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Q8:
Try to collect all the currently accepted meanings for the word ‘species’. Discuss with your teacher the meaning of species in case of higher plants and animals on one hand, and bacteria on the other hand.
Solution :
1. Definitions of Species: Multi-Dimensional Perspectives
The term "species" serves as the fundamental unit of biological classification, yet its definition varies based on the organism's mode of reproduction and evolutionary history.
| Concept | Definition | Primary Criterion |
|---|---|---|
| Biological Species Concept (BSC) | Groups of interbreeding natural populations that are reproductively isolated from other such groups. [Proposed by Ernst Mayr, 1942]. | Reproductive Isolation |
| Phylogenetic Species Concept | The smallest diagnosable cluster of individual organisms within which there is a parental pattern of ancestry and descent. | Monophyly |
| Ecological Species Concept | A set of organisms adapted to a particular set of resources, called a niche, in the environment. | Ecological Niche |
2. Analysis of Species: Higher Plants and Animals vs. Bacteria
The definition of species deviates significantly when comparing eukaryotes (plants/animals) and prokaryotes (bacteria).
- Higher Plants and Animals: These organisms predominantly reproduce sexually. The Biological Species Concept (BSC) is the standard metric. Species are defined by their ability to interbreed to produce fertile offspring. Reproductive isolation is the key mechanism maintaining the integrity of these species.
- Bacteria: Bacteria reproduce primarily via binary fission (asexual reproduction) and engage in horizontal gene transfer (HGT) via transformation, transduction, and conjugation. Because they lack sexual reproduction, the BSC is inapplicable. Instead, microbiologists use the Polyphasic Approach.
3. Taxonomic Criteria for Bacterial Species
Due to the lack of sexual markers, bacterial species are defined using a combination of genetic and phenotypic traits:
- 16S rRNA Sequence Similarity: A threshold of $\ge 97\%$ (or more recently $\ge 98.7\%$) identity in the 16S rRNA gene is used as a proxy for taxonomic classification. [Per Woese & Fox, 1977].
- DNA-DNA Hybridization (DDH): Traditionally, if two strains show $\ge 70\%$ genomic DNA hybridization, they are considered to be in the same species.
- Average Nucleotide Identity (ANI): Modern genomics uses ANI, where a value of $\ge 95-96\%$ is now the accepted standard for defining a bacterial species.
4. Comparative Summary Diagram
Final Solution: The definition of 'species' is inherently contextual; while higher organisms are defined by the ability to interbreed and maintain reproductive isolation, bacteria are defined by genomic sequence similarity and phenotypic characteristics due to their asexual nature and horizontal gene transfer capabilities.
More Questions from Class 11 Biology The Living World EXERCISES
- Q1: Why are living organisms classified?
- Q2: Why are the classification systems changing every now and then?
- Q3: What different criteria would you choose to classify people that you meet often?
- Q4: What do we learn from identification of individuals and populations?
- Q5: Given below is the scientific name of Mango. Identify the correctly written name. Mangifera Indica Mangifera indica
- Q6: Define a taxon. Give some examples of taxa at different hierarchical levels.
- Q7: Can you identify the correct sequence of taxonomical categories?
- Q9(i): Define and understand the following terms: (i) Phylum
- Q9(ii): Define and understand the following terms: (ii) Class
- Q9(iii): Define and understand the following terms: (iii) Family
- Q9(iv): Define and understand the following terms: (iv) Order
- Q9(v): Define and understand the following terms: (v) Genus
- Q10: Illustrate the taxonomical hierarchy with suitable examples of a plant and an animal.
CBSE Solutions for Class 11 Biology The Living World
Chapters in CBSE - Class 11 Biology
- Biological Classification
- Plant Kingdom
- Animal Kingdom
- Morphology of Flowering Plants
- Anatomy of Flowering Plants
- Structural Organisation in Animals
- Cell – The Unit of Life
- Biomolecules
- Cell Cycle and Division
- Transport in Plants
- Mineral Nutrition
- Photosynthesis
- Respiration in Plants
- Plant Growth and Development
- Digestion and Absorption
- Breathing and Exchange of Gases
- Body Fluids and Circulation
- Excretory Products and Elimination
- Locomotion and Movement
- Neural Control and Coordination
- Chemical Coordination and Integration
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