2. Systematics of Living Organisms
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Can
you recall?
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What is five kingdom system of classification?
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There is great diversity of organisms around us.
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Since time immemorial, we humans have been exploiting
this wealth for our own benefit.
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During this process man tried to differentiate between
and identify the organisms.
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Eventually this evolved into a branch of biology known
as systematics or classification.
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The methods of classification dates back to ancient
time when Indian, Greek and Roman philosophers have contributed their might to
systematize science.
2.1
Systematics:
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‘‘Systematics is the study of kinds and diversity of
organisms and their comparative and evolutionary relationship’’(G. Simpson,
1961).
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Taxonomy:
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Taxonomy means classification following certain rules
or principles.
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Word Taxonomy comes from two Greek words,
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taxis meaning arrangement and
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nomous meaning law or rule.
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The term taxonomy was first introduced by A. P. de
Candolle (Swiss Botanist) [1778-1841].
2.2
Classification:
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It is the arrangement of organisms or groups of
organisms in distinct categories in accordance with a particular and well
established plan.
• This classification is based on similarities and dissimilarities among the organisms.
• Artificial:
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It is the classification that is based on few easily
observable and non-evolutionary features such as habit, colour, form, etc.;
often irrespective of their affinity (relationship) with other organisms.
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e.g., Linnaeus system of classification.
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Natural:
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It is the classification which is based on objectively
significant rather than being selected for convenience like artificial system
of classification
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e.g., Bentham and Hooker's system of classification.
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e.g., Engler and Prantles classification
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In the system of classification the terms like ‘taxa’
and ‘categories’ are often used.
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Each category is referred to as a unit of
classification.
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In fact, it represents a rank and is commonly termed
as taxon.
2.3 Three domains
of life:
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It is believed that the life originated on earth in
its very simple form.
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Constant struggle of the early living beings gave rise
to more and more perfect forms of life.
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This struggle and progress is evolution which led to
formation of diverse life forms.
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Carl Woese in 1990 proposed three domains of life to
classify life forms.
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They are Archaea, Bacteria and Eukarya.
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Domain is an unit larger than Kingdom in the system of
classification.
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Bacteria and Archaea both have prokaryotic cells where
as Eukarya have eukaryotic cell.
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All the three domains have very unique ribosomal RNA
(rRNA).
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Archaea are known for their survival in very extreme
conditions like high temperature, salinity, acidic conditions, etc.
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Bacteria, though are prokaryotes differ from Archaea
in structure of cell wall.
2.4
Chemotaxonomy:
• It is method of biological classification based on similarities and differences in structure of certain compounds present among the organisms being classified.
• In short, it is the classification based on chemical constituents of organisms.
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e.g., Archaea cell wall is without
peptidoglycan and that of Prokarya is with peptidoglycan.
• Among Eukarya, fungi have chitinous cell wall while plants have cellulosic cell wall.
2.5
Numerical taxonomy:
• The system is based on quantification of characters and develops an algorithm for classification.
• The basic aim of this taxonomy was to create a taxonomy using numeric algorithms like cluster analysis rather than using subjective evaluation of their properties.
• This system was first proposed by Sokel and Sneath in 1963.
• 2.6 Cladogram :
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It is a typical branching pattern.
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[As shown on previous page, a diagram of three domains
of life is a cladogram.]
• It represents a hypothetical relationship denoting a comparison of organisms and their common ancestors.
• 2.7 Phylogeny :
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It is evolutionary relationship of organism.
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It is an important tool in classification as it takes
into account not merely the morphological status but also the relationship
of one group of organism with other groups of life.
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The system helps to understand the evolution
and also focuses on the similarities of their metabolic functioning.
• Woese’s three domain concept as well as Whittaker’s five kingdom system are very good examples of phylogenetic relationship.
• 2.8 DNA barcoding :
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DNA barcoding, is a new method for the identification
of any species based on its DNA sequence from a tiny tissue sample
of the organism under study.
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It helps
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to study newly identified species
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to understand ecological and evolutionary
relationships between living beings.
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The process of DNA barcoding includes two basic steps:
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(a) collecting DNA barcode data of known species and
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(b) matching the barcode sequence of the unknown
sample against the barcode library for identification.
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DNA barcoding has many applications.
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A few to mention are,
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protection of endangered species,
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preservation of natural resources,
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pest control in agriculture,
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identifying disease vectors,
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authentication of natural health products and
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identification of medicinal plants
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Can
you tell?
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1. Which characters of organisms are visible
characters?
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2. Name the recent approaches in taxonomy.
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3. What is DNA barcoding?
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4. What is evolution?
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5. Enlist uses of taxonomy.
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2.9 Taxonomic Categories:
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Classification is not a single step process.
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It involves hierarchy of steps.
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Each step represents a rank or category.
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Since the category is a part of overall
taxonomic arrangement it is called taxonomic category.
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All categories together constitute the taxonomic
hierarchy.
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Kingdom, division, class, order, family, genus,
species are the categories in hierarchial sequence.
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These are compulsory categories.
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Besides, there are some facultative categories
like sub-order, sub-family, etc. to be used as per need.
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2.10
Taxonomic Hierarchy :
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Taxon :
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A taxon is the taxonomic group of any rank in the
system of classification (H.J. Lam 1948)
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e.g.,
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In plant kingdom each one of the following such as Angiosperms,
Dicotyledonae, Polypetalae, Malvaceae represents a
taxonomic group i.e. a taxon.
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2.11
Units of Classification:
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Species:
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Species is the principal natural taxonomic unit.
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It ranks below a genus and denoted by Latin
binomial.
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It is considered as the basic unit of
classification.
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It is a group of
organisms that can interbreed under natural condition to
produce fertile offspring.
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It was thought to be an indivisible, stable
and static unit.
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However, in the modern taxonomy, subdivision of
species such as sub-species, varieties and populations are seen and given more
importance.
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Genus :
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Genus is a taxonomic rank or category larger than
species used in the biological classification of living and fossil
organisms.
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Genus is a group of
species bearing close
resemblance to one another
in their morphological characters, but they do not interbreed.
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e.g., 1. Tiger, Leopard, Lion all three
belong to same genus Panthera.
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They have common characters yet are different from
each other because their genus is same, but species is different.
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e.g., 2. genus Solanum.
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Brinjal and potato both belong to this genus.
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Family :
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It is one of the major hierarchial taxonomic rank.
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A family
represents a group of closely related genera.
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e.g., 1. Genera like Hibiscus, Gossypium, Sida,
Bombax are included in same family Malvaceae.
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e.g., 2. Cat also belongs to family
of leopards, tigers and lions, family Felidae but dog
belongs to different family Canidae
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Cohort / Order:
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It is taxonomic rank used in the
classification of organisms and recognised by nomenclature codes.
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An order is a
group of closely related families showing definite affinities.
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Order thus is a step above family in taxonomic
hierarchy.
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Members belonging to same order, but different
families may show very few dis similarities.
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e.g., 1. Family - Papavaraceae, Brassicaceae,
Capparidaceae, etc., with parietal placentation are grouped in
order Parietales.
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e.g., 2. Families of dogs and cats though
are different, they belong to same order Carnivora.
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Class :
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The class is the distinct taxonomic rank of biological
classification having its own distinctive name.
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A group of higher taxonomic rank than order.
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Class is the
assemblage of closely allied orders.
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e.g.,
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Orders Carnivora and order Primates
belong to class Mammalia.
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Thus monkeys, gorillas, gibbons (Primates)
and dogs, cats, tigers (Carnivora) belong to same class.
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Division
/ Phylum :
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The division is a category composed of
related classes.
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e.g., Division Angiospermae includes two
classes - Dicotyledonae and Monocotyledonae
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In
animal classification division is a sub-unit of Category / Phylum.
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Sub-kingdom :
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Different divisions having some
similarities form sub-kingdom.
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e.g., The divisions Angiospermae and Gymnospermae
will the sub-kingdom Phanerogams or Spermatophyta.
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Kingdom :
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It is the highest taxonomic category composed of
different subkingdoms.
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e.g., sub-kingdom Phanerogams and
Cryptogams form the Plant kingdom or Plantae which includes all the plants
while all animals are included in kingdom Animalia.
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The taxonomic categories we have considered so far are
broad categories.
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Scientists have added sub-categories to these in order
to place organisms in more scientific manner.
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You will observe that as we go higher in taxonomical
ladder, number of common characters go on decreasing.
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If we are comparing two organisms that are related to
each other only at division or phylum level, their classification may become
difficult.
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Can
you tell?
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1. Why horse and ass are considered to be two
different species or animals?
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2. Make a flow chart showing taxonomic hierarchy.
