NCERT Textbook: Organisms & Populations Notes | Study Biology Class 12 - NEET

 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 13
Organisms and Populations
Chapter 14
Ecosystem
Chapter 15
Biodiversity and Conservation
Chapter 16
Environmental  Issues
2020-21
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 13
Organisms and Populations
Chapter 14
Ecosystem
Chapter 15
Biodiversity and Conservation
Chapter 16
Environmental  Issues
2020-21
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)
2020-21
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 13
Organisms and Populations
Chapter 14
Ecosystem
Chapter 15
Biodiversity and Conservation
Chapter 16
Environmental  Issues
2020-21
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)
2020-21
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 13
ORGANISMS AND POPULATIONS
13.1 Organism and Its
Environment
13.2 Populations
2020-21
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 13
Organisms and Populations
Chapter 14
Ecosystem
Chapter 15
Biodiversity and Conservation
Chapter 16
Environmental  Issues
2020-21
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)
2020-21
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 13
ORGANISMS AND POPULATIONS
13.1 Organism and Its
Environment
13.2 Populations
2020-21
220
BIOLOGY
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 organismic and population levels.
13.1 ORGANISM AND ITS ENVIRONMENT
Ecology at the organismic level is essentially physiological ecology which
tries to understand how different organisms are adapted to their
environments in terms of not only survival but also reproduction.  You
may have learnt in earlier classes how the rotation of  our planet around
the Sun and the tilt of its axis cause annual variations in the intensity
and duration of temperature, resulting in distinct seasons.  These
variations together with annual variation in precipitation (remember
precipitation includes both rain and snow) account for the formation of
major biomes such as desert, rain forest and tundra (Figure 13.1).
Figure 13.1 Biome distribution with respect to annual temperature and precipitation
Regional and local variations within each biome lead to the formation of a
wide variety of habitats. Major biomes of India are shown in Figure 13.2.
On planet Earth, life exists not just in a few favourable habitats but even
in extreme and harsh habitats – scorching Rajasthan desert, rain-soaked
Meghalaya forests, deep ocean trenches, torrential streams, permafrost
(snow laden) polar regions, high mountain tops, thermal springs, and
stinking compost pits, to name a few.  Even our intestine is a unique
habitat for hundreds of species of microbes.
2020-21
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 13
Organisms and Populations
Chapter 14
Ecosystem
Chapter 15
Biodiversity and Conservation
Chapter 16
Environmental  Issues
2020-21
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)
2020-21
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 13
ORGANISMS AND POPULATIONS
13.1 Organism and Its
Environment
13.2 Populations
2020-21
220
BIOLOGY
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 organismic and population levels.
13.1 ORGANISM AND ITS ENVIRONMENT
Ecology at the organismic level is essentially physiological ecology which
tries to understand how different organisms are adapted to their
environments in terms of not only survival but also reproduction.  You
may have learnt in earlier classes how the rotation of  our planet around
the Sun and the tilt of its axis cause annual variations in the intensity
and duration of temperature, resulting in distinct seasons.  These
variations together with annual variation in precipitation (remember
precipitation includes both rain and snow) account for the formation of
major biomes such as desert, rain forest and tundra (Figure 13.1).
Figure 13.1 Biome distribution with respect to annual temperature and precipitation
Regional and local variations within each biome lead to the formation of a
wide variety of habitats. Major biomes of India are shown in Figure 13.2.
On planet Earth, life exists not just in a few favourable habitats but even
in extreme and harsh habitats – scorching Rajasthan desert, rain-soaked
Meghalaya forests, deep ocean trenches, torrential streams, permafrost
(snow laden) polar regions, high mountain tops, thermal springs, and
stinking compost pits, to name a few.  Even our intestine is a unique
habitat for hundreds of species of microbes.
2020-21
221
ORGANISMS AND POPULATIONS
What are the key elements that lead to so much variation in the
physical and chemical conditions of different habitats? The most
important ones are temperature, water, light and soil.  We must remember
that the physico-chemical (abiotic) components alone do not characterise
the habitat of an organism completely; the habitat includes biotic
components also – pathogens, parasites, predators and competitors – of
the organism with which they interact constantly.  We assume that over a
period of time, the organism had through natural selection, evolved
adaptations to optimise its survival and reproduction in its habitat.
Each organism has an invariably defined range of conditions that it
can tolerate, diversity in the resources it utilises and a distinct functional
role in the ecological system, all these together comprise its niche.
13.1.1 Major Abiotic Factors
Temperature: Temperature is the most important ecologically relevant
environmental factor.  You are aware that the average temperature on
land varies seasonally, decreases progressively from the equator towards
the poles and from plains to the mountain tops. It ranges from subzero
levels in polar areas and high altitudes to >50
0
C in tropical deserts in
summer. There are, however, unique habitats such as thermal springs
and deep-sea hydrothermal vents where average temperatures exceed
100
0
 C.  It is general knowledge that mango trees do not and cannot grow
Figure 13.2 Major biomes of India : (a) Tropical rain forest; (b) Deciduous forest;
(c) Desert; (d) Sea coast
 (a) (b)
 (c) (d)
2020-21