The Living World — NCERT Solutions

Definition of Taxon:

A taxon (plural: taxa) is a unit of classification that represents a category or rank in the taxonomic hierarchy. It refers to any level of grouping of organisms that share certain common characteristics and are given a name. Each taxon represents a real biological entity.

Key point: A taxon can be defined at any level of the taxonomic hierarchy — from species to kingdom.

Examples of taxa at different hierarchical levels:

Conclusion: Each category in the taxonomic hierarchy — species, genus, family, order, class, phylum/division, and kingdom — represents a taxon. All taxa together constitute the taxonomic hierarchy.

Correct Answer: (c) Species → Genus → Order → Phylum

Concept — Taxonomic Hierarchy (from lowest to highest):

The correct ascending sequence of taxonomic categories is:

$$\text{Species} \rightarrow \text{Genus} \rightarrow \text{Family} \rightarrow \text{Order} \rightarrow \text{Class} \rightarrow \text{Phylum/Division} \rightarrow \text{Kingdom}$$

Evaluation of options:

• (a) Species → Order → Phylum → Kingdom: ❌ Incorrect — 'Genus' and 'Family' and 'Class' are skipped, and the jump from Species directly to Order is wrong in sequence.

• (b) Genus → Species → Order → Kingdom: ❌ Incorrect — Species comes before Genus (Species is lower than Genus), so Genus → Species is a descending step, making the sequence inconsistent.

• (c) Species → Genus → Order → Phylum: ✓ Correct — This follows the correct ascending order. Species is the lowest, followed by Genus, then Order (Family and Class are in between but not listed), and then Phylum. The relative sequence of the listed categories is correct.

Conclusion: Option (c) represents the correct sequence of taxonomical categories in ascending order.

Meanings/Concepts of 'Species':

The term 'species' has been defined in several ways by different biologists:

1. Morphological/Typological Species Concept: A species is a group of organisms that share the same morphological (structural) characteristics and can be distinguished from other groups. (Linnaeus's concept)

2. Biological Species Concept (Ernst Mayr, 1942): A species is a group of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups. This is the most widely accepted definition.

$$\text{Species} = \text{group of interbreeding individuals} + \text{reproductively isolated from others}$$

3. Phylogenetic/Evolutionary Species Concept: A species is the smallest group of organisms that share a common ancestor and form one branch on the tree of life (a distinct lineage).

4. Ecological Species Concept: A species is a group of organisms that occupy the same ecological niche.

5. Genetic Species Concept: A species is defined by its unique gene pool.

Species in Higher Plants and Animals vs. Bacteria:

Conclusion: The concept of 'species' is straightforward for sexually reproducing higher organisms but becomes complex for bacteria due to asexual reproduction and horizontal gene transfer. For bacteria, molecular methods (especially 16S rRNA gene sequencing) are now the standard for species identification.

Definitions of Taxonomic Categories:

(i) Phylum:

• Definition: Phylum is a taxonomic category that is higher than Class and lower than Kingdom. It is used for animals (the equivalent rank in plants is called Division).
• A phylum consists of one or more related classes that share certain fundamental body plan characteristics.
• Example: Phylum Chordata includes all animals with a notochord at some stage of their life — fishes, amphibians, reptiles, birds, and mammals.
• Example (Plants): Division Bryophyta includes mosses and liverworts.

(ii) Class:

• Definition: Class is a taxonomic category that is higher than Order and lower than Phylum/Division. It consists of one or more related orders.
• Example: Class Mammalia (in Phylum Chordata) includes all mammals — organisms that have mammary glands, hair/fur, and are warm-blooded.
• Example (Plants): Class Dicotyledonae includes plants with two cotyledons in their seeds.

(iii) Family:

• Definition: Family is a taxonomic category that is higher than Genus and lower than Order. It consists of one or more related genera (plural of genus) that share more characters in common compared to other genera.
• In plants, families are identified based on vegetative and reproductive features.
• Example (Animals): Family Felidae includes all cats — lions, tigers, leopards, domestic cats.
• Example (Plants): Family Solanaceae includes potato (Solanum tuberosum), tomato (Solanum lycopersicum), brinjal, etc.

(iv) Order:

• Definition: Order is a taxonomic category that is higher than Family and lower than Class. It consists of one or more related families.
• Example (Animals): Order Carnivora includes flesh-eating mammals — dogs, cats, bears, etc.
• Example (Plants): Order Polymoniales includes the family Solanaceae.

(v) Genus:

• Definition: Genus is a taxonomic category that is higher than Species and lower than Family. It consists of one or more related species that share a closer degree of similarity with each other than with species of other genera.
• The genus name forms the first part of the binomial scientific name.
• Example (Animals): Genus Panthera includes lion (Panthera leo), tiger (Panthera tigris), and leopard (Panthera pardus).
• Example (Plants): Genus Solanum includes potato (Solanum tuberosum), tomato (Solanum lycopersicum), and brinjal (Solanum melongena).

Summary Table:

Concept: The taxonomical hierarchy is the arrangement of taxonomic categories in a definite order from the highest (Kingdom) to the lowest (Species). Each category is called a taxon.

The sequence from highest to lowest is:
$$\text{Kingdom} \rightarrow \text{Division/Phylum} \rightarrow \text{Class} \rightarrow \text{Order} \rightarrow \text{Family} \rightarrow \text{Genus} \rightarrow \text{Species}$$

**Example 1 — Plant: Wheat (Triticum aestivum)**

Scientific name: Triticum aestivum

**Example 2 — Animal: Housecat (Felis catus) / or Humans (Homo sapiens)**

(a) Housecat:

Scientific name: Felis catus

(b) Human (additional example):

Scientific name: Homo sapiens

Key observations from the hierarchy:

1. As we move from Species to Kingdom, the number of organisms included increases and the number of common characteristics decreases.
2. As we move from Kingdom to Species, the number of organisms decreases and the common characteristics increase.
3. Species is the basic unit of classification.
4. Each level (taxon) in the hierarchy is based on shared characteristics that distinguish it from other taxa at the same level.

Conclusion: The taxonomical hierarchy provides a systematic framework to classify and understand the diversity of life, showing the relative position of each organism in the living world.