Chapter 1: The Living World (Diversity in the Living World)

There are millions of organisms (1.7–1.8 million species known) and it is impossible to study each one separately. Classification groups them into convenient categories on easily observable characters, so that (i) study becomes easy and systematic; (ii) identifying an organism tells us many of its characters at once; (iii) relationships among different groups (including evolutionary ones) become clear; (iv) a sound basis for naming is obtained; and (v) organisms useful in agriculture, forestry, industry and medicine can be recognised.

Classification depends on our knowledge of organisms. New organisms keep being discovered, and new kinds of information (cell structure, biochemistry, development, ecology, DNA and evolutionary relationships) keep becoming available. As knowledge grows, earlier groupings are found to be artificial or incomplete, so systems are revised to reflect natural and evolutionary relationships better.

Possible criteria: sex (male, female); age group (child, teenager, adult, elderly); relation (family, relatives, friends, neighbours); occupation (students, teachers, shopkeepers, doctors); place (school, locality, town); language spoken; physical features such as height or colour of eyes. The criteria chosen depend on the purpose of the classification, just as in biology.

Identification tells us the correct name and hence the taxonomic position of an organism, and so its characters and relationship with other organisms. For populations it tells us about their number, distribution, variation and the biodiversity of an area; it helps in conservation of rare and endangered species, in recognising useful (crop, medicinal) and harmful (pest, pathogen) organisms, and in studying evolution.

Mangifera indica: the genus begins with a capital letter and the specific epithet with a small letter; both are in italics (or underlined separately when handwritten).

A taxon (plural taxa) is a taxonomic group of any rank, i.e. a unit of classification representing a category in the hierarchy. Examples: kingdom Animalia, phylum Chordata, class Mammalia, order Primata, family Hominidae, genus Homo, species sapiens; for a plant: kingdom Plantae, division Angiospermae, class Dicotyledonae, order Sapindales, family Anacardiaceae, genus Mangifera, species indica.

(a) and (c) are both correct ascending sequences (some categories are skipped, but the order is right); (b) is wrong because species is below genus. If one answer is wanted, (c) is the most complete correct sequence; (a) is also in correct order.

Morphological (taxonomic) species: a group of individuals with fundamental similarities in form, distinguishable from related species by distinct morphological differences (NCERT). Biological species (Ernst Mayr): a group of actually or potentially interbreeding natural populations, reproductively isolated from other such groups. Evolutionary species: a lineage with its own evolutionary role and tendencies. In higher plants and animals, which reproduce sexually, the biological species concept works well: members interbreed and produce fertile offspring. In bacteria, which reproduce asexually (and exchange genes only occasionally), interbreeding cannot be used; bacterial species are defined by morphology, physiology, biochemical reactions and DNA (e.g. rRNA sequence) similarity.

(i) Phylum: a category of related classes sharing a few common features, e.g. Chordata (notochord, dorsal hollow nerve cord); in plants, division. (ii) Class: a group of related orders, e.g. Mammalia includes Primata and Carnivora. (iii) Family: a group of related genera with fewer similarities than a genus, based on vegetative and reproductive features in plants, e.g. Solanaceae, Felidae. (iv) Order: an assemblage of families showing a few similar characters, e.g. Polymoniales, Carnivora. (v) Genus: a group of closely related species with more characters in common than with species of other genera, e.g. Panthera, Solanum.

Animal (man): Kingdom Animalia → Phylum Chordata → Class Mammalia → Order Primata → Family Hominidae → Genus Homo → Species sapiens. Plant (wheat): Kingdom Plantae → Division Angiospermae → Class Monocotyledonae → Order Poales → Family Poaceae → Genus Triticum → Species aestivum. Going from species to kingdom, each category includes more organisms that share fewer characters (see the figure in the concept notes).