The Living World

Living organisms are classified for the following reasons:

• To Organize Diversity: There are millions of species on Earth. Classification groups them into categories (like plants, animals, bacteria) making it possible to study them systematically.
• Ease of Identification: It helps in the correct identification of a specific organism.
• Understanding Relationships: It reveals the evolutionary relationships (phylogeny) between different groups of organisms.
• Universal Communication: It provides a standard scientific name (Nomenclature) for every organism, avoiding confusion caused by local names.

Classification systems change because:

1. Evolution of Knowledge: As we discover new tools and techniques (like electron microscopy, DNA sequencing), our understanding of organisms improves.
2. Shift in Criteria: Early systems were based on superficial morphological characters (Artificial systems). Later, internal structure, embryology, and cell structure were included (Natural systems).
3. Phylogenetic Approach: Modern systems focus on evolutionary relationships and genetic similarities (Phylogenetic systems), necessitating changes in older groupings.

To classify people we meet often, we can use various criteria depending on the context:

• Physical Characteristics: Height, eye color, hair texture, gender.
• Relationship: Family members, friends, colleagues, neighbors.
• Profession/Occupation: Teachers, doctors, students, shopkeepers.
• Geography: People from the same city, state, or country.
• Language: Based on their mother tongue.

• Recognition: We can recognize specific traits that define a species.
• Variation: We learn about the variation present within a population (e.g., different skin tones in humans).
• Relationships: It helps in establishing reproductive relationships (who can interbreed).
• Distribution: We understand the geographical distribution and habitat preferences of populations.

Correct Name: Mangifera indica

Definition: A taxon (plural: taxa) is a scientific term used to represent a category or rank at any level in the hierarchy of classification. It represents a real biological object or group.

Examples:

• At the level of Kingdom: Plantae is a taxon.
• At the level of Phylum: Chordata is a taxon.
• At the level of Class: Mammalia is a taxon.
• At the level of Species: Homo sapiens is a taxon.

The correct hierarchical sequence (ascending order) is:

(c) Species $\to$ Genus $\to$ Order $\to$ Phylum

Option (a) is also correctly ordered (ascending) but skips intermediate categories. Option (c) represents a closer sequence found in the hierarchy.

General Definition: A group of individual organisms with fundamental similarities that can interbreed to produce fertile offspring.

Discussion:

• Higher Plants and Animals: The Biological Species Concept applies. Species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups. (e.g., All human beings belong to Homo sapiens).
• Bacteria: Bacteria reproduce asexually (binary fission). The biological concept of interbreeding does not apply. Here, a species is defined based on DNA similarity, biochemical properties, and morphology. A group of bacterial strains with high genomic similarity is considered a species.

• (i) Phylum: A taxonomic category higher than Class and lower than Kingdom. It consists of one or more related classes with common features. (e.g., Phylum Chordata includes classes Amphibia, Reptilia, Mammalia).
• (ii) Class: A category made of one or more related orders. (e.g., Class Mammalia includes Order Primata and Order Carnivora).
• (iii) Family: A group of related genera with fewer similarities as compared to genus and species. (e.g., Family Felidae includes genera Panthera (lion) and Felis (cat)).
• (iv) Order: A higher category that identifies a group of families which exhibit a few similar characters. (e.g., Order Polymoniales includes families Solanaceae and Convolvulaceae).
• (v) Genus: A group of related species which has more characters in common in comparison to species of other genera. (e.g., Genus Panthera includes lion, tiger, and leopard).