these are set off--the
evolution of that difference in
body size is really driven by
contests, scrambles and
rivalries.
 
And during these contests
females can be sitting there,
looking at the males go at it,
fighting for access to them,
and they could be choosing
males for their competitive
ability.
 
So the fact that the males are
fighting isn't ruling out female
choice, it's making possible
perhaps another kind of female
choice.
 
That does explain large,
well-armed males,
and the most striking examples
of this dimorphism are in
pinnipeds.
 
So here are two southern male
elephant seals,
okay?
 
Bull elephant seals.
 
By the way, these guys are very
superior divers.
They will go out offshore,
oh 100 miles or so,
and dive down to a depth of oh
between 1000 and 3000 feet,
to fish for squid,
and they're very good at it;
they're kind of like little
mini whales cruising around out
there.
 
And a male elephant seal will
spend about nine months a year
storing up a lot of food,
because then he's going to haul
up on a beach and try to protect
a harem on the beach,
and try to chase off other
males and fight vigorously.
And during that three months he
doesn't eat.
He gets extremely grouchy.
 
Now it is possible for these
guys to control a harem of about
forty or fifty female seals on
the beach.
And it's not easy to run down a
beach if you're a 4000 pound
male elephant seal.
 
You are built for swimming
sleekly through the water,
chasing squid,
not for humping along the beach
with your four flippers.
 
Okay?
 
But these guys do,
for three months,
and they get all beaten up,
chasing off juvenile males that
are coming in and trying to
sneak copulations with their
harems.
 
And, because of this spatial
situation, if you're a juvenile
male, you can actually give the
females a little chance to make
another choice.
 
So the juvenile males hang out
sort of on the boundaries
between the harems of the big
dominant males,
and try to sneak copulations.
 
This sets off a situation of
controlled chaos that goes on
for three months,
and everybody gets exhausted
fighting.
 
If you look at the ratio of
male body length to female body
length,
and look at the harem size that
a pinniped can control--
what you have here is data on
different pinniped species.
 
This, I think,
is a harbor seal,
which is virtually monogamous;
so harbor seals tend to mate
for life.
 
But an elephant seal will be
about 1.6 times as long as a
female, and that means it's how
much heavier than a female?
What do you do with that 1.6 to
estimate the difference in
weight?
 
Can't hear you.
 
Student:  Cube it.
 
Prof: You cube it.
 
That's right.
 
So that means that the male
elephant seal is on the order of
five or six times heavier than
the female.
So if he's 4000 pounds,
she's about 850 pounds,
something like that.
 
And that's a pretty strong
relationship in biology,
for sexual dimorphism versus
harem size,
and that illustrates the
importance of competition for
mates in controlling the
evolution of size differences
between males and females.
 
Now the one that's probably a
bit more fun is mate choice.
I had a graduate student in
Switzerland who went to a
behavioral ecology meeting in
Sweden,
at a time when mate choice was
a hot topic in the literature.
And it is a standard feature of
scientific meetings that in the
evening there's a bar,
and people are talking at the
bar, and so she quietly went
around and decorated males with
various things,
like feathers or whatnot,
and then stood back and took
notes to see how long they got
to chat up a woman,
depending upon what kind of
decoration they were wearing.
 
And she showed a very
significant effect,
and the ones that looked
weirdest actually got the most
attention.
 
>
 
So, just a tip. Okay?
 
>
 
Now if you're really choosey
and you're able to detect high
quality territories or good
genes or sexy sons or something
like that,
you can improve your fitness by
being choosey.
 
But remember to be choosey you
have to take time.
Choosiness is essentially
shopping.
Right?
 
Shopping takes time.
 
If you shop too long,
you may miss the opportunity
because the shelf will be empty.
 
Okay?
 
So organisms should be careful,
but not too careful.
There's kind of an optimal
waiting time,
and after awhile,
as the season progresses,
for a seasonally breeding
organism,
there's going to come a time
when mating with anybody is
better than not mating at all.
 
So this business of being
choosey is constrained by time.
Now choice based on an
immediate phenotypic benefit--
that means I'm going to choose
this mate because he's got a
great territory and I'm going to
get a lot of food,
or I can see that my babies
will have a lot of food;
that would be an immediate
phenotypic benefit--
that can explain a lot,
but it won't explain
extravagant male morphology or
leks,
and very often extravagant male
morphology and leks are
associated with each other.
 
You just saw some in the video
at the beginning.
That Bird of Paradise was on a
lek, and I'll show you some more
in a minute.
 
So things that lek are
peacocks, sage-grouse,
Birds of Paradise.
 
And a lek--it's one of the few
Swedish words which has been
appropriated into English.
 
Lek in Swedish means--has two
meanings.
Meaning number one is a sports
place.
So you can actually drive down
a road in Sweden and see a
little sign that says Lek,
and you'll see a sports field.
But it also means the same
thing that it does in behavioral
ecology in English,
which is a traditional display
ground where males come year
after year to advertise and try
to attract females to mate with
them.
Okay?
 
And this is a mating system in
which the males are then not
going to go off and take care of
the babies.
The males sit there and display
and fight with each other.
Females come and mate with
them, and the only thing that
the female gets from the male is
genes.
These are some things that lek.
 
This is a sage-grouse at
Malheur National Wildlife Refuge
in Oregon.
 
This is a Bird of Paradise and
this is a peacock.
Now these males--these are all
males--
have pretty extravagant
morphology,
but they not only have
extravagant morphology,
they have extravagant behavior.
 
You saw what that Rifle Bird
was doing, waving its wings back
and forth.
 
Anytime you see something like
this, you can bet that if you
were out there and you saw it in
action that there would be
feathers in motion;
there would be dances going on;
there would be really elaborate
stuff that these birds were
doing with their behavior to
attract mates.
For example,
the male sage-grouse in Malheur
in Oregon go to the lek in
January.
In January, in Eastern Oregon,
the temperature can be 20
degrees below 0.
 
There's snow on the ground.
 
These guys are getting up in
the dark, before sunrise,
to get out there on the lek,
to try to get a one-up on the
competition.
 
They puff up their breasts and
they make a popping sound when
they do so, and then after they
pop they coo.
So they go pop-pop,
coo, coo, pop-pop,
coo.
 
You can hear them a kilometer
away.
And they make themselves as
visible as possible.
Around the lek there are
lurking coyotes.
Overhead, cruising through the
air, there are Golden Eagles.
These guys are taking major
risks to get up at five in the
morning to go out and try to
make love in the snow.
You know?
 
>
 
That's a serious modification
of male behavior.
Talk about being a prisoner of
your hormones.
These guys are in jail.
 
>
 
So what kind of an experiment
could you do in the field to try
to decide what's a female
looking for?
Well Malte Andersson had a
great idea.
He wanted to work on the
African Widowbird.
African Widowbirds have--the
males have naturally long tails,
and they control territories
within which two,
three, four,
five females might nest.
So what Malte did--by the way,
he did this in Masai Mara,
in Kenya--he shortened tails on
some Widowbirds by simply
cutting them off with scissors.
 
On his control group he cut the
tail off and glued it back
together, so it didn't change in
length.
And then on his experimental
group he cut the tail off--he
took the cut tails from the
short-tailed ones and glued them
on to make super long tails.
 
Okay?
 
So he had three groups.
 
He had short-tailed controls
and real long tails.
Now the ones with shortened
tails only averaged half a nest
on their territory,
and the ones with the
lengthened tails averaged nearly
two nests on their territory.
Individuals were assigned at
random to these different
groups.
 
And so the data indicate that
female Widowbirds were building
nests on territories of males
with longer tails.
And then the question is,
if that's such a great thing,
if you're going to double your
reproductive success with a
longer tail,
then why don't you already have
it?
 
Darn it, why hasn't evolution
done that to you?
Well the answer is probably
that natural selection is
preventing a further increase in
male tail length because females
are preferring much longer tails
than are found in natural
populations.
 
It's fun to give this part of
the lecture in German,
because those of you who know
German know the double meaning
of Schwanz in German;
it's what you imagine.
Okay, there's another
hypothesis, and that is what is
a female looking for?
 
Well she's looking for an
indication of good genes,
under this hypothesis,
and that would mean that a
female should prefer a male
displaying an honest costly
signal--
notice here I've put
honest and costly
together--
an honest costly signal that
they contain genes for superior
survival ability;
for example,
genes for parasite and pathogen
resistance, even for different
MHC alleles.
 
Now the vertebrate immune
system is partially integrated
into the vertebrate nervous
system.
The two systems can send each
other information.
If there was a way that your
sensory system could pick up
information on the composition
of the MHC alleles,
in a potential partner,
and send it to your brain,
that would affect your mate
choice.
This leads into something that
you may enjoy looking at.
If you're into ISI Web of
Science, type in T-shirt
Experiment, and look at the
impact of body odor on how
attractive a potential mate
smells.
Okay?
 
It turns out that if you do
that, and then you do the DNA
sequencing to see whether or not
the people who are reporting
attractive or unattractive have
similar or dissimilar MHC
alleles,
the ones that are reporting
that a smell is attractive are
the ones who have different
immune genes,
and the ones who are reporting
that a smell is repugnant have
similar immune genes.
And the way the immune system
works in the offspring,
to resist infectious disease,
is by generating diversity
within the body,
and it can only do that if the
genes are different.
 
So you have to find a mate with
different MHC alleles if you
want to have disease resistant
offspring.
And there is some evidence in
humans that in fact we do react
to scent and that there is
information in scent.
By the way, this is well
established again--
it's not so well established in
humans because we can't do
manipulation experiments--
but it is well established in
mice,
that mice do this.
And where people make mistakes
and they do mate with people who
have similar MHC genes,
they get into a situation where
there are multiple spontaneous
abortions.
So it appears not only that
there is a level of selection at
mate choice,
but there is also a level of
rejection of zygotes that are
potentially not going to resist
infectious disease.
 
That's work by Carole Ober at
the University of Chicago,
working on Hutterites.
 
So interesting stuff here.
 
Now what kind of evidence have
we got?
Well if a male produced an
ornament in order to advertise
that he was resistant to
disease,
then you would expect that male
fitness would decrease with
increased parasite infection.
 
So that would be an assumption
behind it;
that would be the selection
that was driving it.
The condition of his ornaments
should decrease with increased
parasite burden.
 
So the less he was able to
resist the parasites,
the less dramatic an ornament
he would be able to express.
So that means that that
ornament's got to be costly.
Then there must be some
heritable variation in
resistance, or there wouldn't be
any response to selection;
just go back to the first basic
four conditions for selection:
that must be there.
 
And if this holds,
then females should be choosing
the most ornamented and the
least parasitized males.
And there are three cases that
are pretty well worked out where
that's exactly what appears to
be going on.
So what female guppies appear
to be looking for are orange
spots in their males.
 
What female pheasants appear to
be looking for are red irises
around the eyes of the males.
 
And what female barn swallows
appear to be looking for are
nice long symmetric tails;
they like the symmetry of the
male's tail.
 
Now that would be a good genes
argument.
The male's got a gene for
parasite resistance,
so I'm going to make with him.
 
That's a good gene.
 
And that is where the Fisherian
process of sexy sons would
start.
 
Okay?
 
So I'm now shifting into the
argument for the third
hypothesis,
which is you choose a male
because you think that if you
have a son by him,
that son will get a lot of
matings.
So preference for good genes
will select for the preference
itself,
and that makes the preferred
trait an object of selection,
and it explains the evolution
of ornamentation.
 
You can see it like this.
 
Suppose the reason a female
guppy likes a male with an
orange spot is that the only way
he can make that orange spot is
if he gets carotenoids out of
the crustacea that he eats.
So if he's really good at
finding high quality food,
he can make a bigger orange
spot.
So it's advertising his
foraging ability.
Okay?
 
That's a good thing.
 
So that gene for foraging
ability comes together in the
offspring with the gene for the
preference,
and because the male has better
foraging ability,
the gene for the preference
will hitchhike on the
reproductive success of the gene
for the foraging ability,
and females will develop
stronger and stronger
preferences for a male with
orange spots.
So that's how the process gets
going, at the beginning.
It's thought that initially the
preference develops because it's
a preference for a gene that
actually affects reproductive
success in your offspring.
 
What if now that guppy
population moves into a new
habitat that doesn't have any
crustacean in it,
and it becomes difficult
perhaps for the males to make
carotenoids?
 
But they still can,
they can still make orange
spots.
 
But the females have the
preference.
The male is no longer giving
off a signal that's reliable in
terms of good genes.
 
All he is signaling is that
he's attractive to females.
Well now evolution has a new
reason to maintain that
selection.
 
It's a selection simply for the
attractiveness of the offspring,
because a component of
reproductive success is mating
success,
and by choosing a male that has
an orange spot,
the female is also choosing
mating success in her sons.
 
So that leads to runaway
selection for sexy sons.
So females are preferring males
with higher fitness.
Their preference genes get
united in their offspring,
with the male's genes for
higher fitness.
The female preference genes
then hitchhike on the male's
fitness genes.
 
Once their preferences are
established,
they work on male traits that
are otherwise neutral,
or maybe even disadvantageous,
except that they are preferred
by females.
 
So they lead to success in
mating.
So then, if all that happens,
then mothers will gain in
fitness by selecting fathers
with heritable traits that make
their sons attractive to females
in the next generation.
In the threespine stickleback,
the intensity of the son's red
coloration is correlated with
the daughter's preference for
red.
 
So what I've just told you
about the genes for preference
and the genes for the male trait
coming together in the offspring
happens to be true,
in sticklebacks.
But when you then ask,
"What's the deal with red
color on the belly of the male
stickleback?",
you discover that it's not only
genetically correlated with
female preference;
bright red males resist
parasites.
 
So by choosing them,
females are avoiding
parasitized males,
and that's also satisfying a
good genes hypothesis.
 
So you can see that in this
case, the data in fact do not
distinguish between the two.
 
It's not like it's either/or.
 
It looks like,
hey, both things are going on
at the same time.
 
Females are choosing sexy sons
who are also parasite resistant.
Now there's a third
possibility, and that is let's
suppose this kind of thing has
been going on in the past and
females have developed certain
sensory abilities,
to perceive the potential mates.
 
You guys are not particularly
good at perceiving potential
mates in the ultraviolet.
 
Okay?
 
Bees are, but you're not.
 
So your sensory capacity is
limited to certain windows,
both with your eyes and your
ears and your taste buds and
everything else.
 
And the idea is that the
sensory capacity inherited from
ancestors would bias the traits.
 
Females might just be selecting
males they can see or hear
especially well.
 
So one well worked out example
is that the female eardrum in a
Tungara frog is tuned to receive
some frequencies better than
others.
 
Here is a male Tungara frog in
Panama calling,
and his call signal is being
picked up by the ear of the
female,
and he can't change his call
signal out of the frequency
range that that female ear can
hear.
 
If she is attracted to him,
he has reproductive success.
But unfortunately,
that's a perfectly fine
frequency for a bat to hear.
 
And in fact a fringe-lipped bat
is a frog eating bat,
and it does exceptionally well
during frog breeding season by
swooping in and munching up the
males who are dutifully calling
to try to attract their females.
 
Now the idea here is that the
male can't evolve out of a
frequency range that the bat can
hear because the female's
eardrum is constrained to be a
certain size;
that's the sensory bias
hypothesis.
Now, so I've stepped through
the main hypotheses that are
thought to drive female choice.
 
So female choice could simply
be for things that she can hear
or see particularly well,
as well as for sexy sons or
direct benefits or good genes.
 
Now what is the actual context
in which this is going on?
Well it's when mating is
happening, when choice is being
made.
 
And that's where the
operational sex ratio comes into
play.
 
It basically is determining the
opportunity for selection,
and it varies with mating
system and parental care.
So sexual selection,
which produces striking
differences in the behavior and
morphology of males and females,
is correlated in ecology and
evolution with mating systems
and patterns of parental care.
 
These things all go along with
each other to form syndromes of
traits.
 
In monogamy,
equal numbers of males and
females have offspring,
and each sex has one partner,
and there's not very much
difference in the operational
sex ratio and not much
opportunity for sexual
selection.
 
In polyandry there are more
males than females,
that have offspring,
and each female has two or more
male partners.
 
So there is great variance in
reproductive success among
females;
there's less variance in
reproductive success among
males.
In the reverse pattern,
in polygyny,
more females than males have
offspring, and each male has two
or more female partners;
that leads to harems and leks.
And then there are things like
polygynandry where each sex may
have several partners.
 
There's a little brown bird,
an LBB, called the Dunnock.
It looks a little bit like a
hedge sparrow.
It lives, among other places,
in the botanical garden at
Cambridge University in England,
where Nick Davies has studied
it for a long time,
and it had the notoriety of
being regarded as an exemplar of
marital fidelity,
until Nick studied it.
 
And once Nick got in there with
his DNA fingerprinting and did
paternity analysis,
he discovered that Dunnocks are
polygynandous.
 
So each sex is mating with
several partners.
A female sitting on a nest will
have eggs in it that were
inseminated by several males,
and the father who's bringing
food to that nest has his genes
in eggs that are in several
other nests.
 
So that's polygynandry,
and well exemplified in the
Dunnock.
 
Now the effects of mate choice
are thought to be especially
strong in these two
circumstances here.
That's where one sex has
relatively little variance in
reproductive success;
the other has large variance in
reproductive success.
 
One sex is limiting;
the other sex is competing.
One sex is choosey;
the other sex is fighting.
I told you that I would show
you some polyandrous birds.
This is a Wattled Jacana,
and the Jacanas--
we ran into them on the
Amazon--a Jacana female will
hold a territory within which
three,
four or five males will build
nests,
and she will go around and
lay--they'll fertilize her and
she'll lay eggs into each of
their nests.
But she doesn't spend any time
raising the babies,
she just fights off the females
on the neighboring territories.
So males take care of the
babies and mom fights for the
territory;
and that happens in the Wattled
Jacana.
 
This is a Phalarope;
this is a female Phalarope.
They are basically related to
the Sandpipers;
so they're shore birds.
 
They live in the far Arctic.
 
And a similar pattern.
 
The male sits on the eggs.
 
A female holds a territory
within which three,
four, five males have nests.
 
And there've been some very
interesting studies done on the
mating physiology of Phalaropes.
 
When a female Phalarope is
displaying, in her courtship
display, and attracting males,
her ovaries are expressing
testosterone.
 
You could imagine that the
courtship display of the female
Phalarope had evolved in an
ancestral male,
and that all of the control
machinery for that was all set
up,
and all you had to do to get it
expressed in the other sex was
turn on the testosterone at the
right time;
which is what her ovaries do
when she's displaying.
 
When she then goes over to the
nest to lay the eggs,
her ovaries secrete estrogen,
and she--
her physiology is switching
back and forth between a male
display in competitive cycle,
driven by testosterone,
and a female lay egg and make
yolk and lay egg cycle,
which is controlled by estrogen.
 
So these sorts of secondary
sexual traits are not only
associated with mating systems
and patterns of parental care,
they also set off a cascade of
physiological integration
throughout the organism,
and uncovering that leads one
into a whole host of interesting
physiological questions.
So to summarize sexual
selection.
It's a component of natural
selection in which mating
success trades off with
survival.
It's not a separate kind of
selection, it's part of natural
selection.
 
It accounts for many of the
attractive ornaments of plants
and animals.
 
It raises the interesting issue
of aesthetics and why our brains
see things as beautiful that
other things find attractive.
There is contest competition
for mates which are the scarcer
reproductive resource,
and that will explain a lot of
sexual size dimorphism--
bull elephant seals,
things like that--
particularly in polygenous
species.
 
We've seen there that the
degree of dimorphism is directly
related to the degree of
polygamy;
the bigger the harem,
the bigger the size dimorphism.
There has been a lot of
documentation actually of active
choice of the non-limiting sex
sometimes.
So that's going on.
 
And whether or not this
indirect mechanism--
this Fisherian runaway process,
that leads to preference for
sexy sons--
whether that is really needed
to explain ornaments in lekking
species,
it seems to be logically the
only possible explanation left
standing.
 
It's as though you have
ruled--you're Sherlock Holmes
and you've ruled out all of the
other hypotheses.
But getting positive evidence
for that has been difficult.
Okay?
 
So this looks plausible.
 
I think it's likely,
but at this point the positive
evidence indicating that that
has been under selection is
still out there.
 
Getting whole genomes of some
of these organisms might solve
it,
because you can then look for
signatures of selection,
if you can identify the
preference genes;
that's a long-term project.
So let's go back to this.
 
I'd like to leave you with this.
 
This is you guys.
 
Here you are;
you're probably somewhere
around this age group.
 
And the mortality rates of the
males in the audience,
on average--every insurance
company in the country knows
this;
every time you apply for car
insurance this difference
manifests itself in your monthly
bill--
the mortality rates for male
humans at this age are several
times those of female humans,
and they appear to be taking
risks.
So I'll leave you with the
unanswered question of whether
that's a product of sexual
selection or not.
And next time we will talk
about speciation.
