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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.
