Chapter
1
The
living World
Syllabus: Biodiversity; Need for classification; three domains of life (Archae, eubacteria and eukaryotes); taxonomy and systematics; concept of species and taxonomical hierarchy; binomial nomenclature.
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.
Let us try to understand each of these.
1. All living organisms grow. Increase in mass and
increase in number of individuals are twin characteristics of growth.
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.
2. 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.
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.
3. 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.
The sum
total of all the chemical reactions occurring in our body is metabolism. No
non-living object exhibits metabolism.
4. Cellular
organisation of the body is the defining feature of life forms.
By definition, cellular
organization is the components that make up the cell and how
they are arranged inside it. Each component called an organelle, performs a specific function vital for the
cell.
4.
Consciousness.
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.
Consciousness therefore, becomes the defining property of living organisms.
1.2 DIVERSITY IN THE LIVING WORLD
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.
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). 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 (The generic
name is the genus to which the species belongs, and the specific epithet refers
to the species within that genus. For example, in the name Homo sapiens,
Homo is the genus and sapiens is 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.
Let us take
the example of mango
The
scientific name of mango is written as Mangifera
indica.
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 origin. Like Mangifera indica
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 origin. Mangifera
indica
4. The
first word denoting the genus starts with a capital letter while the specific
epithet starts with a small letter.
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.
Classification
and Taxonomy
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’. 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.
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.
1.3
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 concrete objects
which can be classified, and thus were given a rank or category.
Homo sapiens is the scientific name of humans, and each level (like class Mammalia
or order Primates) is a taxonomic category (taxon).
Homo sapiens, (Latin:
“wise man”) the species to which all modern human beings belong.
1.3.1
Species
|
Taxonomic Hierarchy (from
broad to specific) Kingdom → Plantae (All plants) ↓ Division → Angiospermae (Flowering plants) ↓ Class → Dicotyledonae (Plants with two seed leaves) ↓ Order → Sapindales (Includes mangoes, maples, etc.) ↓ Family → Anacardiaceae (Mango family) ↓ Genus → Mangifera (All types of mango trees) ↓ Species → indica (Indian mango) ➡ Scientific
Name: **Mangifera indica** |
A biological species is a group of
organisms that can reproduce with one another in nature and produce fertile offspring. Taxonomic
studies consider a group of individual organisms with fundamental similarities
as a species. Mangifera indica, Solanum tuberosum (potato) and Panthera leo
(lion). All the three names, indica, tuberosum and leo, represent the
specific epithets, while the first words Mangifera, Solanum and Panthera are
genera and represents another higher level of taxon or category. The generic name is the genus to which the
species belongs, and the specific epithet refers to the species within that
genus.
1.3.2
Genus
Genus
comprises a group of related species which has more characters in common in
comparison to species of other genera.
Example:
|
Genus |
Species |
similarity |
|
Solanum |
Nigrum (black berry) |
Phyllotaxy
(arrangement of leaves), venation(arrangement of system of veins),
inflorescence (a cluster of flowers on a branch or a system of branches) |
|
|
Melongena (brinjal) tuberosum (potato) |
|
|
Panthera |
Leo (lion) |
The
so-called “big cats” (genus Panthera), especially the lion, often
roar, growl, or shriek |
|
Pardus (leopard) |
||
|
Tigris (tiger) |
1.3.3
Family
The next
category, Family, has a group of related genera with still less number of
similarities as compared to genus and species.
|
Family |
Genus |
Similarity |
|
Felidae ( Similar fur
characteristics) |
panthera |
The
family Felidae consists of at least 36 wild cat species. These felids are
morphologically similar with rounded, flat faces, facial whiskers,
large eyes, and large ears |
|
Felis (cats) |
1.3.4
Order
Order being
a higher category, is the assemblage of families which exhibit a few similar characters.
The similar characters are less in number as compared to different genera
included in a family.
|
Order |
Family |
similarity |
|
Carnivore ( Elonagted and pointed
canines) |
Canidae
(dogs and related species) |
Most
carnivores are meat eaters. Their jaws only move vertically, or up and down.
Most carnivores have sharp side teeth, or canines, that help them sever meat. |
|
Felidae
(cats) |
||
|
Ursidae
(bears) |
1.3.5
Class
This
category includes related orders. For example, order Primata comprising monkey,
gorilla and gibbon is placed in class Mammalia along with order Carnivora that
includes animals like tiger, cat and dog.
1.3.6
Phylum
Classes
comprising animals like fishes, amphibians, reptiles, birds along with mammals
constitute the next higher category called Phylum. All these, based on the
common features like presence of notochord and dorsal hollow neural system, are
included in phylum Chordata. In case of plants, classes with a few similar
characters are assigned to a higher category called Division.
1.3.7
Kingdom
All animals
belonging to various phyla are assigned to the highest category called Kingdom
Animalia in the classification system of animals. The Kingdom Plantae, on the
other hand, is distinct, and comprises all plants from various divisions.
Henceforth, we will refer to these two groups as animal and plant kingdoms.
1.4
TAXONOMICAL AIDS
It is used for classification of an
organism, and the information gathered is also stored along with the specimens.
Biologists have established certain procedures and techniques to store and
preserve the information as well as the specimens.
1.4.1 Herbarium
1.4.2 Botanical Gardens
These specialised gardens have
collections of living plants for reference. Plant species in these gardens are
grown for identification purposes and each plant is labelled indicating its
botanical/scientific name and its family. The famous botanical gardens are at
Kew (England), Indian Botanical Garden, Howrah (India) and at National
Botanical Research Institute, Lucknow (India).
1.4.3 Museum
Biological museums are generally set
up in educational institutes such as schools and colleges. Museums have
collections of preserved plant and animal specimens for study and reference.
Specimens are preserved in the containers or jars in preservative solutions.
1.4.4 Zoological Parks
These are the places where wild
animals are kept in protected environments under human care and which enable us
to learn about their food habits and behaviour. All animals in a zoo are
provided, as far as possible, the conditions similar to their natural habitats.
Children love visiting these parks, commonly called Zoos.
1.4.5 Key
1. Key is a kind of Book and is
another taxonomical aid used for identification of plants and animals based on
the similarities and dissimilarities.
2. The keys are based on the
contrasting characters generally in a pair called couplet.
3. It represents the choice made
between two opposite options.
4. This results in acceptance of only
one and rejection of the other. Each statement in the key is called a lead.
Separate taxonomic keys are required for each taxonomic category such as
family, genus and species for identification purposes. Keys are generally
analytical in nature.
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📝 MCQ:
Which of the following statements is correct about taxonomic keys?
A) A key contains multiple unrelated characters for identification. Download
B) Each statement in a key is called a lead and is part of a couplet. Download
C) Keys are used only for identifying species, not genus or family. Download
D) A key always presents more than two options at each step. Download
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