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Module 1: Introduction to Ecology and Evolution

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Ecology and Evolution

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We are the products of Evolution? Life began on this earth roughly four billion years
back. And whatever life forms we observed now, whether it is trees, whether it is birds,
animals, even us, we are all the products of evolution. In this lecture, we are going to
have a look whether ecology has any relation with evolution.
(Refer Slide Time: 00:37)

Let us begin with some key terms. As we saw before, ecology is the scientific study of
interaction among organisms and their environments. And here, we need to emphasize
on the word interactions. We are studying the interactions among the organisms and their
environment, whereas evolution, the process through which we have all been made, it is
the genetic adaptation of organisms to their environment.

(Refer Slide Time: 01:15)

In this case we need to look into these two terms - genetic adaptation, what is genetics?
what is adaptation? and how are these related to evolution? Now, adaptation is any
alteration in the structure or function of an organism by which the organism becomes
better able to survive and multiply in its environment. Adaptation is any alteration.
Alteration is changes; any change in the structure of an organism or the function of an
organism.
For instance if in place of hands, if I develop wings, so that would be a change in the
structure or for instance a change in the function would be in place of using my hands for
writing, if maybe I start using it for something else. Probably for instance, I develop
some other sense organs on this hand, so that I can smell these objects by touching them,
so that would be a change in the function of an organ.
Any such changes in any organism or any organ of the organism by which the organism
becomes better able to survive, and multiply in its environment. Any changes will not be
adaptation. A change or an alteration is an adaptation only when it permits the organism
to better survive in that environment, better survive and to better multiply in that
environment.

(Refer Slide Time: 02:39)

For instance, a classical example of adaptation is, a camel living in a desert environment.
In that desert environment, you have mounds and mounds of sand, and there you have a
camel. Now, what are the kinds of adaptations that you will find in this animal? One is
that it has a hump, now why does it have a hump? The hump stores energy in the form of
fats and water; why is that required? Because, when you are living in a desert
environment, you do not have very ready access to water and food.
If you have ever seen a camel drinking, it would drink buckets and buckets of water and
store all of that into its body. And the urine that this animal releases out is a very
concentrated urine, because it is trying to save all of that water inside its body. If it loses
out that water, it would not have access to that water again.
Similarly, if you look at the blood of the animal, it has the characteristic that even if it
has lost quite a lot of water, it will still be able to pump this blood into the body and be
able to bring nutrients to the cells, and take out the waste materials. Similarly, if you look
at the legs, so the legs are padded. Now, why are they padded? Because, if you consider
a leg that is say like this, and a leg that is padded and has a larger surface area.
Now if this area is “a” and this area is capital “A”, we are talking about these areas. And
if the weight of the animal is say “x” kg, so the amount of downwards force, that is being
put on the legs is [“x” multiplied by “g”], where g is the acceleration due to gravity,
approximately 9.8 meter per second square.

This much amount of force that is acting downwards, is divided in by the four legs. The
pressure that would be exerted by this force that is [x multiplied by g divided by 4] on
each interface between the leg and the sand would be given by [x multiplied by g
divided by 4a] in the first case and [x multiplied by g divided by 4A] in the second case.
In this particular example if A is large, so in that case the pressure would be less. If you
have less amount of pressure, how it helps is that, if you have this sand, and if you have a
pointed leg, it will go inside the sand, whereas, if you have a padded leg, then because
the amount of pressure is less, so the animal will be able to walk on this sand. This is
also another adaptation that is there in the animal.
(Refer Slide Time: 05:51)

Then, if you look at the eyes of the animal, you would observe that the eyes have very
large eyelashes. Now, these eyelashes prevent sand from getting into the eyes of the
animal. If you look at its tongue or even its mouth, it will be very well suited to eat the
kinds of vegetation that are present in the desert environment. These are all different
kinds of adaptations that this animal has and all these adaptations are permitting this
animal to better survive in this environment.
Genetics means relating to genes or heredity. Basically, all of these adaptations, they
should be of such a manner that they get passed on from one organism to its offspring, to
their offspring, and so on; which means that all of these adaptations have to be heritable
adaptations. So, if you have a camel that, for instance, has feet that are even better

adapted, then most of its companions. And if this trait is not able to be passed down to its
offspring, it would not be called a genetic adaptation. So, what we want in the case of
evolution is genetic adaptations or inheritable fitness that permit the animal to better
survive and better reproduce.
(Refer Slide Time: 07:23)

We have introduced this term fitness. And fitness refers to the ability of a particular
organism to leave descendants in future generations, relative to other organisms.
Evolution tends to maximize fitness through the process of natural selection. Basically
fitness is the ability of an animal or an organism to leave descendants in the future
generations relative to other organisms; which means that it should be able to leave more
number of descendants as compared to any other organism of the same species that is
there in the environment.

(Refer Slide Time: 07:57)

For instance you have two individuals, you have this individual A, and this individual B.
And suppose you have individual A that has produced 10 offsprings, individual B has
produced 100 offsprings. And all of these are able to survive to their maturity. So, here
you have 10, and here you have 100. So, in this case we would say that organism B has a
better amount of fitness as compared to organism A, because it left 100 offsprings,
whereas A was only able to leave 10 offsprings.
But, suppose out of these 10 offsprings, 9 were able to survive, and out of these 100
offsprings only 7 were able to survive. Why? Because organism A was able to devote all
its time and attention to all of its 10 offsprings, so that 9 survived. Whereas, B just
produced more number of offsprings, and it did not give it any parental care and so only
7 survived to the next generation.
So, in that case we would say that A is having more amount of fitness as compared to B,
because it left more number of offsprings to the next generation. Why is that important?
It is important because evolution tends to maximize the fitness through the process of
natural selection. What we mean by this is that, evolution prefers better fitness. Why?
Because if organism A has those characteristics which are inheritable, and because of
which it was able to leave more number of offsprings. So, all of these 9 offsprings will
also be getting those characters from A and so all of these 9 or most of these 9 organisms

will be able to leave even more number of offsprings in the next generation as compared
to B.
So, in the case of B, out of 100 only 7 survived; out of these, only a very few numbers
would survive. So, after a while we would observe that in this system, we will be having
more number of organism with A kind of characteristics as compared to B kind of
characteristic. So, evolution tends to maximize the fitness that is present.
(Refer Slide Time: 10:23)

Now, what are the characteristics of this fitness? Fitness is environment-specific. We do
not have an absolute value of fitness, it is environment-specific. So, for instance, in the
case of our organisms A and B; in a single environment it is possible. Let us consider an
environment in which there is more amount of predation.
In this environment, if you are able to protect your offsprings, you will be able to have
more number of offsprings in the next generation whereas, if you are not able to protect
your offsprings, most of the offsprings would die off. But, then in an environment in
which you do not have any predation and you have ample resources as compared to the
population; in that case you do not require very much amount of parental care that needs
to be given to the offsprings.
So, in that case this organism B that was able to have more number of offsprings would
be said to be more fit as compared to organism A that only gave 10 individuals, because

in the absence of predation, in the absence of diseases, when you have ample amount of
resources available, most of the offsprings are able to survive. So, in such situations just
producing more number of offsprings would give you a bit of amount of fitness. So, in
this example we saw that fitness is environment specific, it depends on how harsh the
environment is.
Secondly, fitness is species-specific. So, we do not compare fitness between two
different species. High reproductive rate alone does not mean higher fitness, but higher
survival of more progeny, does. So, as we saw before, if you have more number of
offspring, it does not mean that you have more amount of fitness. What is important is
how many of those offsprings are able to survive to the next generation.
Then fitness should be measured across several generations; it is a long-term measure.
So, we cannot determine fitness in just one or two generations, it has to be determined
over a long period of time. And it works at the level of complete organism; not on
individual traits such as size or speed.
Essentially if you have two organisms, if one organism is faster than the second
organism, it does not mean that the organism will be more fit, because we will have to
look at all the characteristics that are present in that organism. So, it is possible that the
organism that has speed also has more amount of blood pressure, and so it dies off
quickly as compared to the second organism. In that case, we will say that speed is alone
insufficient to give fitness to the organism. So, all the characteristics of the organism
need to be looked in totality.

(Refer Slide Time: 13:11)

Next, we said that natural selection is the mechanism through which more fit organisms
are selected. How do we define natural selection? It is the process in nature by which
only those organisms that are best adapted to their environment tend to survive and
transmit their genetic characteristics to the succeeding generations, while those less
adapted tend to be eliminated. Natural selection is the process through which nature is
selecting those organisms that are better fitted to the environment.
(Refer Slide Time: 13:45)

And there are five stages in natural selection. The first stage is called variation. All
individuals are not identical. They have different characteristics. For instance, if we look
at a class of students; we would find that we have students of different heights or we
have students of different weights or different skin color or different color of the hair or
different eye color. These are all variations that are found in a population. Natural
selection, when it wants to select those organisms that are the best fit; in essence, it also
means that you need to have some variations. If all the organisms are one and the same,
then you cannot select between these two organisms.
(Refer Slide Time: 14:27)

We have variations that are present in organisms, and a classical example is that of
peppered moth.
This is a moth, and it is present in two varieties. One is this dark colored moth, and the
second is this light colored version. These belong to the same species, but they have
different colors.

(Refer Slide Time: 14:47)

The second step of natural selection is over population. Over population means that
organisms tend to produce excess number of offsprings. So, for example female
mosquitoes may lay 500 to 1000 eggs.
(Refer Slide Time: 15:07)

Now, if you had a situation in which every two organisms after mating, they only
produced two offsprings, which upon mating again produced only two offsprings. In
such a situation, we will observe that the population is not growing, the population is
static; because for every two of organisms in this generation, say G1, you only have two

offsprings in the second generation G2. For these two organisms in the second
generation, you only have these two offsprings.
In this case you will have a situation in which, the number of organisms will remain
constant with time, there will not be any change. However, it is observed that if you
provide ample amount of resources to any organism, it tends to overpopulate. So,
overpopulation means that from two organisms in the first generation, the second
generation may be having say 10 organisms.
The first generation had two organisms whereas the second had 10 organisms. So, it was
a multiplication factor of 5. If you do this multiplication factor of 5 again, then in place
of ten, you will be having 50 organisms; next you will be having 250 organisms, and so
you will have a curve that is arising exponentially.
In nature, what we observe is that organisms tend to produce excess offspring. So most
of the organisms tend to go for an exponential curve. But, the problem with that
exponential curve is that you do not have ample resources to accommodate all of these
organisms. So, there will be a struggle for existence. The resources are limited, so, not all
offsprings will be accommodated. And when that is a situation, then you will have some
individuals that will have to be eliminated.
(Refer Slide Time: 17:25)

We went to Kruger national park in 2018, and they will observe cheetahs that were
hunting. Now, let us have a look at how this hunting happens to understand better, what
is the struggle for existence.
So that is the voice of our tour operator. And we are observing cheetahs that are hunting
impalas. Impalas are deer that are found in South Africa. And as we can observe here,
this cheetah is moving in a stealth pattern; it is moving very slowly; it is moving very
cautiously towards the impalas which are the prey for this animal. And now it has started
running and the prey or the impalas are also running, and there we see another cheetah.
In fact, these cheetahs were hunting in groups. We had four cheetahs in this particular
group. The impalas were also in a group. The cheetahs tried to run after the impalas. But,
even after running and even after spending quite a large amount of energy doing this
stealth operation and running, they were not able to catch any impala. That tells us the
struggle for existence. We have four cheetahs here, but they will not get food every day.
Out of this struggle for existence, if there is; out of these four cheetahs, if there is one
cheetah that is not able to tolerate hunger or falls prey to a disease, because it is not
getting enough amount of food, it will be eliminated from the nature.
Only those that are the best fitted will survive to the next generation, which brings us to
the fourth step of natural selection, which is survival of the fittest. Only those
individuals that are best able to obtain and use resources will survive and reproduce. For
instance, even in the case of these four cheetahs, after hunting a prey, if you figured out
that one of these four were able to get the largest amount of meat and there was another
one that was not able to get enough amount of meat. In that case, you will have the first
cheetah that would be able to survive better as compared to the second cheetah which
does not get enough amount of food. So, survival of the fittest means that only those
individuals that are best even to obtain and use resources will survive and reproduce.

(Refer Slide Time: 20:15)

So, getting resources is crucial for the survival of an organism. And only when this
organism survives, breeds and produces more number of offsprings, will we say that this
organism is fit, and will be selected in the process of natural selection. The fifth step in
natural selection is changes in the gene pool. Inherited characters increase the frequency
of favorite traits in the population. What is changes in the gene pool? So, we come back
to this example of the peppered moth. We saw before that this peppered moth is present
in two color variations. One is the dark color, and one is the light color.
This example comes from England. And before the industrial revolution, this area was
very pollution free. So, trees had a lot of lichens on their surface, and these whitish color
are the lichens. Lichens provided the bark of the trees, lighter shade. And on this lighter
colored bark, we can observe this insect, but we cannot observe this insect as easily. So,
we also have a lighter colored insect, a lighter colored peppered moth that is there on this
bark, but we are not able to see it very easily.
Now, when industrial revolution came, there was quite a lot of air pollution in that area,
and pollution killed off the lichens. So, if this lichen gets removed from the bark, so the
barks get exposed. And probably you will also find some amount of soot on these barks.
When that happens, this lighter colored version which was earlier very much
camouflaged on the lichens is now clearly visible, whereas this dark colored version,

now it becomes camouflaged. So, in this image as well we have two peppered moths,
and we are very easily able to see this peppered moth, but not this peppered moth.
Now, how is that important? That is important, because in these situations when you
have an unpolluted atmosphere, when you have quite a lot of lichens on the trees, these
dark colored individuals are preferentially predated upon. If there is a bird that feeds on
peppered moths, and if it visits this tree, it would be able to observe this peppered moth,
but not this one, so it will eat up this one, and this one will be saved. Whereas, in the
case of a polluted environment, we would observe that this one is very clearly visible,
but this one gets camouflaged, so this will be preferentially eaten.
(Refer Slide Time: 22:55)

When it was observed that across generations with time, what was the proportion of this
dark colored allele, and what was the proportion of the light colored allele in the
population. This is how it went, number or let us say proportion of alleles. So, let us
divide this period into stages. The first stage is, before industrial revolution. The second
one is, during and after industrial revolution. And the third one is, after clean air act was
passed.
In the first period, before industrial revolution, we had a situation like this one. In this
situation, it so happened that most of the dark colored peppered moths were eaten up,
and so their numbers were very less. Most of the peppered moths that you would observe
would be light in color. So, before industrial revolution, you have this dark color variety

which is very less, and you have the light colored variety that is very high. Now, during
and after the industrial revolution, the light ones were preferentially eaten up, and the
black ones more spared. So, after a few generations, it so happened that the number of
dark colored moths in the population increased, and the light colors reduced.
Once they had quite a lot of air pollution and had situations of public outcry especially
after the great London smog, they had the passage of a Clean Air Act, through which the
amount of pollution in the air was regulated. Once that happened and, once the air
cleaned up again, the situation again reverted back to this situation. There was less
amount of pollutant, so lichens again came up on the trees, and again we had situations
in which the dark colored moths became preferentially eaten.
In that case, we again got to a situation in which the number of dark colored moths
reduced in proportion, and the number of light colored moths increased in proportion.
What we are observing here is changes in the gene pool. This is a very good example of
how natural selection operates in principle. In this example, we can see that there is
variation in the organism. So, different individuals have got different colors. Now,
peppered moths, like most other organisms also produce a number offsprings. So, there
is overpopulation, there is struggle for resources.
Now, if this was the only tree that was available, and if these two peppered moths were
the only two peppered moths that were available, so this peppered moths would have
resided here, and this peppered moth would have moved to this location in which case
both of these peppered moths would have been spared from predation, and both would
have been able to live equally well.
However, because there is a struggle for resources; because there is a dearth of resources
as compared to the number of organisms that are produced, there was a struggle for
existence. Not every peppered moth could get into a place where it would hide out. So,
there was a struggle for existence.
In this struggle for existence, there was survival of the fittest. So, in the presence of
predation, in this sort of an environment, this one survived better. So, this would be said,
survival of this organism was preferred by natural selection. In this environment, this one
was preferred. So, there was a survival of the fittest. And this also resulted into changes
in the gene pool. Now, here it is important to note that whenever there are these changes

in the gene pool, in most of the situations we do not have a situation in which you have
100 percent organisms that are of one variety, and no organism that is there of the second
variety.
Coming back to the drawing board, here we observe that even in the first situation, we
had a very few number of individuals that were dark in color, but they still remained
there in the system. Variation is very crucial for the system to survive, because if so
happened that this number went to 0, so they would not have been any more variation
that was remaining in the system.
And once the system changed, once it moved to a polluted scenario; In such a situation if
you only had the light colored moths, all of those light colored moths would have been
eaten up and so no peppered moths would have existed today. Whereas, nature always
prefers to have these variations, and so, even in these situations we observe that we will
have some number of individuals that still persist in the system even though they are not
the best suited.
(Refer Slide Time: 28:29)

How does this selection occur? We have three different kinds of selections which are
called as directional selections, stabilizing selection, and disruptive selection.
In this example what we are observing is that, here we have the frequency of individuals,
and here we have different colors that are present in the population. Here we have an

organism that is very light in color, here we have an organism that is very dark in color,
and these are variations in between. Now, suppose the original population was something
like this, so the most preferred or the most fit organism was there in the center.
(Refer Slide Time: 29:13)

Now, in the case of a directional selection, this curve would shift either to the right or to
the left. What we are seeing here is that, here we have the frequencies of individuals, and
here we have the color. Let us call these shades as 1, 2, 3, 4 and 5. And the earlier
population was something like this. So, in this case, we had most of the organisms that
had this color of three, so this is the most preferred one.
Now, if the situation changes, and if this curve shifts to the right, so it becomes
something like this. So, in this case, we will have that, the organisms of shade four are
more selected, so this is a directional shift. So, essentially the peak of the curve shifts
from this to this or it can move to the other side as well. So, this is a directional selection.

(Refer Slide Time: 30:25)

The second selection is called a disruptive selection. In the case of a disruptive selection,
we have a situation in which these organisms are selected, the middle ones are not
selected, and then the larger ones are selected. In this situation we have that the light
ones are preferred, and the dark ones are preferred, but the middle ones are not preferred,
now when do we have a situation like this.
Suppose you have a forest in which you have some trees that are dark in color, and then
you also have some trees that are light in color. Now, in such a forest if the light colored
individual goes and sits on the light colored bark, and the dark colored individual goes
and sits on the dark colored bark, both of these are spayed from predation. But, the
middle color whether it goes to the dark tree or whether it goes to the light tree, it is not
that much camouflaged, so it becomes apparent and it gets predated upon. Such a
selection in which nature prefers the two extremes, but not things in the middle goes by
the name of a disruptive selection.
And third is a stabilizing selection. So, in the case of a stabilizing selection, we have a
situation in which the earlier curve was like this,. In the later generation, this curve
becomes even more narrow down. So, for instance earlier we had these shades 1, 2, 3, 4
and 5 in generation 1. But, in the second generation, the shades 1 and 5 get completely
removed and now this whole curve has become even more towards the center; so, in this
case we have lost two traits, and we have even shifted the system towards a center point.
So, this would be called a stabilizing solution.
(Refer Slide Time: 32:45)

We observe examples of all three of these in the nature. For instance, this is a study of
directional selection. There is a set of islands that go by the name of Galapagos islands,
and here we have birds that are called finches. Now, these birds have beaks, and their
beak size was studied. There was a draught in 1977, and before the draught we had this
sort of a curve. What we observe here is that this beak depth of 8.8mm is the most
preferred one. Now, after the draught what happened was that during this period most of
the nuts that were there became even more harder to break open. So, here we have this
chart of seed hardness versus the beak depth.
Now, if you have a seed that is harder to break open, so you require a larger sized beak to
break open that seed. Now, in this draught what we observed was that before the draught,
we had this pattern in which the beak size of 8.8mm was more preferred. After the
draught it shifted from 8.8 to 9.8mm. So, there was a directional shift towards larger
sized beaks because of the draught. So, this is an example of directional selection.
(Refer Slide Time: 34:11)

An example of stabilizing selection is the weight of human baby at birth. Here we well
observe that if the weight is around 8 pounds, so we have minimum amount of mortality
that is there in the system, and highest amount of survival rate. If it shifts to the right or
to the left, so in those situations these babies die off more easily.
This is obviously a very old paper and our advances in medicine have enabled other
babies to survive today. But, then if you look at this graph, if you only concentrate on

this graph, we will observe that babies of eight pounds are selected, so this is the most
optimum weight. This is an example of a stabilizing selection. So, if you shift to the right
or to the left, you have a lesser probability of survival.
(Refer Slide Time: 35:01)

This is an example of a disruptive selection. This is again an example from Galapagos
islands in which we had a bird population in which these beak sizes were more preferred,
and then, these beak sizes were more preferred, but the center ones were less preferred.
We can have a situation like this if we have an environment in which you have, say these
nuts that are hard to crack, and these nuts that are easy to crack, but you do not have any
nuts that come in between.
So, if you have a bird that comes here, it won’t be able to crack a larger sized or a harder
nut. But, if it tries to crack open these softer nuts, it will face a lot of competition from
their already existing birds, which have smaller beak sizes and are probably more adapt
or more amenable to crack open those softer nuts. In such a situation, we will have a
disruptive selection, so we will observe two modes in the curve.

(Refer Slide Time: 36:07)

Next, we have a look at coevolution, which is a situation in which there are two species
that are evolving at the same time. This is the evolution of two or more species that
interact closely with one another, with each species adapting to changes in the other. A
good example is bee hummingbird that is feeding on these flowers.
(Refer Slide Time: 36:43)

In these flowers, they have an elongated shape much as a funnel, and there is nectar on
the inside. The flower produces nectar to attract these birds, so that if this is a flower, and
here we have the nectar. This bird reaches, and tries to feed on the nectar, and in that

process, it gets the pollens from the flower onto its beak. When it goes to another flower
of the same species, it is able to transport these pollens from one flower to another
flower.