Diversity in the Living World Books Notes Study Material
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Diversity in The Living World Books Notes Study Material
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Diversity in The Living World Books Notes Study Material
What is Living?
Life is a unique, complex organisation of molecules that expresses itself through chemical reactions which lead to growth, development, responsiveness, adaptation and reproduction. The objects exhibiting op tics of life are designated as living beings. They have their own specific form and structure. Based on these characteristics, the living objects can be recognised as trees, shrubs, cattle, birds, fungi, bacteria, etc. However, a specific shape, size and structure is also present in many non-living objects such as brick or rock. No single trait of life can distinguish a living being from a non-living object. Therefore, a number of traits are examined simultaneously to differentiate living beings.
Characteristics of Living Beings
- Cellular Structure. It is defining property of living beings. Each living being is entity which is formed of one or more cells. The cells are made of protoplasm, a complex entity which is formed popularly called living matter. Composition of living matter is known. However, we have not yet been able to create protoplasm because of lack of organisation of biomolecules. Protoplasm and cellular structure are absent in viruses.
- Metabolism. All organisms operate a network of thousands of chemical reactions. The sum total of all chemical reactions occurring in an organism due to specific interactions amongst different types of molecules within the interior of cells is called metabolism (Gk. Metabole-change). Metabolism is defining property of living beings. All activities of an organism including growth, movements, development, responsiveness, reproduction, etc. are due to metabolism. No nonliving object shows metabolism.
However, metabolic reactions can be carried out outside the body of an organism in cell free systems. Such reactions are neither living nor nonliving. The isolated in vitro metabolic reactions can, however, be called biological reactions or living reactions as they involve biochemicals.
Types. Metabolism is of two kinds, catabolism and anabolism. Anabolism includes all the “building up” reactions. It is also called constructive metabolism since it involves the synthesis of complex substances from simpler ones, e.g., synthesis of organic compounds from CO2 and H2O during photosynthesis, formation of starch from glucose, production of proteins from amino acids, formation of lipids from fatty acids and alcohols. Energy is stored (as potential energy) in the process. (Diversity in the Living World Books Notes Study Material)
Catabolism (= katabolism) constitutes “breakdown reactions”. It is also known as destructive metabolism because it involves breaking of complex substances into simpler ones. Potential energy present in the complex substances is converted into kinetic energy Respiration is an example of catabolism. It releases energy for performing different body activities. (Diversity in the Living World Books Notes Study Material)
- Growth. Growth is irreversible increase in mass of an individual. A multicellular organism increases its mass by cell division. In plants growth continues throughout life as they have meristematic areas where cell divisions occur regularly. In animals, growth occurs to a certain age after which cells divide only to replace worn out and lost cells. Unicellular organisms also grow by cell division. However, cell division is also a means of reproduction in them. In higher animals and plants, growth and reproduction are mutually exclusive.
Living organisms show internal growth due to addition of materials and formation of cells inside the body. Such a method is called intussusception (L. intus — within, suscipere- to receive). A dead organism does not grow. However, some nonliving articles can increase in size, e.g., mountains, boulders, crystals, stones. It is due to addition of similar materials to their outer surface. The process is called accretion (L. accrescere — to increase).
In living beings growth producing substances are of two types, protoplasmic and apoplasmic. Protoplasmic substances are components of living matter like cytoplasm and nucleus. Apoplasmic substances (Gk. apo- away, plastos – formed) are nonliving materials formed by the cells which become component of tissues, e.g., cell wall, fibres of connective tissue, matrix of bone and cartilage.
Chemically growth is a result of difference between anabolism and catabolism. Growth occurs when anabolism exceeds catabolism. There will be no growth if anabolism and catabolism are equal. Degrowth or negative growth can occur when catabolism exceeds anabolism.
- Reproduction. It is the formation of new individuals of the similar kind – life arises from pre-existing life. Reproduction is not essential for survival of the individuals. It is required for perpetuation of a population. Ability for reproduction develops when a young individual becomes mature. Reproduction is of two types, asexual and sexual. Asexual reproduction is uniparental while sexual reproduction is generally biparental.
Asexual reproduction is the formation of new individuals from specialised or unspecialised parts of a single parent without the formation and fusion of gametes. It occurs by spores, binary fission, multiple fission, fragmentation and regeneration. Sexual reproduction involves the formation and fusion of two types of sex cells or gametes. The fusion product or zygote gives rise to an offspring.
In unicellular organisms, growth and reproduction are synonyms. Many organisms do not reproduce, e.g., mules, sterile worker bees, infertile human couples. Therefore, reproduction is not an all inclusive characteristic of living organism. However, no nonliving object has the power to reproduce or replicate. (Diversity in the Living World Books Notes Study Material)
- Consciousness. It is awareness of the surroundings and responding to external stimuli. The external stimuli can be physical, chemical or biological. The stimuli are perceived by sense organs in higher animals, e.g., eyes, ears, nose. Plants do not possess such sophisticated sense organs. However, they do respond to external factors such as light, water temperature, pollutants, other organisms, etc. Photoperiods (duration of daily exposure to light) influence reproduction in those animals and plants which breed during particular season (seasonal breeders).
All organisms, from primitive prokaryotes to most advanced and complex eukaryotes, are able to sense and respond to environmental factors. Organisms also handle chemicals entering their bodies. Human beings have an additional faculty of self consciousness (awareness of self). Consciousness is said to be the defining property of living organisms. (Diversity in the Living World Books Notes Study Material)
If a patient is lying in coma and is supported by machines for various functions, self consciousness and consciousness to external environment are supposed to be absent. Some of these patients never come back to normal life. They can neither be called living nor nonliving or dead.
- Organisation. A living being has an organisation, that is, the living being consists of several components and subcomponents which cooperate with one another for the well being of the whole organism. A living being has multiple level organisation. Each level of organisation has its own properties which are not found in its constituents. A cellular organelle develops a property not found in its interacting molecular components. A living cell has its own characteristics not found in its organelles. A tissue is able to have a trait not found in its constituent cells.
- Energy. Living beings constantly require energy not only to perform various activities of the body but also to overcome entropy or tendency to randomness. The source of energy is food. It is required by every cell of the body.
- Homeostasis (Homoeostasis). A favourable internal environment suitable for the functioning of body organs is present in every living being. It is quite different from the external environment. Changes in external environment do not have much impact on the internal environment as the living beings have a self regulated system to adjust and maintain the internal environment. The phenomenon is called homeostasis (Gk. homois — alike, stasis standing). Homeostasis is also present in each cell of a multicellular organism.
- Variations. Living beings possess variations and have the ability to evolve with time.
- Adaptations (L. ad-toward, apt-adjust). Useful inheritable variations or changes in form, function and behaviour which help an organism to adjust well and successfully in its environment are called adaptations. An organism is considered best adapted to an environment when it possesses inherited traits that enhance its survival and ability to reproduce in that environment. Adaptations allow the organisms to overcome seasonal and other changes in the environment. (Diversity in the Living World Books Notes Study Material)
They are of two types, short term adaptations (e.g., hibernation in most amphibians and reptiles and some mammals) and long term adaptations (e.g., the claws of different birds are well adapted to suit their perching habits).
- Healing and Repair. Living beings can repair and heal the broken and injured parts.
- Disposal of Wastes. Wastes generated by living beings are regularly disposed off.
- Movements. Living beings show movements of their parts. Some are able to move from place to place. The phenomenon is called locomotion.
- Life Span. Every living organism has a definite life span of birth, growth, maturity, senescence and death.
- Death. The stoppage of various life activities by an individual organism accompanied by increase in entropy is called death. Death occurs due to ageing, disease, accident and predation. Ageing normally occurs in all organisms after a period of reproductive maturity. It is, however, absent in some cases where the organism multiplies by binary fission, e.g., Amoeba, bacteria. A fully grown Amoeba, or a bacterium divides into two daughters. In the process it loses its independent existence. Here, natural death is absent and the organism is immortal.
Living organisms are, therefore, self replicating, evolving and self regulatory interactive systems capable of responding to external stimuli, sharing a common genetic material to varying degree both horizontally and vertically. (Diversity in the Living World Books Notes Study Material)
DIVERSITY IN THE LIVING WORLD (BIODIVERSITY)
Diversity in the living world or biodiversity is the occurrence of a wide variety of life forms differing in morphology, size, colour, anatomy, habitats and habits. Each different kind of plant, animal or microorganism represents a species. Currently there are some 1.7-1.8 million living organisms known to science. Out of these 1.25 million are animals. The plants number about 0.5 million. The single group of insects, however, outnumber all the plants and other animals. This group contains about 1.025 million species. Every year about 15000 new organisms used to be discovered. The number has increased since the launching of projects like Global Biodiversity Information Facility and Species 2000.
It is estimated that any number between 5 to 30 million species of living organisms are present on earth. Most of the unknown occur in the dense tropical rain forests and underwater reefs. Tropical rain forests are less than 8% of the total land. Their area is shrinking due to human exploitation. It is feared that if this exploitation is not stopped forthwith, many of species will become extinct forever before coming to light. Because of this, the Silent Valley rain forest of Kerala has been saved from submergence under proposed dam across river Kuntizha.
The past organisms have also left their impressions or remains in the rocks. They are called fossils. The term microfossils is used for impressions and remains of microorganisms as well as microscopic remains of larger organisms. It is believed that the extinct species ma out-number the living ones by 50-100 times. With such a large number of living and extinct organisms, it is essential to have a proper universal system of nomenclature, identification and classification that can bring out their true relationships. They are all domains of systematic.
- Systematics (Gk. systema- order, sequence). Systematics is a term often used interchangeably with taxonomy. According to Simpson (1961), systematics is the science that deals with diversity of organisms and all their comparative and evolutionary relationships based on study of comparative anatomy, development, comparative biochemistry, comparative physiology and comparative ecology by grouping of organisms at every level of classification right from species to the kingdom. (Diversity in the Living World Books Notes Study Material)
- Taxonomy (Gk. taxis- arrangement, nomos- law, de Candolle, 1813). It is the branch of study that deals with principles and procedures of identification, nomenclature and classification of organisms. (Diversity in the Living World Books Notes Study Material)
- Classification. Classification is the arrangement of organisms into convenient categories or groups on the basis of their similarities and differences in certain easily observable but fundamental characters. Then a hierarchy of categories is raised keeping in mind the affinities or relationships of various groups. Therefore, categories or taxonomic categories represent distinct biological entities and not merely morphological aggregates. The various categories are kingdom, phylum (or division in plants), class, order, family, genus and species.
- Nomenclature (L. nomen-name, calare-call). It is the science of providing distinct and proper names to organisms so that they can be easily recognised and differentiated from others. Through nomenclature each organism is given a two word name, generic and specific, e.g., Mangifera indica (Mango). (Diversity in the Living World Books Notes Study Material)
- Identification. Identification is the finding of correct name and place of an organism in a system of classification. Identification is done with the help of keys. This is carried out by determining similarity with already known organisms. Suppose there are 3 types of animals belonging to different species- x, y, z. An animal t is found in a locality being surveyed. It is found to resemble the species y. The finding that the animal t belongs to species y is its identification. (Diversity in the Living World Books Notes Study Material)
- Key. It is a list of alternate characters arranged in such a manner that by selection and elimination one can easily identify an organism as to its name and position. (Diversity in the Living World Books Notes Study Material)
Systematics is a branch of Biology that deals with cataloguing plants, animals and other organisms into categories that can be named, remembered, compared and studied. Study of only one organism of a group provides sufficient information about the remaining members of that group. Scientists connected with the study of systematics are called systematists or taxonomists. The terms systematics, taxonomy and classification are often held as synonyms but technically they carry different meanings. Simpson, (1961) has defined systematics as the branch of biology that deals with the diversity of organims at every level of classification. (Diversity in the Living World Books Notes Study Material)
Taxonomy, systematics or classification of organisms is based on the study of their comparative morphology (form, external and internal structure), cytology, embryology, fossil relatives, biochemical analysis and ecological relationships. The knowledge is required by all biologists working in different fields, e.g., agriculture, forestry, industry, ecology, medicines, genetics, physiology, etc. It also helps in developing evolutionary relationships, with or without the help of taxonomic studies of fossils. (Diversity in the Living World Books Notes Study Material)
History. Early classifications were concerned entirely for easy identification of useful and harmful plants and animals. Hippocrates (460-377 BC, father of medicine) and Aristotle (384-322 BC, father of zoology) arranged animals on the basis of habitat into aquatic, terrestrial, aerial animals. On the basis of single character, Greek scholars divided animals into four major groups, insects, birds, fishes and whales. Theophrastus (father of botany, 370-285 BC) divided plants on the basis of form, texture and habit into four groups— trees, shrubs, undershrubs and herbs. He described 480 plants in his book ‘Historia Plantarum’. (Diversity in the Living World Books Notes Study Material)
Pliny the Elder (28-79 A.D.) introduced the first system of artificial classification. His book, Historia Naturalis (c75 AD), mentions over 1,000 economic plants with about 2,000 items. More and more organisms were discovered and named. John Ray (1627–1705), English naturalist, described about 18600 plants in three volumes ‘Historia Generalis Plantarum’ between 1686-1704. The naturalist introduced the word “species” in its present sense for the first time. (Diversity in the Living World Books Notes Study Material)
John Ray defined species as an assemblage of individuals with similar parentage and having ability to pass the parental traits to the offspring.
Swedish naturalist Carolus Linnaeus developed the scientific system of naming species. It is known as binomial system of nomenclature. Linnaeus described 5900 species of plants in his book Species Plantarum (1753) and 4326 species of animals in Systema Naturae (1758). The word systematics is derived from Latin word ‘systema’ which means systematic arrangement of organisms. Linnaeus used systerma Naturae as the title of his book.
Right from Aristotle to Linnaeus, every systematist employed limited number of traits for classification of organisms. Therefore, the systems proposed by them remained artificial. Later on with increased indepth study of various biological domains, more and more characters were taken into consideration by taxomonists. It brought out natural affinities amongst organisms. This represented the phase of classical taxonomy which produced natural systems of classification. A modification of this system is numerical taxonomy or phenetics which came into existence during 1950s.
Simultaneously biologists began to find out evolutionary and genetic relationships. This resulted in development of phylogenetic classification or cladistics (Gk. klados-branch, L. clados-branch). In cladistics organisms are arranged in historical order in which they evolved as branches of the parent stock. This phase is known as new systematics or biosystematics. Father of new systematics is Sir Julian Huxley (1940).
Basics of Systematic Study
- Characterisation. The organism to be studied is described for all its morphological and other characteristics.
- Identification. Based on the studied characteristics, the identification of the organism is carried out to know whether it is similar to any of the known group or taxa. (Diversity in the Living World Books Notes Study Material)
- Classification. The organism is now classified on the basis of its resemblance to different taxa. It is possible that the organism may not resemble any known taxa or groups. A new group or taxon is raised to accommodate it. (Diversity in the Living World Books Notes Study Material)
- Nomenclature. After placing the organism in various taxa, its correct name is determined. If the organism is new to systematics, it is given a new name based on rules and conventions of nomenclature. (Diversity in the Living World Books Notes Study Material)
It is taxonomy based on observable morphological characters with normal individuals considered to be expression of the same while their variations are believed to be imperfect expressions. Classical taxonomy originated with Plato followed by Aristotle (father of Zoology), Theophrastus (Father of Botany) upto Linnaeus (father of Taxonomy) and his contemporaries.
- Species are delimited on the basis of morphological characters.
- Only a few characters are employed for classification.
- A few individuals or their preserved specimens are used for study. It is called typological concept.
- Species are believed to be static or immutable.
- Species is centre stage of study. Its subunits are not important.
New Systematics or Modern Taxonomy
The term new systematics was coined by Julian Huxley (1940). New systematics is systematic study which takes into consideration all types of characters including those from classifical morphology, anatomy, cytology, physiology, biochemistry, ecology, genetics, development (embryology), behaviour, etc. of the whole population instead of a few typological specimens.
In contrast classical systematics is based on the study of mainly morphological traits of one or a few specimens with supporting evidences from other fields. New systematics is also called population systematics and biosystematics. It strives to bring out evolutionary relationships amongst organisms.
- New systematics is based on the study of all types of variations in the species.
- Alongwith morphological characters, other investigations are also carried out to know the variety of traits.
- Delimitation of species is carried out on the basis of all types of biological traits. It is also called biological delimitation.
- Traits indicating primitiveness and advancement are found out.
- Inter-relationships are brought out.
- Species are considered dynamic.
Two types of names have been given to organisms, common and scientific.
Vernacular or Common Names
They are names given to the organisms in a particular language and region of the world. There are several types of names like English, Hindi, Gujarati, Marathi, Bengali, Tamil, etc. The names have been in use since times immemorial. New names are added whenever outside organisms are introduced into the area. (Diversity in the Living World Books Notes Study Material)
The vernacular names have their uses:
(i) They are based on some minor or major characteristics of organism, e.g., Kandiali (after the occurrence of spines), Dodhak (after milky latex).
(ii) They are brief.
(iii) The residents of an area become familiar with them since their childhood.
(iv) The vernacular names are easier to pronounce and remember by the residents of an area. Even then the vernacular names cannot be used by biologists due to the following reasons. (Diversity in the Living World Books Notes Study Material)
- All organisms do not occur in an area.
- Many organisms occurring in a particular area have not been given common names because they are either microscopic or are unimportant to human affairs. (Diversity in the Living World Books Notes Study Material)
- Certain common names have no significance. Rather they are misleading, e.g., Love-in-a-Mist (Nigella damascena), Hen-and-Chicken (Sempervivum soboliferum), Forget-Me-Not (Myosotis sylvatica), Widow’s Tears (Tradescantia or Rhoeo species), Yesterday-Today-Tomorrow (Brunfelsia hopeana). (Diversity in the Living World Books Notes Study Material)
- Some common names have incorrect meaning, e.g., Silver Fish, Jelly Fish, Cuttle Fish, and Star Fish. They belong to different phyla and have no relationship with true fishes Silver fish is not even aquatic. (Diversity in the Living World Books Notes Study Material)
- Common names cannot be used in communications amongst scientists of even the different regions of same country because the same organism has different local names in different parts. For example, Rose is called gulab in Hindi, golap in Bangla and Rojapo in Tamil. Butterfly is titli in Hindi, prajapati in Bangla and vannathu poochi in Tamil. (Diversity in the Living World Books Notes Study Material)
- Sometimes a single organism is known by several names in the same language, e.g., Water Lily has 81 Dutch names, 44 French names and 15 English names. Likewise, Prickly Poppy has 8 Hindi names. (Diversity in the Living World Books Notes Study Material)
- Different regions can have opposing names. Corn is Maize in Commonwealth countries while it implies wheat and other grains in U.S.A.
- A single name is often used for two or more species. Touch-me-not is the name for both Impatiens balsamifera (Balsam) and Mimosa pudica (Sensitive Plant). Dodhak is the name of many plants that possess milky latex e.g., Euphorbia, Sonchus, Launaea, etc.
- A wrong common name cannot be easily corrected.
Scientific or Technical Names
A scientific name is the one which is given by biologists and is understood to represent a particular organism in every part of the world. Scientists ensure that a name being given by them had not been used earlier for any other organism. The system of providing scientific and technical names is known as binomial nomenclature. (Diversity in the Living World Books Notes Study Material)
The system was developed by Linnaeus (Philosophia Botanica, 1751). The technical names recognised internationally are the ones given by Linnaeus in “Species Plantarum” (1753) and the 10th edition of his book “Systema Naturae” (1758). Binomial nomenclature is the system of providing organisms with appropriate and distinct names consisting of two words, first generic and second specific. The first or generic word is also called genus. It is like a noun and its first letter is written in capital form. (Diversity in the Living World Books Notes Study Material)
The second word or specific epithet represents the species. It is like an adjective. Its first letter is written in small form except occasionally when it denotes a person or place. To the two word name is appended the name of taxonomist who discovered the organism and provided it with its scientific name, e.g., Ficus bengalensis L., Mangifera indica Linn., Homo sapiens Linneaus.
The name of taxonomist can be written in full or in abbreviated form. There are several technical names which have three words, e.g., Homo sapiens sapiens, Acacia nilotica indica, Gorilla gorilla gorilla. Here the first word is generic, the second specific while the third word represents variety (mostly in botanical literature) or subspecies (mostly in zoological literature)
Rules of Binomial Nomenclature
There are five codes of nomenclature: (i) International Code of Botanical Nomenclature (ICBN), (ii) International Code of Zoological Nomenclature (ICZN), (iii) International Code of Bacteriological Nomenclature (IC Bac N), (iv) International Code of Viral Nomenclature (ICVN) and (v) International Code of Nomenclature for Cultivated Plants (ICNCP). International conferences are held from time to time to update the codes and resolve the controversies, if any. The rules of nomenclature framed under these codes as well as the rules set by Linnaeus are as follows:
- Each organism is given only one name consisting of two words, generic and specific.
- Though the codes are separate for plants, animals, bacteria, etc. and the same generic name can be given to different organisms belonging to these domains, it should be avoided. However the same specific name can be given to organisms belonging to different genera. Two species belonging to the same genus cannot have similar specific names. (Diversity in the Living World Books Notes Study Material)
- The generic name is written first. It is followed by specific epithet and then the name of the discoverer in full or in abbreviation.
- The specific epithet can be single or compound. Usually it begins with a small letter.
- The scientific name is printed in italics. It’s two words are separately underlined in handwritten description. An exception is made when the biological name is written as title of paragraph. (Diversity in the Living World Books Notes Study Material)
- The name of the author is kept in Roman script.
- The original names were taken from Latin and Greek languages. New names are now derived either from Latin language or are latinised. This is because Latin language is dead and, therefore, it will not change in form or spellings with the passage of time.
- Barring obvious error or misprint, a scientific name retains its original spellings.
- No names are recognised prior to those used by Linnaeus in 1753 for plants in “Species Plantarum” and in 1758 for animals in the 10th edition of “Systema Naturae”. (Diversity in the Living World Books Notes Study Material)
- The names of families and subfamilies should be based on name of type genus.
- The names of subfamilies, families and other categories are not printed in italics. They can, however, be written in bold letters.
- When a species is transferred or revised the name of the original worker is retained but in parenthesis, e.g., Syzygium cumini (L) Skeels.
- In publishing a new name the type specimen of the material is kept.
- A new scientific name is thought of on the basis of its characteristic, a personality or place. The selected name is such that it has no resemblance with any previously published name. (Diversity in the Living World Books Notes Study Material)
Advantages of Scientific Names
(i) Every species has a single and specific name consisting of two (or rarely three) words.
(ii) Every organism known to science has been provided with a scientific name irrespective of its importance.
(iii) There is no possibility of any change in the spellings of a scientific name as the latter has been derived from dead Latin language.
(iv) The names are of universal application for all the countries and the languages.
(v) They are generally descriptive.
(vi) The names indicate relationship of a species with others present in the same genus.
(vii) They are comprehensive and are easier to recollect.
(viii) A wrong name can easily be corrected.
(ix) A newly discovered organism can be easily provided with a new scientific name.
Revision of Group
It is the grouping of species into distinct taxa on the basis of their resemblances and differences, development of complexity or simplicity and hence evolutionary relationships. First of all criteria are selected for delimiting a species. In case of sexually reproducing organisms, interbreeding is used as the basic criterion. In case of others, morphology, physiology, cytotaxonomy and molecular biology including DNA-matching is resorted to. The species are then grouped into higher taxonomic categories on the basis of certain common features called correlated characters.
TAXONOMIC CATEGORIES (Taxonomic Hierarchy)
It is also called Linnaean hierarchy because it was first proposed by Linnaeus. Hierarchy of categories is the classification of organisms in a definite sequence of categories (taxonomic categories) in a descending order starting from kingdom and reaching upto species or an ascending order from species to kingdom. The number of similar characters of categories decreases from lowest rank (species) to highest rank. The hierarchy includes seven obligate categories- kingdom, division or phylum, class, order, family, genus and species.
The categories are arranged in descending sequence keeping the kingdom at the top. In order to make taxonomic position of species more precise, certain categories have been added to this list. They are called intermediate categories, e.g., subkingdom, superphylum or superdivision, subdivision, superclass, subclass, superorder, suborder, superfamily, subfamily, tribe, subspecies, variety, etc.
There are seven obligate categories and some intermediate categories. The seven obligate categories are as follows:
- Species. Species (used both as singular and plural) is a natural population of individuals or group of populations which resemble one another in all essential morphological and reproductive characters so that they are able to interbreed freely and produce fertile offspring. Mango is species indica of genus Mangifera (Mangifera indica). Potato is species tuberosum of genus Solanum (Solanum tuberosum). Lion’s species is leo of genus Panthera(Panthera leo) while Tiger’s species is tigris of genus Panthera (Panthera tigris).
Each species also called genetically distinct and reproductively isolated natural population. Mayr(1964) has defined species as “a group of actually or potentially interbreeding populations that are reproductively isolated from other such groups”. However there are two objections for using interbreeding as a sole criterion for delimitation of a species. (Diversity in the Living World Books Notes Study Material)
(i) Occasional hybrids occur in nature due to breakdown of mechanical, physiological and seasonal barriers between species. Interspecific hybrids have been obtained since ages artificially by man, e.g., mule. Hybrids between closely related species are often obtained in captivity. These hybrids are generally sterile, but some are fertile as well. A few examples are given below:
(a) Mule (sterile) – Female horse and male donkey
(b) Hinny (sterile) – Male horse and female donkey
(c) Tigon (fertile) – Male tiger and female lion
(d) Liger (fertile) – Male lion and female tiger
(ii) Sexual reproduction is absent in procaryotes and some protists. In such cases and fossils morphological differences, cytotaxonomy and chemotaxonomy are resorted. (Diversity in the Living World Books Notes Study Material)
Principles for Delimiting a Species
(i) The different members of a species whether morphologically similar or dissimilar are able to interbreed freely and produce fertile offspring,
(ii) All the members whether present in one population or different populations found in remote areas of the globe are derived from a common ancestor. (Diversity in the Living World Books Notes Study Material)
(iii) The members resemble one another more than they resemble individuals of any other species.
(iv) There is a complete anatomical similarity.
(v) All the members of a species have similar karyotype (cytotaxonomy)- there is similarity in the number, size, shape and behaviour of meiotic chromosomes. (Diversity in the Living World Books Notes Study Material)
(vi) All the individuals of a species contain similar genetic material.
(vii) There is a broad similarity in morphological characters.
(viii) At the molecular level there is similarity in the types of proteins, enzymes, hormones and other biochemicals (chemotaxonomy).
- Genus. It is a group or assemblage of related species which resemble one another in certain correlated characters. Correlated characters are those similar or common features which are used in delimitation of a taxon above the rank of species. All the species of genus are presumed to have evolved from a common ancestor. (Diversity in the Living World Books Notes Study Material)
It is not essential for a genus to have several species. There are genera which have only one species. They are called monotypic. Others are known as polytypic. Thus the genus Solanum has a large number of closely related species, e.g., S. tuberosum (Petato), S. melongena (Egg Plant, Brinjal), S. nigrum (Black Night-shade), S. surratense. (Diversity in the Living World Books Notes Study Material)
Similarly Lion, Tiger, Leopard, Jaguar are closely related species which have been placed in the genus Panthera. The animals are respectively named as Panthera leo, P. tigris, P. Pardus and P. onca. These ferocious animals are related with some differences to cats included in the genus Felis, e.g., F. domestica (Common Cat), F. bengalensis (Leopard Cat),F.marmorata (Marbled Cat), E. chaus (Jungle Cat), F. viverrina (Fishing Cat) and F. temminki (Golden Cat). (Diversity in the Living World Books Notes Study Material)
- Family. It is taxonomic category which contains one or more related genera. All the genera of a family have some common features or correlated characters. They are separable from genera of a related family by important and characteristic differences in both vegetative and reproductive features. (Diversity in the Living World Books Notes Study Material)
Thus the genera of cats (Felis) and leopard (Panthera) are included in the family felidae. The members of family felidae are quite distinct from those of family canidae (dogs, foxes, wolves). Similarly, the family solanaceae contains a number of genera like Solanum, Withania, Datura, Petunia and Nicotiana. They are distinguishable from the genera of the related family convolvulaceae (Convolvulus, Ipomoea).
- Order. The category includes one or more related families. Thus the family solanaceae is placed in the order polemoniales alongwith four other related families (convolvulaceae. boraginaceae, hydrophyllaceae and polemoniaceae). Similarly, the families felidae and canidae are included under the order carnivora alongwith hyaenidae (hyaenas) and ursidae (bears). (Diversity in the Living World Books Notes Study Material)
- Class. A class is made of one or more related orders. For example, the class dicotyledoneae (dicotyledonae, dictoyledons) of flowering plants contains all dicots which are grouped into several orders (e.g., rosales, passiflorales, polemoniales, sapindales, ranales, etc.) Likewise, class mammalia of animals includes all mammals which range from bats (order chiroptera), kangaroos (order marsupialia), rodents (order rodentia), whales (order cetacea), carnivores (order carnivora) to great apes and man (order primata).(Diversity in the Living World Books Notes Study Material)
- Division or Phylum. It is a category higher than that of class. The term phylum is used for animals while division is commonly employed for plants. A division or phylum is formed of one or more classes. The phylum chordata of animals contains not only the class mammalia but also aves (birds), reptilia (reptiles), amphibia (amphibians), cyclostomata, chondrichthyes, osteichthyes (fishes), etc. The division spermatophyta in plants has similarly seven classes of gymnosperms and two classes of angiosperms.(Diversity in the Living World Books Notes Study Material)
- Kingdom. It is the highest taxonomic category. All plants are included in kingdom plantae while all animals belong to kingdom animalia.
Taxon (plural-taxa, Gk, taxis-arrangement)
It is a unit in classification which may represent any level of grouping of organisms based on certain easily observable common characteristics like Maize (species), roses (genus), grasses (family), conifers (order), dicots (class), seed plants (division), etc. The term was introduced for the first time by ICBN during 1956. (Diversity in the Living World Books Notes Study Material)
Mayr (1964) has defined taxon to be a taxonomic group of any rank that is sufficiently distinct to be worthy of being assigned to a definite category. Simpson (1961) recognises taxon to be a group of real organisms recognised as a formal unit at any level of hierarchical classification.
There is some confusion in the use of taxon and category. Bryophyta is a taxon while division is a category. Similarly Zea mays is a taxon while species is a category. Thus while category represents an abstract term, taxon represents the real organisms. Certain common names represent species while other represent genus, family, order, class or phylum.(Diversity in the Living World Books Notes Study Material
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