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3. Evolution, Ecology and Behavior: Adaptive Evolution: Natural Selection


Poziom:

Temat: Nauka i technologia

Prof: Today we're going to talk about adaptive
evolution, and that means that today is
going to be all about the different kinds of natural
selection that there are.
It's going to be about the vocabulary that evolutionary
biologists use to describe selection.
It's going to be about rates of evolution,
why evolution is sometimes very fast,
sometimes very slow, and it's going to be about the
different contexts in which selection occurs.
So we'll talk a little bit about sexual selection.
We'll talk a little bit about group and species selection,
things like that.
All of these things that I mention today are going to be
coming up again and again.
So this is just part of the intellectual toolkit for dealing
with the course.
This is an outline of the lecture, and since it's a whole
lecture it's in pretty small type, and I don't expect you to
read that off the board.
But I do want you to have it so it makes it easy for you to
review this when you download it and you look at it in your
notes, because it does summarize the
main points.
Basically what I'm going to do is tell you that evolution can
be either adaptive, in which case it has been
driven by and shaped by natural selection.
It can be neutral, in which case it's been
dominated by drift.
Or it can be maladaptive.
So evolution does not only produce things that work well.
Evolution produces things in which stuff can go wrong,
and sometimes evolution just wanders around.
Now adaptive evolution is not about the survival of the
fittest.
That is a phrase invented by Herbert Spencer,
in the nineteenth century, that has had a long shelf life,
and it's wrong.
Adaptive evolution is about a design for reproductive success.
It's all about how many children and grandchildren you
have, and whether you do it better than somebody else that's
in the population.
It's always relative.
Natural selection is like the tale about the Buddhist monk and
the disciple who were attacked by the tiger and the disciple
says to his master, "Oh Master,
we're going to be killed because we cannot possibly
outrun the tiger," and the master says,
"No, I just have to run faster than you."
Well, selection is always relative;
it always depends on what the picture is of reproductive
success in that population at the time that it's happening.
Now I'm then going to discuss when selection is strong and
when it can be slow, and I will tell you something
about the rate-- the units in which evolutionary
rates are measured, and then I'll run through types
of selection.
Now there are going to be two questions that pose puzzles that
come up in this lecture.
One is going to be what will happen,
if directional selection continues for a long time,
can that continue, and if it has to stop,
then why should it stop?
And the other question will be how can we explain that even
though evolution can really be extremely fast,
that sometimes things don't change for hundreds of millions
of years?
So you have to be able to come up with enough intellectual
tools to be able to handle that range of variation in
evolutionary outcome.
Both of those things really do happen: nothing for a long
period of time, or incredibly fast.
So here's incredibly fast: antibiotic resistance.
It is a curious and striking cultural fact that in the United
States, when people talk about
antibiotic resistance in television and in the
newspapers, they almost never mention the
word 'evolution.'
They say it emerges or it develops.
But, in fact, this is the poster child for
rapid evolution.
You can see here roughly the years in which antibiotic
resistance emerged, evolved, in these diseases.
If we develop a new drug and release it in the UK in 2009,
resistant strains of bacteria will have evolved and will be in
hospitals in the UK within six months,
and those resistant strains of bacteria will be observed in
hospitals in Hong Kong within two years.
The bacteria will have moved around the earth as people move
around the earth.
The drug industry is in a co-evolutionary arms race to try
to keep up with the bacteria that evolve resistance,
and we have gradually been losing the arms race.
So if you read in the newspapers about multiply
resistant staphylococcus aureus, which is MRSA,
it is starting to crop up now out in the community.
It's not just confined to hospital emergency wards and
intensive care units anymore, it's starting to spread.
And if staph aureus picks up resistance to vancomycin,
which is one of our last lines of resistance against it,
it's going to be very difficult for surgeons to do operations in
the confidence that they can keep their patients from dying
after they have surgery.
So this is serious stuff.
Most resistant bacteria live in hospitals,
because that's where most antibiotics are used,
and the number of hospital acquired infections is about two
million per year in the United States,
and it's estimated that about 90,000 people who did not have a
bacterial infection when they went into the hospital,
got into the hospital and then died from that bacterial
infection.
And, in fact, it looks like this is a serious
underestimate because this is the official report,
but if you look at what the hospitals asked for,
in terms of money from the insurance industry,
it's about ten times higher than that,
OK?
So, by comparison, this is how many people are
dying in 2005 from AIDS in the United States,
influenza and breast cancer, and you take that and you
multiply it to the planet and you can see that the evolution
of antibiotic resistance is a pretty serious issue.
The economic burden in the US four years ago was eighty
billion a year, and this is a problem which is
caused by strong directional natural selection,
eliciting a rapid evolutionary response.
So in the next few slides I'm going to be talking about what I
mean by directional and what I mean by rapid.
By rapid, in this case, if you have a normal sized
bacterial population in your body and I give you an
antibiotic, the probability is that if you
don't do your antibiotic treatment correctly,
within about a week or two you will have resistant bacteria in
your body.
Finish your antibiotic treatments, never stop in the
middle, OK; kill them all off.
Here's a good example of rapid evolution in nature in a fish in
ecological time.
It's one of a series of cases that accumulated in the 1970s
and 1980s that demonstrated that evolution isn't all about
dinosaurs, and millions of years,
and slow, steady change.
Evolution in stuff like the color of the male's body,
the number of babies the female has,
how fast they grow--all kinds of ecologically and behaviorally
important properties can happen real quick.
This was done by doing an experiment in which guppies
interacted with predators.
This is a cichlid fish Crenicichla.
This is the pike killifish Rivulus, and it was done in
Trinidad, by David Reznick who is at Riverside.
Now the setup in Trinidad basically is that there is a
mountain range on the north end of the island,
and there are lots of little streams that are going down the
mountain range into a river and they go over a waterfall.
And the fact that the stream goes over a waterfall has
prevented the large predators from getting up above the
waterfall, and above some of the
waterfalls there were no fish at all.
So what Reznick did was he took fish that had evolved for a long
time, with predators below the
waterfalls, and he put them above the
waterfalls, and he did replicates.
It was a nice system.
There were lots of streams.
You could do it four or five times, to make sure it was a
consistent pattern.
And these are the results.
The life history traits--that means how big they are when
they're born, how old, and how large they are
when they mature, how many babies they have and
how long they live-- all evolved rapidly.
So they responded quickly.
The fastest rates of evolution were measured in things that
occur early in life.
So the number of babies in the first brood, how big the babies
were in the first brood, how fast the babies grew,
that all changed quickly.
And basically the pattern was this.
If the guppies are under a high predation regime,
they mature earlier and they have more smaller offspring,
they have a shorter life--this all has something to do with the
evolution of aging and why we grow old and die--
and they had more smaller offspring.
Okay?
The males were less colorful and they displayed more
discretely.
Guppy courtship is normally a fairly elaborate thing.
The male, who you can see is really brightly colored,
also has an elaborate display behavior,
and he will dance up in front of the female and he will wave
his fins back and forth and then he will dart in and try to mate.
And the females prefers males who have bright orange spots.
The bright orange spots probably were originally a
direct indication that the male was good at catching
crustaceans, because crustaceans have
carotenoids in them.
So they catch amphipods and shrimp and things like that,
and then reprocess the chemicals and they can make
orange with it.
That was an indication that a male was a good forager,
and so the female might select that male because then her
babies also would be good at catching food.
However, the male is dancing in front of the female,
and that makes him a sitting duck for Crenicichla (Pike
Cichlids) And as we'll see a little bit later in the lecture,
sexual selection involves a direct tradeoff between mating
success and survival, and these guys were displaying
frantically to get mating success,
at the risk of being snapped up by a predator,
and the ones that survived were the ones that simplified their
display behavior.
Okay?
So this all happened pretty quick.
Now how do we measure it?
Well currently--and there's been a bit of controversy about
this-- but currently the preferred
unit of measurement is a haldane,
and a haldane is a change in the mean value of the population
by one standard deviation per generation.
So I'm going to tell you what a standard deviation is,
and I'm going to tell you who Haldane was.
Haldane was the son of the Lord Admiral of the British Navy who
commanded the British Navy in World War One.
And he was a brilliant polymath.
He was fluent in Greek and Latin, as well as mathematics
and biology, and he did foundational work in
biochemistry and on the origin of life,
as well as in population genetics.
For many years he was a professor at University College,
London.
He was a Communist and a Socialist,
and a social reformer, who had a romance with the
Soviet Union and then became bitterly disillusioned when he
discovered what had gone on in the gulags in the 1950s.
When he got intestinal cancer he retired from his position in
London and he took a job in India and he taught a whole
generation of population geneticists in India before he
died in 1962.
A very interesting guy, and actually there is a lot
about the social impact of science that you can learn from
reading about J.B.S. Haldane.
The biography is just called JBS.
This is a standard deviation.
It is an empirical observation, supported by an elementary
theorem of mathematical statistics,
called the Central Limit Theorem, that most population
distributions look like a bell-shaped curve.
It's called the normal distribution.
It was formalized by Gauss; sometimes it's called a
Gaussian distribution.
And the shape of the curve and its spread are basically
measured by the standard deviation.
So the mean value is at the center here and the distribution
is theoretically symmetrical-- in practice it's
quasi-symmetrical-- and the degree of spread is
measured by the standard deviation units.
So within 1 standard deviation you will find,
on each side, 34.1, or with both 68.2% of all
of the individuals observed in the population.
So 1 haldane basically would take a population that say had a
mean value-- suppose it was for body size,
maybe a body size of 10 grams-- and if it had a standard
deviation of 2 grams, and it was evolving at a rate
of 1 haldane, it would move that mean from
here to here; 1 standard deviation unit up,
and the population mean in the next generation,
instead of being 10 grams, would be 12 grams.
So that's the meaning of the haldane.
Here are some measured haldanes.
Okay?
So for those guppies in Trinidad, that were evolving
pretty fast, the number of spots in the area
of orange spots-- when you took away the predator
and suddenly being brightly colored wasn't risky anymore and
females liked it; so it was good to be brightly
colored--those spot numbers increased quickly.
They were increasing at about .7 haldanes.
In the Galapagos finches that Peter and Rosemary Grant
studied, they go through El Nino,
and during El Nino-- it's a strong selective event,
so about every ten years there's a strong selective event
on the Galapagos finches-- and during El Nino they were
evolving at about .7 haldanes in body size;
they're getting bigger.
And then in the other years they were getting smaller.
So they fluctuate, they go up and down,
depending upon the El Nino conditions in the Galapagos.
There have also been lots of measurements of slower rates;
for example, since the extinction of their
competitors in the late nineteenth century the surviving
Hawaiian honeycreeper, the I'iwi has been evolving a
shorter bill, and that's been a very slow
rate of evolution.
The migratory timing of Columbia River salmon has been
changing as a result of the human fishery on them.
All of the fished populations of the world are evolving under
the pressure of human fishing.
Most of the fish in the world are getting smaller.
Many of the stocks are collapsing.
It's producing a change in the time of year that the Columbia
River salmon run up the Columbia.
This is also due to the building of dams on the
Columbia.
So this is a human induced selection process.
These are fairly slow rates.
So what does this mean, if we just try to think about
these rates and evolutionary times?
A Galapagos finch is about 25 grams, about the size of a house
sparrow.
They evolved during El Nino at about half a gram a year.
What if the El Nino conditions persisted forever?
What if it wasn't the southern oscillation that was driving the
rainfall pattern in the Galapagos?
What if it just stayed warm and wet for a long time in the
Galapagos?
Well that would produce directional selection,
and if you did it for a hundred years, it would turn a 25 gram
finch into a 75 gram finch.
Basically it would take a finch and turn it into a small robin.
Okay?
If you did it for 10,000 years, it would turn it into a turkey.
Now finches as big as turkeys don't do very well in a finch
habitat.
They are living in a place where they hop around in bushes.
They are living in an environment in which food is
sometimes very hard to come by.
I've been observing the turkeys that live near my garden in
Hamden, trying to get up into the trees
next to Lake Whitney to pick the berries off as winter has come
on and it's gotten very cold.
They're pretty clumsy.
So what will happen if you keep a strong directional selection
going on finches?
What would happen to humans if there were strong directional
selection on humans to increase in body size?
What would happen if we got turned from say 50 to 80 kilo
primates into three-ton primates?
How long could that go on?
One of the fastest rates of evolution ever measured in the
fossil record was when elephants went onto islands in the
Mediterranean and turned from twelve-ton elephants down into
things about the size of a Saint Bernard.
Okay?
They did it in less than 100,000 years.
They did it because they were food limited and they'd been
released from predation pressure.
Okay?
So how far can that process go?
These are quick changes that we're describing.
The finches are moving pretty fast.
The guppies are moving pretty fast.
The elephants change pretty quickly.
But if you look over the whole spread of evolutionary time,
over hundreds of millions of years,
things stay within a fairly narrow envelope of body sizes.
Why does that happen?
So if we look at microevolutionary rates--and by
the way there are good papers on this.
If you're interested in rates, this is a good paper topic.
Umm, lots of measurements, lots of argument about why.
They vary from very fast to very slow.
The fastest are in the finches and in the Trinidad guppies.
There have been lots of rates measured in Hawaiian
mosquitofish and Hawaiian honeycreepers.
So there are lots of estimates available.
And interestingly, the shorter the period over
which the rate is measured, the greater the maximum rate.
So if you measure a rate by making comparisons between two
populations that have been separated for hundreds of years
or hundreds of generations, it's usually fairly slow,
and if you focus in and you just look at a brief period,
it can be very fast.
Why do you think that might be?
Why might we measure a faster rate when we do so over a
shorter period of time?
If we measure it over a short period of time,
sometimes it's faster.
If we measure it over a long period of time,
it's slower.
Does that suggest anything about what the pattern might
look like that I'm about to draw on the board?
Yes?
Student: >
So it can go up and down, and >
Prof: You got it, that's all it takes.
It just has to go up and down.
If I measure it over this period, it looks pretty fast.
If I measure it over this period, it looks pretty slow.
That's all it is.
Okay, the take-home message, from many studies done in the
'70s, '80s and '90s,
is that evolution can be very fast when populations are large
and selection is strong.
And the reason for that is that big populations have lots of
genetic variation.
So there's a potential for a big response to selection.
Small populations don't have so much genetic variation.
So even though selection might be strong, they can't respond so
well.
This point, the shorter the time interval over which you
measure the rate, the higher the maximum rate.
And here's one reason why you can't take Galapagos finches and
turn them into turkeys and then turn the turkeys into ostriches
and then turn the ostriches into moas and then have the moas turn
into tyrannosaurus rex.
Okay?
As you push things very far, in any direction,
there's an internal process that converts the directional
selection into stabilizing selection.
And those are the tradeoffs, the linkages among traits.
If you try to make a finch very large,
then although it may be gaining something in terms of say food
capturing ability, it is giving up maneuverability.
If you try to take elephants and make them very small,
then at some point they are not going to be able to compete with
other elephants for food supply, even though there may not be
any predators there.
There are all kinds of biomechanical linkages within
bodies where tradeoffs are involved.
So if you look within the organism,
you see that it's a bundle of linkages and compromises,
and every time you try to change one trait you have a
byproduct, you have an implicit selection
going on, on other traits.
So although you may be realizing a benefit in one,
or a place, you are paying a cost in the others.
The most striking example we've seen of it in the lecture so far
is the guppy, the male guppy.
If he evolves to be bright and a wonderful dancer,
so that females just love to mate with him,
he will get killed by a predator.
That is about as straightforward and brutal a
tradeoff as you can imagine.
Okay?
But these go on all over the place and some of them are very
subtle.
Now why is it that sometimes traits evolve very fast and
sometimes very slow?
This is a picture of clubmoss, lycopodium.
If I were to take you out into the woods of Connecticut in the
springtime, you would see them all over the
place, and if I were to put you a time
machine and take you back 400 million years,
they wouldn't look any different.
This is latimeria, this is a Coelacanth.
If I were to put you into a research submarine off the
Comoro Islands in the strait between South Africa and
Madagascar, between Malawi and Madagascar,
and we went down at night to a depth of 300 to 600 feet off the
volcanic slope of the island, we would find these guys
cruising around in mid-water.
They have spent the day in caves and they come out at
night, into the mid-waters of the
earth's oceans, and apparently they have been
doing this now for going on 150 million years.
They haven't changed at all.
By the way, they have an egg the size of an orange.
They're interesting.
They're, they're pretty effective predators too.
They are, uh, ambush predators.
They drift around and then they suck things into their mouths by
a big kind of vacuum suction device.
It's a common method of fish feeding.
So they're living fossils.
Now why haven't they changed?
Look at what's happened to their relatives.
The clubmosses had relatives at the time that looked about like
them, that since then have turned
into redwood trees, orchids, wheat fields--you name
it, these guys still look the same.
Latimeria had relatives that since then have turned into
marlin and reptiles and birds, mammals;
it hasn't changed.
So we have these two things to understand.
We have to understand how evolution can go really
fast--antibiotic resistance, guppies, finches--and why
sometimes it is so slow.
Any ideas on this one?
Is this the first time you've hit this problem of why
evolution is sometimes so slow?
Student: It finds a pretty stable way of living and
surviving, and sometimes way down below depths of the ocean
>.
Might not that change the effect of latimeria?
And the clubmoss are in >
for hundreds of millions of years, while
>.
Prof: Right.
Okay, that's one kind of explanation, and I think it's
certainly a plausible one.
It's not the only one, but it's certainly one kind.
So his argument is the reason these guys haven't changed is
that they're really good at always finding the same kind of
environment, so that they are never exposed
to change.
So if their environment moves around the globe,
they track it.
Now remember, between 140 million years ago
and now, the earth went through a huge meteorite strike,
the dinosaurs went extinct.
Heavy stuff happened back there at the end of the cretaceous,
and latimeria just cruised around and it hasn't changed
very much.
Now the argument is actually probably most convincing for
marine invertebrates, that make larvae that can go
out and spread through the ocean for thousands of kilometers.
And, in fact, we know from the behavior of
marine invertebrate larvae-- so now I'm talking about worms,
barnacles, clams, stuff like that--that
they like to settle on places where there are successfully
growing adults of their own species.
They smell that out very carefully, and that's where they
settle.
So basically the larvae are selecting the habitat in which
the adults will be selected by natural selection.
That means that they manage themselves to generate
stabilizing selection over hundreds of millions of years.
That's, and arguably latimeria has done the same thing.
It's been living in lava tubes on the sides of submarine
volcanoes at 300 to 1000 feet, for a long time,
and that habitat's always been around.
Any idea for another explanation of stasis?
That's an externalist explanation.
Okay?
It relies on aspects of the habitat and the way natural
selection is operating on the organisms.
Anybody got an idea for an internalist explanation?
Yes?
Student: There are genetic mechanisms that will
regulate DNA copying and improve >
application.
Prof: I very much doubt that a lack of mutations was
ever the reason that things didn't change.
You've got 4.6 in you that're new since your mom and dad,
for example.
Yes?
Student: All populations are a >
Prof: Well, yeah, the problem with that
over a long period of time Greg is that if it's really a small
population it's more likely to go extinct,
and these things are out there for hundreds of millions of
years.
So that one's a little difficult.
Other ideas?
Well there's a whole school of thought that says that this kind
of thing is due to developmental constraints;
that development has constrained the organisms so
that they couldn't evolve in certain ways.
And that's plausible for certain major features of the
body plan, that are determined very early
in development, and involve developmental
tissue relationships and things like that,
that are obviously hard to change.
It's not so plausible for some of the smaller details of these
creatures.
So I think that the actual explanation is probably a
mixture of these things.
There probably is some phylogenetic or developmental
constraint.
Things that happened a long time ago,
in the way organisms were built, are hard to change,
and they've been constraining the things that can change more
rapidly.
But I think you'll find, if you get into this,
that it's a huge and controversial literature on it.
Okay.
Kinds of selection.
Now we go through another one of these vocabulary building
exercises, and I'll try to illustrate a few of these.
But I just want to get these words out there and I want to
get them into your minds so that you can start to think about the
fact that natural selection comes in lots of different
flavors.
We can talk about directional, stabilizing and disruptive
selection; natural and sexual selection;
frequency dependent selection; and then selection acting on
individuals, on kin, on groups and on species.
So each of these is cutting the selection cake in a different
direction; but it is all of these
different things.
So, directional, stabilizing and disruptive.
Basically what's going on with directional selection,
that's making the Galapagos finch into a turkey,
is that the fitness gradient is linear.
That means that if the fitness of something over here is low
and up here is high, that means that natural
selection is selecting for say bigger things--
this body size on the X-axis is going to the right--
and it will take a distribution that looks like this and it will
move it to the right.
So if this is 1 standard deviation, then this amount of
movement is 1 haldane, right here.
Stabilizing selection is actually what we were just
invoking to argue that the coelacanth didn't change.
It was living in a habitat where it was always good to be
like a coelacanth, and natural selection was
selecting out things that didn't look like coelacanths;
whether they were larger or smaller, or their fins were
different shapes, or things like that.
So they tended to stay the same.
That means that we were selecting for the mean of the
population and we were discarding the extreme values.
Who in the room is under 5'5, and also who in the room is
over 6'1?
Raise your hands please.
Okay, if there's stabilizing selection on human height--you
guys have no grandchildren.
Can I see the hands of everybody else?
Hey, you made it.
Okay?
That's stabilizing selection.
It means selection for the mean value, and it's selection
against the extremes.
Be happy that that doesn't appear to be the only thing
going on in humans.
Disruptive selection is selection against the mean and
for the extremes, and it will take a bell-shape
curve like this, it will knock out the mean
value, and then the next generation it will push it apart
like that.
Okay?
So if we look for examples, strong directional selection
will produce very rapid evolution.
We saw that with antibiotic resistance and the guppies.
It can't continue.
It usually gets converted into stabilizing selection.
Disruptive selection causes, historically,
things like the conversion of similar looking gametes into
quite different gametes.
So disruptive selection was involved in the origin of eggs
and sperm, back in the day,
about a billion years ago, and it may play a role in
sympatric speciation; which we will come to,
um, probably in mid-February.
So just remember that.
Disruptive selection is selection to take a population
that has a certain mean value and split it in half and turn it
into two different things.
Now, natural and sexual selection.
We've referred to sexual selection with the guppy.
The classic example of sexual selection is a peacock's tail.
This is actually what inspired Darwin to come up with the
concept.
He said, "Look at that peacock.
There isn't any reason, from the point of view of
survival, for a male peacock to be that
colorful, and have that big a tail,
and have this absolutely exotic behavior of dancing around,
waving its tail."
And, in fact, if you look at the birds of
paradise, the amazing thing about the
birds of paradise is not really their feathers,
it's what they do with their feathers.
They do fan dances with their feathers.
They can do the rumba, they can shake,
they can rock and roll.
They do all kinds of stuff, and they're all dangerous,
because they're out there displaying and predators could
come along and eat them.
Okay?
In fact, peacocks are eaten by tigers, or they were eaten by
tigers before the tiger just about went extinct in India;
they're down to a few hundred in India, and the Siberian tiger
is under threat right now in Siberia.
But the tigers traditionally ate peacocks.
They really did.
So the display behavior was dangerous.
So what the male was doing was he was trading off survival for
mating success.
He was a victim of female preferences.
>
Don't tell that to the fraternity guys,
okay?
So, sexual selection is a component of natural selection.
Natural selection is all about variation in reproductive
success, and you can achieve
reproductive success by mating and by surviving and by doing
other things.
Okay, so it's a component of natural selection.
And the tradeoff involved is survival versus mating.
It's driven by two things.
Either ma--either the males are competing with each other for
access to females, or the females are conniving
against each other for access to males--
one of those processes may drive intelligence a little bit
more than the other-- and it's also driven by members
of one sex choosing mates of the other sex.
So we're going to have a whole lecture on sexual selection.
It's often fun to write a paper on this topic.
There are several criteria that one sex might use in choosing a
mate.
One is a direct benefit.
So with birds that would be, "Oh, that male's got a
really good territory, it's got a lot of food in it;
therefore I could have a lot of babies and raise them there,
so I'll go live in that territory."
Not so directly looking at the male, just saying,
"Oh, he happens to hold that territory."
That would be a direct benefit.
Or you could say, "Oh my goodness,
isn't he sexy?
If I mate with him, my sons are going to be sexy
too."
>
That's called the sexy-son hypothesis,
and actually it does appear to drive some of the more
extravagant displays, and is probably responsible for
the evolutionary shaping of the peacock's tail.
A third hypothesis is, "Oh he's resistant to
disease, and he happens to be wearing a
piece of morphology, that I can detect externally,
that tells me that he's resistant to disease;
because it's expensive to produce and only resistant males
are capable developmentally of producing it."
There's an interesting principle involved in that.
Basically it is that honest signals are costly.
Okay?
And if disease resistance is costly and you can advertise
your resistance with a signal that you are disease resistant,
then that could be something that a female preference might
then evolve to notice.
We'll go into that, but you can see immediately
that if the signal is not costly,
then it can be invaded by cheaters,
and then as soon as there was cheating going on,
the female preference would erode, because there wouldn't be
any point to having that preference;
you were getting cheated on too frequently.
Yes?
Student: I just have a question about the sexy-son
hypothesis.
Prof: Yes.
Student: It seems like it implies a certain psychology
in the mother that's kind of expensive to have.
Prof: Yes it does, doesn't it?
And isn't it interesting that things that we find beautiful
evidently are also preferred by female birds and by bees that
are locating flowers and things like that?
It implies a whole set of innate preferences in choice.
It doesn't imply necessarily consciousness;
I mean, you can build robots that will do this.
But it does imply a fairly costly choice apparatus,
which appears to have evolved.
Frequency dependent selection is another kind of selection,
and that happens whenever the advantage of doing one thing
depends on what the other people in the population are doing.
Okay?
There are some classical examples of this.
One is the classical 50:50 sex ratio, and another is genetic
diversity for immune genes.
I'll just say a few words about genetic diversity for immune
genes, because we're going to come back to sex ratios when we
do sex allocation theory.
Let's suppose that you have a gene that is resistant to a
particular disease, and therefore your offspring
survive better and you have more grandchildren,
and this gene then spreads through the population until
eventually most of the people in the population are resistant.
That means that there's selection operating on the
disease to come up with a variant that can overcome that
resistance, and when that variant comes up,
it will spread until it is common,
and it creates selection to cause the same thing going on in
the host population, and back and forth it goes.
The more frequent something becomes,
the more it's subject to very strong negative selection,
and the less frequent it becomes, the more it's protected
from being selected, because things that are rare
aren't very good resources; things that are common are
great resources.
And so what happens is that you get what is now recognized as a
classical oscillation of virulence and resistance between
the host and the pathogen.
One of the most interesting things about a human immune
system is that the MHC or HLA genes that mediate this kind of
resistance against pathogens have some of the highest genetic
diversity of any genes anywhere.
It looks like variants, rare variants,
have been selected again and again and again.
So every time something becomes frequent,
it becomes useless and another rare one is selected,
and eventually a huge supply of variation builds up in the
population.
So this principle really has had quite a role to play in the
selection of the vertebrate immune system.
Okay, group selection and species selection.
I'll go through this fairly quickly.
We're going to come back to this issue when we do behavior
in, um, April.
Okay?
But group selection--here's an example of group selection.
A bunch of partridges get together in Scotland in late
fall.
They look around.
They notice that there are just a tremendous number of
partridges in Scotland in late fall,
and they think--you know, speaking anthropomorphically--
they think, "Oh, there are too many partridges.
Therefore we will all cut back on our reproduction so that our
population does not go extinct."
That's an example of group selection.
It won't work because over in the corner is sitting Joe
Partridge, who looks at all of these guys and says,
"You're idiots.
You're cutting back on your reproduction.
I'm going to have 50 babies."
>
Group selection is vulnerable to selfish mutations;
selfish mutants invade.
Okay?
So they invade for a variety of reasons, and we'll work through
all of that.
But group selection is not stable.
Selfish mutants invade.
They do so.
There's a lot of selective events in genes and individuals
for each selective event of a whole group.
And if you extend group selection up to species--
how many times have you guys ever heard,
perhaps on Discovery Channel or BBC or National Geographic,
that behavior X or morphology Y exists for the good of the
species?
Have you ever heard that?
Yes, it happens a lot.
It's bullshit, just plain bullshit.
Okay?
Things don't exist for the good of the species.
Things exist because individuals outperformed other
individuals in the competition for reproductive success.
Now there is some large-scale differential species selection
that occurs on the phylogenetic tree,
and it shapes patterns at a big macro scale across the tree.
One of them is sex.
Virtually all asexual things are relatively young and they
had sexual ancestors.
It appears that sex reduces the probability of extinction,
and that asex makes you more extinction prone.
So that is a kind of species selection.
But it's not a selection for a precise adaptation.
There's no way that species selection could have ever
designed the vertebrate eye, the vertebrate brain,
any of the detailed, precise, complicated mechanisms
that we know of in biology.
All that stuff has gone on because of individual and gene
selection.
Some of the big macro evolutionary patterns have been
generated by a kind of species selection.
For example, the fact that dinosaurs aren't
here anymore and that mammals dominate the earth is a kind of
selection.
It doesn't tell you about how fast the mammals run,
why they are warm-blooded, ta-da da-da da-da.
Dinosaurs were warm-blooded too.
Okay, we can classify selection a number of different ways.
Each one, each of the methods of classification,
highlights a distinction.
Selection can be strong and the response can be fast,
but some traits evolve very, very slowly.
And you need to be able to hold those two facts in your mind,
and have intellectual tools that will allow you to deal with
both situations.
Okay, next time Neutral and Maladaptive Evolution.
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