- DIVERSITY IN THE LIVING WORLD:
Biology is the science of life forms and living processes. The living world comprises an amazing diversity of living organisms. Early man could easily perceive the difference between inanimate matter and living organisms. Early man deified some of the inanimate matter (wind, sea, fire etc.) and some among the animals and plants. A common feature of all such forms of inanimate and animate objects was the sense of awe or fear that they evoked. The description of living organisms including human beings began much later in human history. Societies which indulged in anthropocentric view of biology could register limited progress in biological knowledge. Systematic and monumental description of life forms brought in, out of necessity, detailed systems of identification, nomenclature and classification. The biggest spin off of such studies was the recognition of the sharing of similarities among living organisms both horizontally and vertically. That all present day living organisms are related to each other and also to all organisms that ever lived on this earth, was a revelation which humbled man and led to cultural movements for conservation of biodiversity. In the following chapters of this unit, you will get a description, including classification, of animals and plants from a taxonomist’s perspective.
WHAT IS “LIVING”?
When we try to define ‘living’, we conventionally look for distinctive
characteristics exhibited by living organisms. Growth, reproduction, ability
to sense environment and mount a suitable response come to our mind
immediately as unique features of living organisms. One can add a few
more features like metabolism, ability to self-replicate, self-organise,
interact and emergence to this list. Let us try to understand each of these.
All living organisms grow. Increase in mass and increase in number
of individuals are twin characteristics of growth. A multicellular organism grows by cell division. In plants, this growth by cell division occurs
continuously throughout their life span. In animals, this growth is seen
only up to a certain age. However, cell division occurs in certain tissues to
replace lost cells. Unicellular organisms also grow by cell division. One
can easily observe this in in vitro cultures by simply counting the number
of cells under the microscope. In majority of higher animals and plants,
growth and reproduction are mutually exclusive events. One must
remember that increase in body mass is considered as growth. Non-living
objects also grow if we take increase in body mass as a criterion for growth.
Mountains, boulders and sand mounds do grow. However, this kind of
growth exhibited by non-living objects is by accumulation of material on
the surface. In living organisms, growth is from inside. Growth, therefore,
cannot be taken as a defining property of living organisms. Conditions
under which it can be observed in all living organisms have to be explained
and then we understand that it is a characteristic of living systems. A
dead organism does not grow.
Reproduction, likewise, is a characteristic of living organisms.
In multicellular organisms, reproduction refers to the production of
progeny possessing features more or less similar to those of parents.
Invariably and implicitly we refer to sexual reproduction. Organisms
reproduce by asexual means also. Fungi multiply and spread easily due
to the millions of asexual spores they produce. In lower organisms like
yeast and hydra, we observe budding. In Planaria (flat worms), we observe
true regeneration, i.e., a fragmented organism regenerates the lost part of
its body and becomes, a new organism. The fungi, the filamentous algae,
the protonema of mosses, all easily multiply by fragmentation. When it
comes to unicellular organisms like bacteria, unicellular algae or Amoeba,
reproduction is synonymous with growth, i.e., increase in number of cells.
We have already defined growth as equivalent to increase in cell number
or mass. Hence, we notice that in single-celled organisms, we are not very
clear about the usage of these two terms – growth and reproduction.
Further, there are many organisms which do not reproduce (mules, sterile
worker bees, infertile human couples, etc). Hence, reproduction also cannot
be an all-inclusive defining characteristic of living organisms. Of course,
no non-living object is capable of reproducing or replicating by itself.
Another characteristic of life is metabolism. All living organisms
are made of chemicals. These chemicals, small and big, belonging to
various classes, sizes, functions, etc., are constantly being made and
changed into some other biomolecules. These conversions are chemical
reactions or metabolic reactions. There are thousands of metabolic
reactions occurring simultaneously inside all living organisms, be they
unicellular or multicellular. All plants, animals, fungi and microbes exhibit
metabolism. The sum total of all the chemical reactions occurring in our
body is metabolism. No non-living object exhibits metabolism. Metabolic
reactions can be demonstrated outside the body in cell-free systems. An
isolated metabolic reaction(s) outside the body of an organism, performed
in a test tube is neither living nor non-living. Hence, while metabolism is
a defining feature of all living organisms without exception, isolated
metabolic reactions in vitro are not living things but surely living reactions.
Hence, cellular organisation of the body is the defining feature of
Perhaps, the most obvious and technically complicated feature of all
living organisms is this ability to sense their surroundings or environment
and respond to these environmental stimuli which could be physical,
chemical or biological. We sense our environment through our sense
organs. Plants respond to external factors like light, water, temperature,
other organisms, pollutants, etc. All organisms, from the prokaryotes to
the most complex eukaryotes can sense and respond to environmental
cues. Photoperiod affects reproduction in seasonal breeders, both plants
and animals. All organisms handle chemicals entering their bodies. All
organisms therefore, are ‘aware’ of their surroundings. Human being is
the only organism who is aware of himself, i.e., has self-consciousness.
Consciousness therefore, becomes the defining property of living
When it comes to human beings, it is all the more difficult to define
the living state. We observe patients lying in coma in hospitals virtually
supported by machines which replace heart and lungs. The patient is
otherwise brain-dead. The patient has no self-consciousness. Are such
patients who never come back to normal life, living or non-living?
In higher classes, you will come to know that all living phenomena
are due to underlying interactions. Properties of tissues are not present
in the constituent cells but arise as a result of interactions among the
constituent cells. Similarly, properties of cellular organelles are not present
in the molecular constituents of the organelle but arise as a result of
interactions among the molecular components comprising the organelle.
These interactions result in emergent properties at a higher level of
organisation. This phenomenon is true in the hierarchy of organisational
complexity at all levels. Therefore, we can say that living organisms are
self-replicating, evolving and self-regulating interactive systems capable
of responding to external stimuli. Biology is the story of life on earth.
Biology is the story of evolution of living organisms on earth. All living
organisms – present, past and future, are linked to one another by the
sharing of the common genetic material, but to varying degrees. DIVERSITY IN THE LIVING WORLD
If you look around you will see a large variety of living organisms, be it
potted plants, insects, birds, your pets or other animals and plants. There
are also several organisms that you cannot see with your naked eye but
they are all around you. If you were to increase the area that you make
observations in, the range and variety of organisms that you see would
increase. Obviously, if you were to visit a dense forest, you would probably
see a much greater number and kinds of living organisms in it. Each
different kind of plant, animal or organism that you see, represents a
species. The number of species that are known and described range
between 1.7-1.8 million. This refers to biodiversity or the number and
types of organisms present on earth. We should remember here that as
we explore new areas, and even old ones, new organisms are continuously
As stated earlier, there are millions of plants and animals in the world;
we know the plants and animals in our own area by their local names.
These local names would vary from place to place, even within a country.
Probably you would recognise the confusion that would be created if we
did not find ways and means to talk to each other, to refer to organisms
we are talking about.
Hence, there is a need to standardise the naming of living organisms
such that a particular organism is known by the same name all over the
world. This process is called nomenclature. Obviously, nomenclature or
naming is only possible when the organism is described correctly and we
know to what organism the name is attached to. This is identification.
In order to facilitate the study, number of scientists have established
procedures to assign a scientific name to each known organism. This is
acceptable to biologists all over the world. For plants, scientific names are
based on agreed principles and criteria, which are provided in International
Code for Botanical Nomenclature (ICBN). You may ask, how are animals
named? Animal taxonomists have evolved International Code of Zoological
Nomenclature (ICZN). The scientific names ensure that each organism
has only one name. Description of any organism should enable the people
(in any part of the world) to arrive at the same name. They also ensure
that such a name has not been used for any other known organism.
Biologists follow universally accepted principles to provide scientific
names to known organisms. Each name has two components – the
Generic name and the specific epithet. This system of providing a name
with two components is called Binomial nomenclature. This naming
system given by Carolus Linnaeus is being practised by biologists all
over the world. This naming system using a two word format was found
convenient. Let us take the example of mango to understand the way of providing scientific names better. The scientific name of mango is written
as Mangifera indica. Let us see how it is a binomial name. In this name
Mangifera represents the genus while indica, is a particular species, or a
specific epithet. Other universal rules of nomenclature are as follows:
1. Biological names are generally in Latin and written in italics.
They are Latinised or derived from Latin irrespective of their
2. The first word in a biological name represents the genus while
the second component denotes the specific epithet.
3. Both the words in a biological name, when handwritten, are
separately underlined, or printed in italics to indicate their Latin
4. The first word denoting the genus starts with a capital letter
while the specific epithet starts with a small letter. It can be
illustrated with the example of Mangifera indica.
Name of the author appears after the specific epithet, i.e., at the end of
the biological name and is written in an abbreviated form, e.g., Mangifera
indica Linn. It indicates that this species was first described by Linnaeus.
Since it is nearly impossible to study all the living organisms, it is
necessary to devise some means to make this possible. This process is
classification. Classification is the process by which anything is grouped
into convenient categories based on some easily observable characters.
For example, we easily recognise groups such as plants or animals or
dogs, cats or insects. The moment we use any of these terms, we associate
certain characters with the organism in that group. What image do you
see when you think of a dog ? Obviously, each one of us will see ‘dogs’
and not ‘cats’. Now, if we were to think of ‘Alsatians’ we know what we are
talking about. Similarly, suppose we were to say ‘mammals’, you would,
of course, think of animals with external ears and body hair. Likewise, in
plants, if we try to talk of ‘Wheat’, the picture in each of our minds will be
of wheat plants, not of rice or any other plant. Hence, all these – ‘Dogs’,
‘Cats’, ‘Mammals’, ‘Wheat’, ‘Rice’, ‘Plants’, ‘Animals’, etc., are convenient
categories we use to study organisms. The scientific term for these
categories is taxa. Here you must recognise that taxa can indicate
categories at very different levels. ‘Plants’ – also form a taxa. ‘Wheat’ is
also a taxa. Similarly, ‘animals’, ‘mammals’, ‘dogs’ are all taxa – but you
know that a dog is a mammal and mammals are animals. Therefore,
‘animals’, ‘mammals’ and ‘dogs’ represent taxa at different levels.
Hence, based on characteristics, all living organisms can be classified
into different taxa. This process of classification is taxonomy. External
and internal structure, along with the structure of cell, development process and ecological information of organisms are essential and form
the basis of modern taxonomic studies.
Hence, characterisation, identification, classification and nomenclature
are the processes that are basic to taxonomy.
Taxonomy is not something new. Human beings have always been
interested in knowing more and more about the various kinds of
organisms, particularly with reference to their own use. In early days,
human beings needed to find sources for their basic needs of food, clothing
and shelter. Hence, the earliest classifications were based on the ‘uses’ of
Human beings were, since long, not only interested in knowing more
about different kinds of organisms and their diversities, but also the
relationships among them. This branch of study was referred to as
systematics. The word systematics is derived from the Latin word
‘systema’ which means systematic arrangement of organisms. Linnaeus
used Systema Naturae as the title of his publication. The scope of
systematics was later enlarged to include identification, nomenclature
and classification. Systematics takes into account evolutionary
relationships between organisms.
TAXONOMIC CATEGORIES :
Classification is not a single step process but involves hierarchy of steps
in which each step represents a rank or category. Since the category is a
part of overall taxonomic arrangement, it is called the taxonomic category
and all categories together constitute the taxonomic hierarchy. Each
category, referred to as a unit of classification, in fact, represents a rank
and is commonly termed as taxon
Taxonomic categories and hierarchy can be illustrated by an example.
Insects represent a group of organisms sharing common features like
three pairs of jointed legs. It means insects are recognisable
in fact, represents a unit of classification. These taxonomic groups/
categories are distinct biological entities and not merely morphological
Taxonomical studies of all known organisms have led to the
development of common categories such as kingdom, phylum or division
(for plants), class, order, family, genus and species. All organisms,
including those in the plant and animal kingdoms have species as the