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OK, so the last lecture we were
talking about competition,

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and the result of competition
between organisms.

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And I didn't quite finish,
so I just want to finish up on that

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lecture, and then will move on to
predation, an interaction between

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two species which results in
increasing the fitness and one,

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and decreasing the fitness in the
other. But let's just finish up

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with competition.
We were talking about character

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displacement and beak depths.
And we have shown that when you

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have two closely related birds with
the same beak size,

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if they live on islands separate
from one another,

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they compete for the same food.
But if you live on an island

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together, what happens is that you
have this character displacement

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where the beak of what will get
bigger and the beak of the other

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will get smaller.
So they can exploit different food

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resources. And this character
displacement is the beginning of

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species formation.
And this is very common on islands.

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And these are the famous finches
from the Galapagos Islands that

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Darwin first based his theory of
evolution, or it was an important

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part of his theory of evolution,
where he realized that there must

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have been an ancestor finch,
that all of these other finches that

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had different representation on
different islands evolved from

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through the slow change in traits
that is selected as a result of

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different combination of species
being together or alone in different

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islands. And this is what's called
adaptive radiation.

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And this is really a powerful
mechanism for evolution.

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This is the Galapagos finches,
but you also see this in an extreme

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form in Hawaii among the honey
creepers showing this incredible

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diversity of beak types that have
evolved to exploit different

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kinds of food.
OK, so before we go on to predation,

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I just want to walk you through an
evolutionary scenario,

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so that you can see how this might
work on an island,

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Archipelago. So this is the
mainland, and we're going to start

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with an ancestor species,
A, and our Archipelago is going to

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have three islands.
OK, let me draw this.

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Here's the same three islands.

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So this is time passing.
So as we move,

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these are the same three islands as
a function of time passing,

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OK? So at the beginning, we're
going to have our founder species

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flying and colonizing the island.
OK, so A is on this island. And so,

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the first thing that happens is that
A evolves and becomes B.

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So each one of these is just the
name of the species,

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OK? This is species A,
species B, through what's known as

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the founder effect.
And that is when you have a few

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members of the species colonizing on
this island. You have a tiny gene

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pool, and the genetic composition of
this drifts such that it actually

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becomes a different species from the
one on the mainland.

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So, A becomes B.
So the full arrow means becomes B

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on the islands through the founder
effect. And now,

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we're going to have B migrate to a
new island. So,

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A is now B on this island,
right?

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And B is going to colonize this
island. And through that same

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founder effect,
B becomes C. So let me just write

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what's happening here.
B becomes C. I think you can see

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what's happening on one island,
and then it migrates back.

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So, let's let C migrate back to
where B is, and C also founds a new

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island. So, we're going
to let C migrate here.

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So we end up with C and B here now
able to compete with each other,

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C here and C here. OK, so we've got
a new combination of species,

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and we are going to let C become D
here due to the founder effect,

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and D is going to migrate over to
this island.

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So, C becomes E when with B because
of character displacement.

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So, C becomes E here when it's with
B. So you end up with E and B on

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this island. D is going to migrate
to this island.

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So, we have C and D here.
And, C has evolved to be D here.

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So, we also have C becomes D when
alone.

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OK, so this is just a scenario we
are making up.

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OK, you can make up any scenario.
But it's to give you the idea of

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how this adaptive radiation comes
about. So, starting with one single

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species on the mainland,
if you have an island archipelago

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where species can be isolated enough
from each other to restrict,

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but not completely eliminate,
the gene flow you can have this

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rapid adaptive radiation.
So, evolutionary biologists often

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study islands in order to study this
phenomenon. OK,

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so just to summarize for competition,
interspecific

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competition results in

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either competitive coexistence,
which can be achieved either through

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niche differentiation.
Do you remember,

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what was an example of that last
time? The barnacles and the inner

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tidal. One took the high road,
and one took the low road. That's

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called niche differentiation,
or character displacement.

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And this can happen very rapidly.
I hesitate to tell you about really

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interesting things that you don't
have to know, but I will anyway.

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This is really interesting study by
a couple at Princeton of the

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Galapagos finches.
And they've shown that you can have

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character displacement of the
islands over a period of one or two

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years, just depending on
the amount of rainfall.

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So, it can happen very rapidly by
just selecting for different

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character traits,
or competitive exclusion,

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which is the case where the niche
overlap is so great that one of the

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species completely outcompete
the other.

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The example from last time was the
zebra mussels and Gause's paramecia

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in the test tubes excluding the
other. So, these are really

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important ecological and
evolutionary forces.

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The other thing I learned from your
comments, which I was very heartened

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by, is that at least one person
really liked the definition of the

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niche, of the N-dimensional hyper
volume, which I have always loved

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and I think is fundamentally
important because people throw

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around ecological niche in everyday
language and think of it as a place

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in the environment.
And I think that a more robust

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definition is so much more useful.
So, let's move onto predation,

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which is a very strong evolutionary
force. It can control population

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dynamics. It can shape community
structure. We're going to talk

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about each of these.
It actually influences competition,

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which in turn shapes community
structure. And it's a powerful

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evolutionary agent.
In other words,

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it's very important in influencing
naturally selection.

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So, let's start with the classic,
here's a classic predator-prey

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interaction. There is two wolves
and a moose being attacked.

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The wolves are not evil. They're
just getting their meal.

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OK, this is a very famous classic
study of the snowshoe hare,

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and lynx. Lynx is a cat:
populations in northern Canada.

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And these data were collected by
the Hudson Bay Company trapping

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records. So, it's really from the
amount of animals that were actually

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trapped that these abundances come
from. And that's where these

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coupled oscillations,
this is the hare, and this is the

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lynx showing these couples
oscillations with a roughly

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11 year cycle.
And people spend a lot of time

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trying to understand what was
driving that oscillation.

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And mathematicians love these
coupled oscillators.

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And so, the first attempt to model
this kind of thing was using a very

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simple model. Remember we said the
dN/dt equals rN?

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That's our exponential growth
equation. And we also said before

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the dN/dt or that r is equal to the
birthrate (b) minus the

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death rate (d).
OK, so grossly oversimplifying the

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system, the first set of equations
that were used to try to describe

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this, are if you use for the
predator, you call the predator

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dN1/DT equals b1,
1 minus d1,N1. OK,

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so that's just this equation.
And they said,

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well, how do we modify this equation
so that the growth rate of the

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predator is somehow a function of
the density of the prey?

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Well, the easiest thing to do is
just make it proportional,

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right? So what they do is put N2 in
there. So, the prey population is

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going to be dN2/dt,
and we're going to the same thing:

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b2,N2 minus d2,N2,
and the question is how do we modify

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the prey growth rate equation so
that it is somehow a function of the

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density of the predator?
So what would you do? So here,

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this says the birthrate of the
predator is influenced.

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As prey density increases,
the birthrate of the predator

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increases.
How would you modify this equation

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so that the predator density has an
effect on it? It's pretty obvious,

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but I'm trying to get you with me.
Exactly. It would be the death term.

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As the predator numbers increase,
the death term would go up.

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And it turns out that these are two
coupled differential equations that

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make a beautiful oscillatory system,
a couple oscillators, perfect

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oscillators like that. This
is predator density.

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So that would be N1,
N2, and this is time if you chart

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their density with time.
And how many of you have actually

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had this in the course?
Yeah, see? [LAUGHTER] And what

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courses have you had in it?
Differential equations, yeah.

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Mathematicians love it. But real
populations, predator and prey,

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don't really operate this way.
I mean, you see these oscillations,

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but rarely can it be attributed
solely to the interaction between

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the predator and the prey.
There's usually many other factors.

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So, and I'll just give you one
example. So ecologists go ahead and

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say, OK, so what's really going on?
And here's an example. It turns

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out that in many cases,
the food supply of the prey is

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oscillating.
OK, so the availability of food to

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the prey oscillates,
making the prey population oscillate,

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which then can drive an oscillation
in the predator population.

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So, it's more than just the
coupling between the two.

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There are also an external
oscillators driving both of them.

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And again, I'd like to try to
address the role of

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experimentation.
Here is an experiment done with

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rabbits. And I don't know what the
predator was. It might have been,

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I'm not sure the predator was, but
anyway, rabbits,

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in which through experimentation
they increased the food supply to

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the rabbits through fertilization.
And they showed that, so this is

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the control.
And this is the phase of the cycle

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in the hare population relative to
the density in the controls.

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So, showing that when the food
supply was increased,

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the oscillation was still there.
So, it wasn't only relieving the

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rabbits of food limitation,
did not eliminate the oscillation,

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but if you'd excluded the predators,
you still also had the oscillation

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there.
And if you did both of them,

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it increased the amplitude of the
oscillation, relative to the control.

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So the conclusion from this is that
both food supply and the predation

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affected the oscillation.
Another very classic experiment is

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the experiment by Huffaker back when
people began to be enamored of these

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coupled differential equations.
People wanted to test the hypothesis

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of those equations in the laboratory.
And they tried to set up

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predator-prey systems in the lab,
and see if they can get them to go

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in these coupled oscillations for
many cycles. And Huffaker set up a

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system of a predatory mite
and a prey mite.

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Ecologists like to use insects as
experimental systems because they're

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small and you can do it in the lab.
And this one lived on oranges.

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So it was an herbivorous mite.
And the predator obviously lived on

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mites. And he set up a very simple
system in the lab of oranges,

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and introduced the predator and prey,
and inevitably he got this,

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where prey would increase, and then
the predator would increase,

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and would overshoot, and the prey
would die, and the predator

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would die.
So he only got one cycle,

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with a simple system could not get
this to persist,

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well, that doesn't even qualify as a
cycle. So he realized,

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so this is a simple system.
And he had a grid,

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which is shown up here in which he
had oranges. And he had them

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interspersed with rubber balls to
have a little bit of complexity in

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the system. But he found with that
design, he could not get the system

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to persist. So he hypothesized that
the reason it wouldn't persist is

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that it's much too simple,
not close enough to nature.

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So he introduced all kinds of
complexity. He increased the size

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of his grid relative to the
populations, which would give the

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prey mites more of a chance to get
away from the predator.

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He put barriers of dispersal,
like Vaseline moats, and he actually

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put little launching pads for the
prey. I don't know what they look

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like, diving boards,
I don't know what they were,

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but little launching pads that just
increased a lot of complexity so

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that it gave the prey an ability to
move around relative

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to the predator.
And he was actually able to,

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this shows his results over 200 days.
He was actually able to get three

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full cycles of the predator-prey
oscillation. And up here,

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it just shows you that they're sort
of a cat and mouse game going on.

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It shows you the location of the
prey mites relative to the predator

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mites at these different points in
time of this cycle,

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showing that they're moving around
the system, and that the complexity

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allowed this coupled oscillator to
persist. And of course,

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we present this because it was a
classic experiment.

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It was a pioneering experiment,
but I mean, there have been a lot

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more since then.
And of course,

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it has implications for stability of
populations in the natural world.

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As we make the natural world less
and less complex,

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00:24:43,000 --> 00:24:48,000
it reduces the ability of
populations that are engaged in

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00:24:48,000 --> 00:24:52,000
these coupled oscillatory systems to
persist. It increases the

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likelihood of extinction.
So this was like a tiny little

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localized extinction in his
experimental system.

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Another classic example that's often
cited about the role of predation in

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00:25:15,000 --> 00:25:28,000
regulating population dynamics has
to do with rubber plantations over

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here in Malaysia.
Did I spell that right?

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That looks wrong. Oh well,
is that OK? And I don't have any

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data for you. So this is just a
story, but it's very compelling,

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and there is data somewhere but I
don't have a slide.

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But, in the first half of the
century, in its rubber plantations

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in Malaysia they had a tremendous
diversity of insects,

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00:26:04,000 --> 00:26:09,000
but didn't have any real serious
problems with insect pests in the

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00:26:09,000 --> 00:26:15,000
rubber plantations.
And, in 1950, they had a small

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00:26:15,000 --> 00:26:21,000
outbreak of defoliated caterpillars.
And that was right about the time

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that DDT was invented and
synthesized, and became available.

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00:26:27,000 --> 00:26:33,000
And so, entomologists came down and
sprayed the plantation with DDT

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00:26:33,000 --> 00:26:40,000
hoping to get rid of
this caterpillar.

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00:26:40,000 --> 00:26:44,000
In the next year,
the outbreak got bigger.

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00:26:44,000 --> 00:26:48,000
They sprayed more. The next year
the outbreak got bigger.

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00:26:48,000 --> 00:26:52,000
They sprayed more, and finally the
whole thing was out of control.

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00:26:52,000 --> 00:26:56,000
And they said what's going on here?
We are killing these things, and

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00:26:56,000 --> 00:27:00,000
it's getting bigger.
To make a long story short,

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00:27:00,000 --> 00:27:04,000
what they were doing, this is a
cocoon.

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Oh boy. How do you spell cocoon?
That's not right. Is that right?

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00:27:12,000 --> 00:27:21,000
Anyway, you know what I mean.
Caterpillars live in cocoons,

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00:27:21,000 --> 00:27:29,000
and there was a, that's a wasp with
a big, long organ that

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00:27:29,000 --> 00:27:36,000
it lays its eggs.
It needs some legs,

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00:27:36,000 --> 00:27:40,000
doesn't it? OK, I made some front
legs too. There.

250
00:27:40,000 --> 00:27:44,000
OK, so that's a wasp that lays its
eggs in these caterpillar cocoons,

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and in doing so, kills caterpillars.
And what they were doing with the

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00:27:49,000 --> 00:27:53,000
DDT, is that they were killing the
natural predator of the wasp.

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00:27:53,000 --> 00:27:57,000
And the caterpillars themselves,
while they were in the cocoon, were

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00:27:57,000 --> 00:28:02,000
actually protected from the DDT.
So the more they put on,

255
00:28:02,000 --> 00:28:07,000
the more they killed the wasp,
and the caterpillars were released

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00:28:07,000 --> 00:28:13,000
from this controlling predation,
and they had huge outbreaks. So,

257
00:28:13,000 --> 00:28:18,000
it's another example of in nature
it's really hard,

258
00:28:18,000 --> 00:28:23,000
you can't see what's controlling
what until you disrupt it.

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00:28:23,000 --> 00:28:28,000
You have to experiment either
inadvertently or on purpose because

260
00:28:28,000 --> 00:28:34,000
everything is dynamic
and turning over.

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00:28:34,000 --> 00:28:40,000
But it looks relatively stable.
And that's why this is so hard.

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00:28:40,000 --> 00:28:46,000
And this is one of my favorite
examples of this,

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00:28:46,000 --> 00:28:52,000
because it brings together a lot of
concepts that we've been talking

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00:28:52,000 --> 00:28:59,000
about, is predation shapes
community structure.

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00:28:59,000 --> 00:29:12,000
And this is another example of

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00:29:12,000 --> 00:29:22,000
introduced species.
And this is St. John's Wort,

267
00:29:22,000 --> 00:29:32,000
which was introduced at California
from Europe.

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00:29:32,000 --> 00:29:44,000
And, it, so I'm going to draw a
couple of habitats here.

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00:29:44,000 --> 00:29:56,000
This is a forest, and this is a
meadow. So this is grass,

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00:29:56,000 --> 00:30:06,000
OK, this is trees.
And these are St.

271
00:30:06,000 --> 00:30:14,000
John's Wort. So, it could grow
equally well in the meadow and in

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00:30:14,000 --> 00:30:23,000
the forest. And it was getting out
of control so they introduced a

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00:30:23,000 --> 00:30:31,000
beetle from Europe that feeds on St.
John's Wort to try to bring it in to

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00:30:31,000 --> 00:30:41,000
under control.
And what they found was that the

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00:30:41,000 --> 00:30:51,000
beetle, because the beetle's
preferred habitat was the meadow

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00:30:51,000 --> 00:31:01,000
that St. John's Wort persisted in
the forests but was eliminated

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00:31:01,000 --> 00:31:08,000
from the meadow.
Now, because the beetle prefers the

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00:31:08,000 --> 00:31:13,000
sunny habitat of the meadow,
so if you were an ecologist and you

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00:31:13,000 --> 00:31:19,000
didn't know that this beetle had
been introduced,

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00:31:19,000 --> 00:31:24,000
you just walked into this state,
you knew nothing about the history,

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00:31:24,000 --> 00:31:29,000
you wanted to study the ecological
niche of St. John's Wort,

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00:31:29,000 --> 00:31:35,000
you would say, well, I only find it
in the forest.

283
00:31:35,000 --> 00:31:40,000
It must like cooler,
wetter environments because you

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00:31:40,000 --> 00:31:46,000
wouldn't really know that it was
being controlled by the presence of

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00:31:46,000 --> 00:31:52,000
this beetle. So, this
is a perfect example.

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00:31:52,000 --> 00:32:03,000
If we draw the niche or two
dimensions of the ecological niche

287
00:32:03,000 --> 00:32:15,000
of St. John's Wort,
and we say that just on these two

288
00:32:15,000 --> 00:32:27,000
dimensions that this is the
fundamental niche on the moisture

289
00:32:27,000 --> 00:32:36,000
light gradient,
in the presence of this beetle,

290
00:32:36,000 --> 00:32:42,000
the realized niche is only this low
light, high moisture environment

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00:32:42,000 --> 00:32:48,000
that you find in the forest.
OK, so in other words, to really

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00:32:48,000 --> 00:32:54,000
understand what's regulating the
ecology of a particular organism,

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00:32:54,000 --> 00:33:00,000
you have to understand all of the
other organisms that it's

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00:33:00,000 --> 00:33:06,000
interacting with,
and what their effect is on this.

295
00:33:06,000 --> 00:33:11,000
And again, that's why it's so

296
00:33:11,000 --> 00:33:16,000
impossible to do this without some
form of experiment,

297
00:33:16,000 --> 00:33:21,000
either manipulative experiments like
I described, or experiments done in

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00:33:21,000 --> 00:33:25,000
a lab, or inadvertent experiments by
introducing species.

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00:33:25,000 --> 00:33:30,000
OK, now let's talk about a couple
of other really classic experiments

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00:33:30,000 --> 00:33:35,000
that have been done.
And these are experiments that have

301
00:33:35,000 --> 00:33:40,000
illustrated the concept of keystone
predator. There are some predators

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00:33:40,000 --> 00:33:45,000
and ecosystems that are what are
called keystones.

303
00:33:45,000 --> 00:33:50,000
And that is if you remove them,
the entire structure of the

304
00:33:50,000 --> 00:33:55,000
community changes.
There are some that if you move

305
00:33:55,000 --> 00:34:00,000
them, it doesn't have a dramatic
effect, but there are certain ones

306
00:34:00,000 --> 00:34:06,000
that are keystone predators
that it does.

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00:34:06,000 --> 00:34:12,000
And these, of course,
are species that conservation

308
00:34:12,000 --> 00:34:19,000
biologists want to first identify,
and second conserve above others

309
00:34:19,000 --> 00:34:26,000
because there is a cascade of
effects if something happens to them.

310
00:34:26,000 --> 00:34:33,000
A classic example of this was a
study by Robert Payne many years ago

311
00:34:33,000 --> 00:34:41,000
in the inner tidal community.
I'm not going to go into the details

312
00:34:41,000 --> 00:34:51,000
of that, but the rocky inner tidal
community is made up of the starfish,

313
00:34:51,000 --> 00:35:01,000
which is called pisaster,
one of the top predators.

314
00:35:01,000 --> 00:35:09,000
And there are also limpets,
chitins, if you grew up in

315
00:35:09,000 --> 00:35:17,000
California you know probably what
these are, mussels.

316
00:35:17,000 --> 00:35:25,000
These are invertebrates-like
barnacles that stick to the ground,

317
00:35:25,000 --> 00:35:33,000
or stick to the rocks, and also
algae that stick to the rocks.

318
00:35:33,000 --> 00:35:38,000
And Payne hypothesized that the
predator was maintaining this

319
00:35:38,000 --> 00:35:43,000
diversity. And the way to test that
hypothesis was to put a cage over

320
00:35:43,000 --> 00:35:49,000
everything and eliminate the
predator from certain areas.

321
00:35:49,000 --> 00:35:54,000
At what he showed was that that's
what that ecosystem looks like if

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00:35:54,000 --> 00:36:00,000
you eliminate the predator.
You can see here, here's the algae.

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00:36:00,000 --> 00:36:05,000
Here are some articles.
This is a new textbook,

324
00:36:05,000 --> 00:36:10,000
see if you get the full story there,
but there's a lot of diversity of

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00:36:10,000 --> 00:36:15,000
the small barnacle-like
invertebrates.

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00:36:15,000 --> 00:36:20,000
Then you eliminate the predator,
and the mussels just completely take

327
00:36:20,000 --> 00:36:25,000
over. And this is preemptive
competition. They compete with

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00:36:25,000 --> 00:36:30,000
everything else for space and
nothing else can survive there.

329
00:36:30,000 --> 00:36:36,000
Another example of a keystone
predator is the sea otter,

330
00:36:36,000 --> 00:36:42,000
which keeps the sea urchins in check
in the bottom of the substrate,

331
00:36:42,000 --> 00:36:49,000
and if the sea otter is not there
the sea urchins takeover and they

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00:36:49,000 --> 00:36:55,000
exclude the kelp,
the entire kelp forests and all the

333
00:36:55,000 --> 00:37:02,000
fishes that lives in the kelp
forests, and all of the diversity of

334
00:37:02,000 --> 00:37:08,000
the ecosystem relies on the sea
otter's ability to keep the sea

335
00:37:08,000 --> 00:37:15,000
urchin population in check.
OK, so some of the best evidence for

336
00:37:15,000 --> 00:37:22,000
predation as an evolutionary agent,
or something that's driving the

337
00:37:22,000 --> 00:37:30,000
natural selection of organisms,
are these defenses that have evolved

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00:37:30,000 --> 00:37:36,000
to avoid predation.
In other words,

339
00:37:36,000 --> 00:37:41,000
if you're constantly under attack,
your fitness will be increased by

340
00:37:41,000 --> 00:37:46,000
features that reduce your
susceptibility to predation.

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00:37:46,000 --> 00:37:52,000
So in this case, there are a lot of
experiments. I'm going to show you

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00:37:52,000 --> 00:37:57,000
a few, but a picture's worth a
thousand words here.

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00:37:57,000 --> 00:38:03,000
I'll show you some examples.
Cryptic coloration is when a prey

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00:38:03,000 --> 00:38:09,000
organism has color features that
make it blend in to the environment.

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00:38:09,000 --> 00:38:15,000
So here's a very famous example
that was actually in another

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00:38:15,000 --> 00:38:21,000
inadvertent experiment.
This is a moth that comes in two

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00:38:21,000 --> 00:38:27,000
forms. This is called the melanic
form, which is dark black or gray,

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00:38:27,000 --> 00:38:33,000
and this is the other form which is
much lighter.

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00:38:33,000 --> 00:38:37,000
And here it is on a birch tree.
You can see that this one blends it

350
00:38:37,000 --> 00:38:42,000
beautifully, whereas this one,
if I were a predator looking for the

351
00:38:42,000 --> 00:38:47,000
moth, I would see this but I
wouldn't see that.

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00:38:47,000 --> 00:38:52,000
Here's the same two moths on
another tree with darker bark

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00:38:52,000 --> 00:38:56,000
showing that in this case,
this one would be more fit in that

354
00:38:56,000 --> 00:39:01,000
one would be less fit.
And there's a very famous study

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00:39:01,000 --> 00:39:06,000
that I don't have time to go into,
but it's a textbook, that showed

356
00:39:06,000 --> 00:39:11,000
experimentally the relative fitness
between these two forms,

357
00:39:11,000 --> 00:39:16,000
depending on the color of the tree
trunks.

358
00:39:16,000 --> 00:39:21,000
And this whole example is called
industrial melanism because the

359
00:39:21,000 --> 00:39:26,000
reason this was noticed was that in
areas of high industrialization the

360
00:39:26,000 --> 00:39:32,000
tree trunks are darker colored
because of the air pollution.

361
00:39:32,000 --> 00:39:36,000
This is back in England back in the
days when there is a lot more air

362
00:39:36,000 --> 00:39:41,000
pollution. So,
they were able to show a shift in

363
00:39:41,000 --> 00:39:46,000
the frequency of these two forms as
a function of the amount of

364
00:39:46,000 --> 00:39:51,000
pollution and environment because
their susceptibility to predation

365
00:39:51,000 --> 00:39:55,000
was reduced if this form dominated.
That study is actually rather

366
00:39:55,000 --> 00:40:00,000
controversial now,
so I won't teach you the details.

367
00:40:00,000 --> 00:40:06,000
But you get the point.
But these moths exist in these two

368
00:40:06,000 --> 00:40:12,000
forms. Here's another example
that's really convoluted that's in

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00:40:12,000 --> 00:40:18,000
your textbook.
So I'm not going to write it on the

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00:40:18,000 --> 00:40:24,000
board. You can read about it there.
But it's an example of what's

371
00:40:24,000 --> 00:40:30,000
called the evolutionary arms race.
And it has to do with Cottonwood

372
00:40:30,000 --> 00:40:36,000
trees. And Cottonwood trees produce
a defense compound that the name of

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00:40:36,000 --> 00:40:43,000
which is on the next slide:
salicorten.

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00:40:43,000 --> 00:40:50,000
It doesn't matter what it's
called, it's a toxic compound that

375
00:40:50,000 --> 00:40:57,000
makes the tree distasteful to
predator. You don't think of

376
00:40:57,000 --> 00:41:05,000
predators of trees.
The beaver is a tree predator.

377
00:41:05,000 --> 00:41:10,000
It chops it down.
So when a beaver chops it down,

378
00:41:10,000 --> 00:41:15,000
a Cottonwood tree, the Cottonwood
tree sprouts new sprouts.

379
00:41:15,000 --> 00:41:21,000
And the resprouts have much higher
concentrations of this toxic

380
00:41:21,000 --> 00:41:26,000
compound than the parent tree had.
In other words, the tree has a

381
00:41:26,000 --> 00:41:32,000
mechanism. And I'm sure we don't
understand that yet,

382
00:41:32,000 --> 00:41:37,000
but someday we'll understand the
genetic underpinnings of this,

383
00:41:37,000 --> 00:41:43,000
the molecular biology of beaver
defenses.

384
00:41:43,000 --> 00:41:47,000
But if it's been felled by a beaver,
it increases the production of this

385
00:41:47,000 --> 00:41:52,000
toxin in the shoots that come out
saying, OK, fool me once,

386
00:41:52,000 --> 00:41:56,000
but you're not getting to get me the
next time. It means there's beavers

387
00:41:56,000 --> 00:42:01,000
around. But the interesting thing
is that these shoots are much more

388
00:42:01,000 --> 00:42:06,000
susceptible to grazing
by a leaf beetle.

389
00:42:06,000 --> 00:42:10,000
In other words,
this increased toxin,

390
00:42:10,000 --> 00:42:15,000
the shoots that have the increased
toxin, are actually grazed more than

391
00:42:15,000 --> 00:42:19,000
the parent tree by this leaf beetle.
So scientists went in and said, you

392
00:42:19,000 --> 00:42:24,000
know, what's going on here?
This is weird. Why make a defense

393
00:42:24,000 --> 00:42:29,000
that makes you more vulnerable to a
different predator?

394
00:42:29,000 --> 00:42:33,000
And what they showed by experiment
was that these leaf beetles were

395
00:42:33,000 --> 00:42:38,000
using that toxin as a defense
against the ants that

396
00:42:38,000 --> 00:42:43,000
want to eat them.
So, they were less susceptible to

397
00:42:43,000 --> 00:42:49,000
predation by ants because they had
taken in the toxin from the shoots.

398
00:42:49,000 --> 00:42:55,000
So, it's called the evolutionary
arms race between predator and prey.

399
00:42:55,000 --> 00:43:01,000
In these systems get very
complex.

400
00:43:01,000 --> 00:43:07,000
So here's the data that shows that,
that the control trees, trees that

401
00:43:07,000 --> 00:43:13,000
haven't been felled by a beaver
compared to trees that have been

402
00:43:13,000 --> 00:43:19,000
felled by a beaver,
and resprouted trees have a much

403
00:43:19,000 --> 00:43:25,000
higher concentration of this toxin.
And this is the larval survival

404
00:43:25,000 --> 00:43:31,000
time of these leaf beetles that
these are from the control trees

405
00:43:31,000 --> 00:43:37,000
versus the browsed trees in the
presence of the ants.

406
00:43:37,000 --> 00:43:43,000
OK, I'm going to skip that one.
That ones in your book, too, which

407
00:43:43,000 --> 00:43:50,000
has an experiment showing that
anti-predation mechanisms are

408
00:43:50,000 --> 00:43:57,000
induced by the presence of a
predator. So I would encourage you

409
00:43:57,000 --> 00:44:04,000
to look at this in a textbook,
this example.

410
00:44:04,000 --> 00:44:09,000
And then finally I'm just going to
go through a series of pictures that

411
00:44:09,000 --> 00:44:15,000
show all of the types of defenses
that have been evolved as

412
00:44:15,000 --> 00:44:20,000
anti-predation devices.
They are the obvious ones like

413
00:44:20,000 --> 00:44:26,000
cacti with spikes,
or porcupines with spikes,

414
00:44:26,000 --> 00:44:32,000
octopi with ink defense. This is a
caterpillar that has evolved to look

415
00:44:32,000 --> 00:44:38,000
like a snake, at least that's the
way it looks to us.

416
00:44:38,000 --> 00:44:42,000
I mean that's a hypothesis,
that the predation on this would be

417
00:44:42,000 --> 00:44:46,000
reduced because it looks like a
predator itself.

418
00:44:46,000 --> 00:44:50,000
These are two different ones
looking like a snake.

419
00:44:50,000 --> 00:44:54,000
And often, snakes have a bright
colored thing on their tail to draw

420
00:44:54,000 --> 00:44:58,000
attention to the tail.
If you're going to be attacked by a

421
00:44:58,000 --> 00:45:02,000
predator, you'd rather have your
tail attacked than your head.

422
00:45:02,000 --> 00:45:08,000
So, this is a common motif in nature.
And this is an insect.

423
00:45:08,000 --> 00:45:14,000
This is its head. And yet,
if you're a predator you'd probably

424
00:45:14,000 --> 00:45:20,000
think this was its head.
And again, whether that's been

425
00:45:20,000 --> 00:45:26,000
established by experiments,
I don't know. So these are just

426
00:45:26,000 --> 00:45:32,000
examples. Moths often have features
that look like eyes that what tends

427
00:45:32,000 --> 00:45:37,000
to ward off a predator.
This is an interesting story that is

428
00:45:37,000 --> 00:45:43,000
in your readings that shows that
there are just a few genes that

429
00:45:43,000 --> 00:45:48,000
control the phenotype of this
particular butterfly.

430
00:45:48,000 --> 00:45:53,000
I think it's a moth, between having
these spots and not having the spots.

431
00:45:53,000 --> 00:45:59,000
In the fall, when it's dry and it's
more selective advantage to look

432
00:45:59,000 --> 00:46:04,000
like a leaf, they look like this.
But at other times,

433
00:46:04,000 --> 00:46:08,000
they're visible anyway so their
selective advantage is to have these

434
00:46:08,000 --> 00:46:13,000
eye spots that make them look like
they have eyes.

435
00:46:13,000 --> 00:46:17,000
OK, I think it's time to quit,
so we'll pick this up next time,

436
00:46:17,000 --> 00:46:22,000
I promise you. Their wonderful
pictures of the creatures in the

437
00:46:22,000 --> 00:46:26,000
deep sea that are very evil looking
predators that fish with their

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luminescent light organs.