NCERT Textbook: Organisms & Populations | Biology Class 12 - NEET PDF Download

 Page 1


Diversity is not only a characteristic of living organisms but
also of content in biology textbooks. Biology is presented either
as botany, zoology and microbiology or as classical and
modern. The latter is a euphemism for molecular aspects of
biology. Luckily we have many threads which weave the
different areas of biological information into a unifying
principle. Ecology is one such thread which gives us a holistic
perspective to biology. The essence of biological understanding
is to know how organisms, while remaining an individual,
interact with other organisms and physical habitats as a group
and hence behave like organised wholes, i.e., population,
community, ecosystem or even as the whole biosphere.
Ecology explains to us all this. A particular aspect of this is the
study of anthropogenic environmental degradation and the
socio-political issues it has raised. This unit describes as well as
takes a critical view of the above aspects.
Chapter 11
Organisms and Populations
Chapter 12
Ecosystem
Chapter 13
Biodiversity and Conservation
2024-25
Page 2


Diversity is not only a characteristic of living organisms but
also of content in biology textbooks. Biology is presented either
as botany, zoology and microbiology or as classical and
modern. The latter is a euphemism for molecular aspects of
biology. Luckily we have many threads which weave the
different areas of biological information into a unifying
principle. Ecology is one such thread which gives us a holistic
perspective to biology. The essence of biological understanding
is to know how organisms, while remaining an individual,
interact with other organisms and physical habitats as a group
and hence behave like organised wholes, i.e., population,
community, ecosystem or even as the whole biosphere.
Ecology explains to us all this. A particular aspect of this is the
study of anthropogenic environmental degradation and the
socio-political issues it has raised. This unit describes as well as
takes a critical view of the above aspects.
Chapter 11
Organisms and Populations
Chapter 12
Ecosystem
Chapter 13
Biodiversity and Conservation
2024-25
Ramdeo Misra is revered as the Father of Ecology in India. Born on 26 August
1908,  Ramdeo Misra obtained Ph.D in Ecology (1937) under Prof. W. H. Pearsall,
FRS, from Leeds University in UK. He established  teaching and research  in
ecology at the Department of Botany of the Banaras Hindu University,
Varanasi. His research laid the foundations for understanding of tropical
communities and their succession, environmental responses of plant
populations and productivity and nutrient cycling in tropical forest and
grassland ecosystems. Misra formulated the first postgraduate course in
ecology in India. Over 50 scholars obtained Ph. D degree under his supervision
and moved on to other universities and research institutes to initiate ecology
teaching and research across the country.
He was honoured with the Fellowships of the Indian National Science
Academy and World Academy of Arts and Science, and the prestigious Sanjay
Gandhi Award in Environment and Ecology. Due to his efforts, the
Government of India established the National Committee for Environmental
Planning and Coordination (1972) which, in later years, paved the way
for the establishment of the Ministry of Environment and Forests (1984).
RAMDEO MISRA
(1908-1998)
2024-25
Page 3


Diversity is not only a characteristic of living organisms but
also of content in biology textbooks. Biology is presented either
as botany, zoology and microbiology or as classical and
modern. The latter is a euphemism for molecular aspects of
biology. Luckily we have many threads which weave the
different areas of biological information into a unifying
principle. Ecology is one such thread which gives us a holistic
perspective to biology. The essence of biological understanding
is to know how organisms, while remaining an individual,
interact with other organisms and physical habitats as a group
and hence behave like organised wholes, i.e., population,
community, ecosystem or even as the whole biosphere.
Ecology explains to us all this. A particular aspect of this is the
study of anthropogenic environmental degradation and the
socio-political issues it has raised. This unit describes as well as
takes a critical view of the above aspects.
Chapter 11
Organisms and Populations
Chapter 12
Ecosystem
Chapter 13
Biodiversity and Conservation
2024-25
Ramdeo Misra is revered as the Father of Ecology in India. Born on 26 August
1908,  Ramdeo Misra obtained Ph.D in Ecology (1937) under Prof. W. H. Pearsall,
FRS, from Leeds University in UK. He established  teaching and research  in
ecology at the Department of Botany of the Banaras Hindu University,
Varanasi. His research laid the foundations for understanding of tropical
communities and their succession, environmental responses of plant
populations and productivity and nutrient cycling in tropical forest and
grassland ecosystems. Misra formulated the first postgraduate course in
ecology in India. Over 50 scholars obtained Ph. D degree under his supervision
and moved on to other universities and research institutes to initiate ecology
teaching and research across the country.
He was honoured with the Fellowships of the Indian National Science
Academy and World Academy of Arts and Science, and the prestigious Sanjay
Gandhi Award in Environment and Ecology. Due to his efforts, the
Government of India established the National Committee for Environmental
Planning and Coordination (1972) which, in later years, paved the way
for the establishment of the Ministry of Environment and Forests (1984).
RAMDEO MISRA
(1908-1998)
2024-25
Our living world is fascinatingly diverse and amazingly
complex.  We can try to understand its complexity by
investigating processes at various levels of biological
organisation–macromolecules, cells, tissues, organs,
individual organisms, population, communities,
ecosystems and biomes. At any level of biological
organisation we can ask two types of questions – for
example, when we hear the bulbul singing early morning
in the garden, we may ask – ‘How does the bird sing?’
Or, ‘Why does the bird sing ?’  The ‘how-type’ questions
seek the mechanism behind the process while the ‘why-
type’ questions seek the significance of the process. For
the first question in our example, the answer might be in
terms of the operation of the voice box and the vibrating
bone in the bird, whereas for the second question the
answer may lie in the bird’s need to communicate with its
mate during breeding season. When you observe nature
around you with a scientific frame of mind you will
certainly come up with many interesting questions of both
types - Why are night-blooming flowers generally white?
How does the bee know which flower has nectar? Why
does cactus have so many thorns? How does the chick
spures recognise her own mother ?, and so on.
CHAPTER 11
ORGANISMS AND POPULATIONS
11.1 Populations
2024-25
Page 4


Diversity is not only a characteristic of living organisms but
also of content in biology textbooks. Biology is presented either
as botany, zoology and microbiology or as classical and
modern. The latter is a euphemism for molecular aspects of
biology. Luckily we have many threads which weave the
different areas of biological information into a unifying
principle. Ecology is one such thread which gives us a holistic
perspective to biology. The essence of biological understanding
is to know how organisms, while remaining an individual,
interact with other organisms and physical habitats as a group
and hence behave like organised wholes, i.e., population,
community, ecosystem or even as the whole biosphere.
Ecology explains to us all this. A particular aspect of this is the
study of anthropogenic environmental degradation and the
socio-political issues it has raised. This unit describes as well as
takes a critical view of the above aspects.
Chapter 11
Organisms and Populations
Chapter 12
Ecosystem
Chapter 13
Biodiversity and Conservation
2024-25
Ramdeo Misra is revered as the Father of Ecology in India. Born on 26 August
1908,  Ramdeo Misra obtained Ph.D in Ecology (1937) under Prof. W. H. Pearsall,
FRS, from Leeds University in UK. He established  teaching and research  in
ecology at the Department of Botany of the Banaras Hindu University,
Varanasi. His research laid the foundations for understanding of tropical
communities and their succession, environmental responses of plant
populations and productivity and nutrient cycling in tropical forest and
grassland ecosystems. Misra formulated the first postgraduate course in
ecology in India. Over 50 scholars obtained Ph. D degree under his supervision
and moved on to other universities and research institutes to initiate ecology
teaching and research across the country.
He was honoured with the Fellowships of the Indian National Science
Academy and World Academy of Arts and Science, and the prestigious Sanjay
Gandhi Award in Environment and Ecology. Due to his efforts, the
Government of India established the National Committee for Environmental
Planning and Coordination (1972) which, in later years, paved the way
for the establishment of the Ministry of Environment and Forests (1984).
RAMDEO MISRA
(1908-1998)
2024-25
Our living world is fascinatingly diverse and amazingly
complex.  We can try to understand its complexity by
investigating processes at various levels of biological
organisation–macromolecules, cells, tissues, organs,
individual organisms, population, communities,
ecosystems and biomes. At any level of biological
organisation we can ask two types of questions – for
example, when we hear the bulbul singing early morning
in the garden, we may ask – ‘How does the bird sing?’
Or, ‘Why does the bird sing ?’  The ‘how-type’ questions
seek the mechanism behind the process while the ‘why-
type’ questions seek the significance of the process. For
the first question in our example, the answer might be in
terms of the operation of the voice box and the vibrating
bone in the bird, whereas for the second question the
answer may lie in the bird’s need to communicate with its
mate during breeding season. When you observe nature
around you with a scientific frame of mind you will
certainly come up with many interesting questions of both
types - Why are night-blooming flowers generally white?
How does the bee know which flower has nectar? Why
does cactus have so many thorns? How does the chick
spures recognise her own mother ?, and so on.
CHAPTER 11
ORGANISMS AND POPULATIONS
11.1 Populations
2024-25
191
ORGANISMS AND POPULATIONS
You have already learnt in previous classes that Ecology is a subject
which studies the interactions among organisms and between the
organism and its physical (abiotic) environment.
Ecology is basically concerned with four levels of biological
organisation – organisms, populations, communities and biomes.  In this
chapter we explore ecology at population levels.
11.1 POPULATIONS
11.1.1 Population Attributes
In nature, we rarely find isolated, single individuals of any species; majority
of them live in groups in a well defined geographical area, share or compete
for similar resources, potentially interbreed and thus constitute a
population. Although the term interbreeding implies sexual reproduction,
a group of individuals resulting from even asexual reproduction is also
generally considered a population for the purpose of ecological studies.
All the cormorants in a wetland, rats in an abandoned dwelling, teakwood
trees in a forest tract, bacteria in a culture plate and lotus plants in a
pond, are some examples of a population.  In earlier chapters you have
learnt that although an individual organism is the one that has to cope
with a changed environment, it is at the population level that natural
selection operates to evolve the desired traits. Population ecology is,
therefore, an important area  because it links ecology to population genetics
and evolution.
A population has certain attributes whereas, an individual organism
does not. An individual may have births and deaths, but a population has
birth rates and death rates. In a population these rates refer to per capita
births and deaths. The rates, hence, expressed are change in numbers
(increase or decrease) with respect to members of the population. Here is an
example. If in a pond there were 20 lotus plants last year and through
reproduction 8 new plants are added, taking the current population to 28,
we calculate the birth rate as 8/20 = 0.4 offspring per lotus per year. If 4
individuals in a laboratory population of 40 fruitflies died during a specified
time interval, say a week, the death rate in the population during that period
is 4/40 = 0.1 individuals per fruitfly per week.
Another attribute characteristic of a population is sex ratio. An
individual is either a male or a female but a population has a sex ratio
(e.g., 60 per cent of the population are females and 40 per cent males).
A population at any given time is composed of individuals of
different ages. If the age distribution (per cent individuals of a given
age or age group) is plotted for the population, the resulting structure
is called an age pyramid (Figure 11.1). For human population, the
age pyramids generally show age distribution of males and females in
a  diagram. The shape of the pyramids reflects the growth status of
the population - (a) whether it is growing, (b) stable or (c) declining.
2024-25
Page 5


Diversity is not only a characteristic of living organisms but
also of content in biology textbooks. Biology is presented either
as botany, zoology and microbiology or as classical and
modern. The latter is a euphemism for molecular aspects of
biology. Luckily we have many threads which weave the
different areas of biological information into a unifying
principle. Ecology is one such thread which gives us a holistic
perspective to biology. The essence of biological understanding
is to know how organisms, while remaining an individual,
interact with other organisms and physical habitats as a group
and hence behave like organised wholes, i.e., population,
community, ecosystem or even as the whole biosphere.
Ecology explains to us all this. A particular aspect of this is the
study of anthropogenic environmental degradation and the
socio-political issues it has raised. This unit describes as well as
takes a critical view of the above aspects.
Chapter 11
Organisms and Populations
Chapter 12
Ecosystem
Chapter 13
Biodiversity and Conservation
2024-25
Ramdeo Misra is revered as the Father of Ecology in India. Born on 26 August
1908,  Ramdeo Misra obtained Ph.D in Ecology (1937) under Prof. W. H. Pearsall,
FRS, from Leeds University in UK. He established  teaching and research  in
ecology at the Department of Botany of the Banaras Hindu University,
Varanasi. His research laid the foundations for understanding of tropical
communities and their succession, environmental responses of plant
populations and productivity and nutrient cycling in tropical forest and
grassland ecosystems. Misra formulated the first postgraduate course in
ecology in India. Over 50 scholars obtained Ph. D degree under his supervision
and moved on to other universities and research institutes to initiate ecology
teaching and research across the country.
He was honoured with the Fellowships of the Indian National Science
Academy and World Academy of Arts and Science, and the prestigious Sanjay
Gandhi Award in Environment and Ecology. Due to his efforts, the
Government of India established the National Committee for Environmental
Planning and Coordination (1972) which, in later years, paved the way
for the establishment of the Ministry of Environment and Forests (1984).
RAMDEO MISRA
(1908-1998)
2024-25
Our living world is fascinatingly diverse and amazingly
complex.  We can try to understand its complexity by
investigating processes at various levels of biological
organisation–macromolecules, cells, tissues, organs,
individual organisms, population, communities,
ecosystems and biomes. At any level of biological
organisation we can ask two types of questions – for
example, when we hear the bulbul singing early morning
in the garden, we may ask – ‘How does the bird sing?’
Or, ‘Why does the bird sing ?’  The ‘how-type’ questions
seek the mechanism behind the process while the ‘why-
type’ questions seek the significance of the process. For
the first question in our example, the answer might be in
terms of the operation of the voice box and the vibrating
bone in the bird, whereas for the second question the
answer may lie in the bird’s need to communicate with its
mate during breeding season. When you observe nature
around you with a scientific frame of mind you will
certainly come up with many interesting questions of both
types - Why are night-blooming flowers generally white?
How does the bee know which flower has nectar? Why
does cactus have so many thorns? How does the chick
spures recognise her own mother ?, and so on.
CHAPTER 11
ORGANISMS AND POPULATIONS
11.1 Populations
2024-25
191
ORGANISMS AND POPULATIONS
You have already learnt in previous classes that Ecology is a subject
which studies the interactions among organisms and between the
organism and its physical (abiotic) environment.
Ecology is basically concerned with four levels of biological
organisation – organisms, populations, communities and biomes.  In this
chapter we explore ecology at population levels.
11.1 POPULATIONS
11.1.1 Population Attributes
In nature, we rarely find isolated, single individuals of any species; majority
of them live in groups in a well defined geographical area, share or compete
for similar resources, potentially interbreed and thus constitute a
population. Although the term interbreeding implies sexual reproduction,
a group of individuals resulting from even asexual reproduction is also
generally considered a population for the purpose of ecological studies.
All the cormorants in a wetland, rats in an abandoned dwelling, teakwood
trees in a forest tract, bacteria in a culture plate and lotus plants in a
pond, are some examples of a population.  In earlier chapters you have
learnt that although an individual organism is the one that has to cope
with a changed environment, it is at the population level that natural
selection operates to evolve the desired traits. Population ecology is,
therefore, an important area  because it links ecology to population genetics
and evolution.
A population has certain attributes whereas, an individual organism
does not. An individual may have births and deaths, but a population has
birth rates and death rates. In a population these rates refer to per capita
births and deaths. The rates, hence, expressed are change in numbers
(increase or decrease) with respect to members of the population. Here is an
example. If in a pond there were 20 lotus plants last year and through
reproduction 8 new plants are added, taking the current population to 28,
we calculate the birth rate as 8/20 = 0.4 offspring per lotus per year. If 4
individuals in a laboratory population of 40 fruitflies died during a specified
time interval, say a week, the death rate in the population during that period
is 4/40 = 0.1 individuals per fruitfly per week.
Another attribute characteristic of a population is sex ratio. An
individual is either a male or a female but a population has a sex ratio
(e.g., 60 per cent of the population are females and 40 per cent males).
A population at any given time is composed of individuals of
different ages. If the age distribution (per cent individuals of a given
age or age group) is plotted for the population, the resulting structure
is called an age pyramid (Figure 11.1). For human population, the
age pyramids generally show age distribution of males and females in
a  diagram. The shape of the pyramids reflects the growth status of
the population - (a) whether it is growing, (b) stable or (c) declining.
2024-25
192
BIOLOGY
The size of the population tells us a lot about its status in the habitat.
Whatever ecological processes we wish to investigate in a population, be
it the outcome of competition with another species, the impact of a
predator or the effect of a pesticide application, we always evaluate them
in terms of any change in the population size. The size, in nature, could
be as low as <10 (Siberian cranes at Bharatpur wetlands in any year) or
go into millions (Chlamydomonas in a pond). Population size,  technically
called population density (designated as N), need not necessarily be
measured in numbers only. Although total number is generally the most
appropriate measure of population density, it is in some cases either
meaningless or difficult to determine.  In an area,  if there are 200 carrot
grass (Parthenium hysterophorus) plants but only a single huge banyan
tree with a large canopy, stating that the population density of banyan is
low relative to that of carrot grass amounts to underestimating the
enormous role of the Banyan in that community. In such cases, the per
cent cover or biomass is a more meaningful measure of the population
size. Total number is again not an easily adoptable measure if the
population is huge and counting is impossible or very time-consuming.
If you have a dense laboratory culture of bacteria  in a petri dish what is
the best measure to report its density?  Sometimes, for certain ecological
investigations, there is no need to know the absolute population densities;
relative densities serve the purpose equally well.  For instance, the number
of fish caught per trap is good enough measure of its total population
density in the lake.  We are mostly obliged to estimate population sizes
indirectly, without actually counting them or seeing them. The tiger census
in our national parks and tiger reserves is often based on pug marks and
fecal pellets.
11.1.2 Population Growth
The size of a population for any species is not a static parameter.  It keeps
changing with time, depending on various factors including food
availability, predation pressure and adverse weather.  In fact, it is these
changes in population density that give us some idea of what is happening
to the population – whether it is flourishing or declining.  Whatever might
be the ultimate reasons, the density of a population in a given habitat
during a given period, fluctuates due to changes in four basic processes,
Figure 11.1 Representation of age pyramids for human population
2024-25