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At least 10 million kinds of organisms inhabit  the Earth but less than one third of these have been  studied and catalogued  (put in record) by biologists.  Diversity among the Earth's organisms is  more obvious than the fundamental unity of life.  We see that all organisms share  many biological characteristics.  Five principal groups of organisms are  prokaryotes, protists,  fungi, plants, and animals. In this chapter we will focus  on the differences among different groups of organisms. We will also see how are  organisms classified and  named and what are the concerns with the existence of biodiversity 

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Biodiversity  The term "biodiversity"  has been derived from ' bio' and 'diversity'.  "Diversity" means variety within a species and  among species. Biodiversity is a measure of the variety of organisms present in different ecosystems. 

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The diversity of plants' (flora) and  animals' (fauna) in a region depends  on climate, altitude, soils and  the presence of other species. Biodiversity is not distributed  evenly on Earth. It is richest in the tropics. Temperate regions also have  many species while there are  fewer species in the polar regions.  Biodiversity found on Earth today is  the result of 4 billion years of evolution.  The origin of life is not well known  to science, though limited evidence suggests  that until 600 million years ago,  all life consisted of bacteria and  similar unicellular organisms. 

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Importance of biodiversity Biodiversity provides food for humans.  A significant proportion of drugs are  derived, directly or indirectly,  from biological sources.  A wide range of industrial materials e.g.  building materials, fibers,  dyes, resins, gums, adhesives,  rubber and oil are derived  directly from plants.  Biodiversity plays important role in making  and maintaining ecosystems. It plays a part in regulating  the chemistry of our atmosphere and  water supply.  Biodiversity is directly involved  in recycling nutrients and  providing fertile soils. 

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Classification Aims And  Principles  There is a large collection of very dissimilar forms of organisms,  found on Earth.  Over 1.5 million types of animals and  over 0.5 million types of plants are  known to biologists and  these are only a small percentage of the total types estimated  to live on Earth.  They range in complexity from small and  simple bacteria to large and  complex human beings. Some of them live in sea,  some on land; some walk,  others fly, and still others are  stationary.  Each has its own way of life i.e. getting  food, avoiding unfavorable environmental conditions,  finding a place to live , and reproducing its kind.  When there are so many diverse kinds of organisms,  it becomes difficult to learn  about the characteristics of each. 

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To study such a large collection,  biologists classify the organisms into groups and  subgroups and for this task they require  some system. Biological classification is  a method by which biologists divide  organisms into groups and  subgroups. 

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Aims of Classification  The branch of biology which deals  with classification is  called taxonomy and the branch which deals  with classification and  also traces the evolutionary history of organisms is  known as systematic. The main aims of both these branches are; To determine similarities and  differences among organisms so that they can be  studied easily. To find the evolutionary relationships among organisms. 

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Basis of Classification  Classification is based  on relationship amongst organisms and  such relationship is got  through similarities in characteristics. These similarities suggest  that all organisms are  related to one another at some point in their evolutionary histories. However, some organisms are  more closely related than others. For example sparrows are  more closely related to pigeons than to insects.  It means that the former two have  common evolutionary histories. When biologists classify  organisms into groups and  subgroups, the similarities are  seen in external and internal structures and  stages of development. Modern genetics provides  another type of information to taxonomists.  The similarities and differences in the DNA of two studied  organisms can be used for getting  idea about similarities and  differences in their structures and  functions. 

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Taxonomic Hierarchy   The groups into which organisms are  classified are known as taxonomic categories or  taxa (singular "taxon"). The taxa form a ladder,  called taxonomic hierarchy. All organisms are divided  into five kingdoms. So kingdom is the largest taxon. On the basis of similarities,  each kingdom is further divided  into smaller taxa in the following way:  Phylum (Division: for plants and  fungi): a phylum is a group of related classes. Class: A class is a group of related orders.  Order: An order is a group of related families.  Family: A family is a group of related genera.  Genus: A genus is a group of related  species. Species: A species consists  of similar organisms. 

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Species The Basic Unit of Classification Species is the basic unit of classification.  "A species is a group of organisms which can interbreed  freely among them and produce  fertile offspring, but  are reproductively isolated from all other such groups in nature."  Each species possesses its own distinct structural,  ecological and behavioral characteristics. 

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Five-Kingdom Classification System In 1937, E-Chatton suggested  the terms of, "Procariotique"  to describe bacteria and  "Eucariotique" to describe  animal and plant cells.  In 1967, Robert Whittaker introduced  the five-kingdom classification system. This system is based on;  The levels of cellular organization i.e. prokaryotic,  unicellular eukaryotic and  multicellular eukaryotic  The principal modes of nutrition i.e. photosynthesis,  absorption, and ingestion. On this basis, organisms are  classified into five Kingdoms:  monera, protista, fungi,  plantae and animalia.  In 1988, Margulis and Schwartz modified  the five-kingdom classification of Whittaker.  They considered genetics along with cellular organization and  mode of nutrition in classification.  They classified the organisms into the same five kingdoms as proposed  by Whittaker. 

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The Five Kingdoms  1. Kingdom monera: It includes  prokaryotic organisms i.e. they are  made of prokaryotic cells. Monerans are unicellular,  although some types form  chains, clusters, or colonies of cells.  Prokaryotic cells are radically different from eukaryotic cells. Most are heterotrophic but  some perform photosynthesis because they have  chlorophyll in their cytoplasm. Within this kingdom, there are  two different kinds of organisms i.e. bacteria and  cyanobacteria. 

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2. Kingdom protista:  It includes eukaryotic unicellular and  simple multicellular organisms. There are three main types of protists.  Algae are unicellular, colonial or  simple multicellular.  They resemble plant cells with cell walls and  chlorophyll in chloroplasts. Simple multicellular means  that they do not have multicellular sex organs and  do not form embryos during their life cycles.  Protozoans resemble animals whose cells lack  chlorophyll and cell walls. Some protists are fungi-like. 

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3. Kingdom fungi:  It includes eukaryotic multicellular heterotrophs which are  absorptive in their nutritional mode e.g. mushrooms. Most fungi are decomposers. They live on organic material,  secrete digestive enzymes and  absorb small organic molecules formed  by the digestion by enzymes 

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4. Kingdom Plantae : It includes eukaryotic multicellular autotrophs.  Plants are autotrophic in nutritional mode,  making their own food by photosynthesis.  They have multicellular sex organs and  form embryos during their life cycles.  Mosses, ferns and flowering plants are  included in this kingdom. 

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5. Kingdom Animalia:  It includes eukaryotic multicellular consumers.  Animals live mostly by ingesting  food and digesting it within specialized cavities. They lack cell wall and  show movements. 

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Binomial Nomenclature  Binomial nomenclature is  the method of giving scientific names to living  organisms.  As the word "binomial" suggests , the scientific name of a species consists  of two names: the first is  genus name and the second one is  the name of species.  Swedish biologist Carolus Linnaeus (1707-1778 AD) first introduced  and adopted the system of binomial nomenclature. His system spread rapidly and  became popular.  Many of his names are in use today 

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Some of the rules which are  universally adopted while suggesting  and documenting scientific names, are:  Scientific names are usually printed  in italics, such as Homo sapiens. When handwritten they are  underlined.  The first term (generic name) always begins  with capital letter, while species name is  never capitalized (even when derived  from a proper name).  The scientific name is generally written  in full when it is first used.  But when several species from the same genus are being  listed, it may then be  abbreviated by just using  an initial for genus; for example Escherichia coli becomes  E. coli. 

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Significance In biological research,  common names cause many problems.  Different regions have different names for the same organism.  For example; common name of onion in Urdu is  'Piyaz' but in different regions of Pakistan it is  also known as 'ganda' or  'bassal' or 'vassal'. In other countries, it has  other sets of names. In science, it is known  with a single name as Allium cepa.  In some cases, different organisms are  called by the same common name.  For example; the name 'black bird'  is used for crow as well as for raven. 

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Common names have no scientific basis.  For example; a fish is a vertebrate animal with fins and gills. But several common names of ' silver fish', 'cray fish' , 'jelly fish', and 'star fish'  do not fit the biologist's definition of a fish. To avoid all these confusions,  organisms are given scientific names by using  binomial nomenclature . The value of this system is  due to its widespread use and  the stability of its names. In binomial nomenclature,  every species can be unambiguously identified  with just two words.  Same name can be used all over the world,  in all languages, avoiding  difficulties of translation. 

Chapter 3

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