eVidyarthi

Notes Class 11 Chapter 2 Biology Maharashtra Board

Systematics of Living Organisms

Introduction

  • Systematics: The study of the kinds, diversity, and evolutionary relationships of organisms (G. Simpson, 1961). It encompasses classification, taxonomy, and phylogeny.
  • Taxonomy: The science of classifying organisms based on specific rules or principles. The term was coined by A.P. de Candolle (Swiss Botanist, 1778-1841), derived from Greek words taxis (arrangement) and nomos (law/rule).
  • Classification: The arrangement of organisms into distinct categories based on similarities and dissimilarities, following a well-established scheme.

2.1 Types of Classification Systems

  1. Artificial Classification:
    • Based on a few easily observable, non-evolutionary features like habit or color, often ignoring relationships with other organisms.
    • Example: Linnaeus’s system of classification.
  2. Natural Classification:
    • Based on objectively significant features, considering relationships among organisms.
    • Example: Bentham and Hooker’s system of classification.
  3. Phylogenetic Classification:
    • Based on common evolutionary descent, focusing on evolutionary relationships.
    • Example: Engler and Prantl’s classification.
  • Taxon: A taxonomic group of any rank (e.g., species, genus, family). Each category in classification is a taxon.
  • Cladogram: A branching diagram showing hypothetical relationships among organisms and their common ancestors.

2.2 Three Domains of Life

  • Proposed by Carl Woese (1990), life is classified into three domains, a unit larger than a kingdom:
    1. Archaea: Prokaryotic cells, cell wall lacks peptidoglycan, survive extreme conditions (e.g., high temperature, salinity). Known as extremophiles.
    2. Bacteria: Prokaryotic cells, cell wall contains peptidoglycan, includes autotrophs and heterotrophs.
    3. Eukarya: Eukaryotic cells, includes organisms with complex cell structures (e.g., fungi with chitinous cell walls, plants with cellulosic cell walls).
  • Phylogenetic Descent: All domains share a common ancestor, with unique ribosomal RNA (rRNA) distinguishing them .

2.3 Other Taxonomic Approaches

  1. Chemotaxonomy:
    • Classification based on similarities/differences in chemical constituents (e.g., cell wall composition: Archaea lack peptidoglycan, Bacteria have it, Fungi have chitin, Plants have cellulose).
  2. Numerical Taxonomy:
    • Uses quantification of characters and algorithms (e.g., cluster analysis) for classification, proposed by R. Sokal and P. Sneath (1963).
  3. DNA Barcoding:
    • Identifies species using DNA sequences from a small tissue sample.
    • Steps: Collect DNA barcode data of known species, match unknown sample sequences against a barcode library.
    • Applications: Protecting endangered species, pest control, identifying disease vectors, authenticating medicinal plants.
  4. Phylogeny:
    • Studies evolutionary relationships, considering morphological and metabolic similarities.
    • Examples: Woese’s three-domain system, Whittaker’s five-kingdom system.

2.4 Taxonomic Hierarchy

  • Classification involves a hierarchy of steps, each representing a rank or category.
  • Taxonomic Categories:
    • Compulsory: Kingdom, Division/Phylum, Class, Order, Family, Genus, Species.
    • Facultative: Sub-order, Sub-family, etc., used as needed.
  • Taxon: A taxonomic group of any rank (e.g., Angiospermae, Malvaceae).
  • Hierarchy Example (Table 2.2):
    • China-rose (Plant): Kingdom: Plantae, Division: Angiospermae, Class: Dicotyledonae, Order: Malvales, Family: Malvaceae, Genus: Hibiscus, Species: rosa-sinensis.
    • Cobra (Animal): Kingdom: Animalia, Phylum: Chordata, Class: Reptilia, Order: Squamata, Family: Elapidae, Genus: Naja, Species: naja.

2.5 Units of Classification

  1. Species:
    • The lowest taxonomic unit, a group of organisms that can interbreed to produce fertile offspring.
    • Considered stable but can include sub-species, varieties, or populations.
    • Example: Homo sapiens (humans).
  2. Genus:
    • A group of closely related species with similar morphological traits but usually not interbreeding.
    • Example: Panthera (includes tiger, leopard, lion); Solanum (includes brinjal, potato).
  3. Family:
    • A group of related genera.
    • Example: Malvaceae (includes Hibiscus, Gossypium); Felidae (cats, tigers); Canidae (dogs).
  4. Order:
    • A group of related families with definite affinities.
    • Example: Parietales (includes Papavaraceae, Brassicaceae); Carnivora (includes Felidae, Canidae).
  5. Class:
    • A group of related orders.
    • Example: Mammalia (includes Carnivora, Primates).
  6. Division/Phylum:
    • A group of related classes.
    • Example: Angiospermae (includes Dicotyledonae, Monocotyledonae); Chordata (animals).
  7. Sub-kingdom:
    • Groups divisions with similarities.
    • Example: Phanerogams (Angiospermae, Gymnospermae).
  8. Kingdom:
    • The highest taxonomic category, composed of sub-kingdoms.
    • Example: Plantae (plants), Animalia (animals).

2.6 Nomenclature

  • Nomenclature: The science of naming organisms to facilitate communication.
  • Types of Names:
    1. Vernacular/Common Names:
      • Vary by region/language, causing confusion (e.g., Mangifera indica has over 50 Sanskrit names).
      • Limited usage due to lack of universality.
    2. Scientific Names:
      • Universal, systematic names following binomial nomenclature, introduced by Carl Linnaeus (Species Plantarum, 1753).
      • Governed by the International Code of Nomenclature for Algae, Fungi, and Plants (ICNAFP), updated at the 2017 Shenzhen Congress (replacing ICBN).
      • Example: Helianthus annuus (sunflower).
  • Rules of Binomial Nomenclature:
    • Composed of two Latin/Greek words: generic epithet (noun, capitalized) and specific epithet (adjective, lowercase).
    • Underlined (handwritten) or italicized (printed).
    • Each epithet: 3-13 letters.
    • Author’s name may follow (e.g., Mangifera indica L., where L. is Linnaeus).

2.7 Five Kingdom System (R.H. Whittaker, 1969)

  • Proposed to address limitations of Linnaeus’s two-kingdom system (Plantae, Animalia), which couldn’t classify organisms like bacteria, fungi, or Euglena.
  • Based on phylogenetic relationships, it includes:
    1. Monera
    2. Protista
    3. Plantae
    4. Fungi
    5. Animalia

2.7.1 Kingdom Monera

  • Features:
    • Unicellular, prokaryotic organisms.
    • Omnipresent, found in extreme environments.
    • Nutrition: Photoautotrophs, chemoautotrophs, or heterotrophs (mostly).
    • No defined nucleus; DNA in a nucleoid, may have plasmids.
    • Cell wall: Peptidoglycan (murein), except in Archaebacteria.
    • No membrane-bound organelles; 70S ribosomes.
    • Reproduction: Asexual (binary fission, budding); rarely sexual (conjugation).
  • Morphology (Fig 2.3):
    • Coccus (spherical), Bacillus (rod-shaped), Vibrio (comma/kidney-shaped), Spirillum (spiral).

  • Types:
    1. Archaebacteria:
      • Extremophiles (e.g., halophiles, thermophiles).
      • Cell wall lacks peptidoglycan.
      • Example: Methanogenic bacteria (produce methane in biogas plants).
    2. Eubacteria:
      • True bacteria with peptidoglycan cell walls.
      • Autotrophs (photosynthetic like Chlorobium, chemosynthetic like sulfur bacteria) or heterotrophs.
      • Heterotrophs: Decomposers, pathogens (e.g., typhoid, cholera), or useful (e.g., Lactobacilli for curdling, Streptomyces for antibiotics).
  • Cyanobacteria (Fig 2.4):
    • Multicellular, filamentous, prokaryotic.
    • Covered by a mucilaginous sheath, have heterocysts for nitrogen fixation.
    • Photosynthetic pigments: Chl-a, Chl-b, carotenes, xanthophylls.
    • Example: Nostoc.

  • Mycoplasma:
    • Smallest living forms, lack cell walls, often pathogenic, resistant to antibiotics.

2.7.2 Kingdom Protista

  • Features:
    • Unicellular, eukaryotic organisms.
    • Link to Plantae, Fungi, and Animalia.

Types:

Plant-like Protists (Chrysophytes):

  • Photosynthetic, major ocean producers.
  • Diatoms (Fig 2.5): Silica cell walls, form diatomaceous earth (used in polishing/filtration).

  • Dinoflagellates (Fig 2.6): Cellulosic plates, photosynthetic, cause red tide (e.g., Gonyaulax).

  • Euglenoids (Fig 2.7): No cell wall, proteinaceous pellicle, two flagella, photosynthetic or heterotrophic (e.g., Euglena).

  • Animal-like Protists (Protozoans):
    • Heterotrophic, no cell wall.
    • Types:
      • Amoeboid (Amoeba, Entamoeba causes dysentery).
      • Flagellated (Trypanosoma causes sleeping sickness).
      • Ciliated (Paramecium, Fig 2.8, has gullet and cilia).
      • Sporozoan (Plasmodium causes malaria).

  • Fungi-like Protists (Myxomycetes):
    • Saprophytic, form plasmodium (cell mass), tough spores (e.g., Stemonitis).

2.7.3 Kingdom Plantae

  • Features:
    • Multicellular, eukaryotic, mostly autotrophic (chlorophyll-containing).
    • Cell wall: Cellulose.
    • Includes semi-autotrophic (insectivorous, e.g., Venus flytrap) and heterotrophic (parasitic, e.g., Cuscuta).
    • Alternation of generations (two distinct life cycle phases).
  • Groups:
    • Cryptogams (non-seed plants).
    • Phanerogams (seed plants).
  • Detailed study in the next chapter.

2.7.4 Kingdom Fungi

  • Features:
    • Eukaryotic, heterotrophic, extracellular digestion.
    • Found in warm, humid places.
    • Body: Unicellular or mycelium (hyphae network).
    • Hyphae: Septate (with cross-walls) or coenocytic (non-septate, multinucleate).
    • Cell wall: Chitin or fungal cellulose.
    • Nutrition: Saprophytic, parasitic, or predatory.
    • Reproduction: Asexual (fragmentation, fission, budding) or sexual.
    • Symbiotic: Lichens (with algae), mycorrhiza (with plant roots).
  • Uses:
    • Food: Mushrooms, morels, truffles.
    • Industry: Saccharomyces (yeast) for baking/brewing.
    • Medicine: Penicillium (antibiotics).
    • Research: Neurospora (genetics).
  • Harmful: Cause plant/animal diseases (e.g., Puccinia).

Classification:

  • Phycomycetes (Algal Fungi) (Fig 2.9):
    • Aseptate, coenocytic hyphae, grow in moist habitats.
    • Example: Mucor, Rhizopus (bread mold), Albugo (parasitic).

  • Ascomycetes (Sac Fungi) (Fig 2.10):
    • Septate hyphae, multicellular (rarely unicellular, e.g., yeast).
    • Example: Aspergillus, Penicillium, Saccharomyces, Neurospora.

  • Basidiomycetes (Club Fungi) (Fig 2.11):
    • Septate hyphae.
    • Example: Agaricus (mushroom), Puccinia (rusts), Ustilago (smuts).

  • Deuteromycetes (Imperfect Fungi) (Fig 2.12):
    • Asexual reproduction only.
    • Example: Alternaria, Colletotrichum.

Lichens:

  • Symbiotic association of algae (phycobiont) and fungi (mycobiont).
  • Algae provide food, fungi provide shelter and minerals.
  • Pollution indicators, aid in soil formation (e.g., Parmelia).

2.7.5 Kingdom Animalia

  • Features:
    • Multicellular, eukaryotic, heterotrophic (holozoic nutrition).
    • No cell wall or chlorophyll.
    • Determinate growth, capable of locomotion.
  • Detailed study in Chapter 4.

2.7.6 Acellular Organisms (Not in Five Kingdoms)

Viruses (Fig 2.13, 2.14):

  • Named by Louis Pasteur, described as contagium vivum fluidum by M.J. Beijernck.
  • Obligate parasites, inert outside host cells, can be crystallized.
  • Structure: Nucleic acid (DNA or RNA, never both) surrounded by a protein capsid (made of capsomeres).
  • Types: DNA viruses, RNA viruses, bacteriophages (infect bacteria, e.g., Fig 2.14).
  • Diseases: Plant (leaf curling, mosaic), animal (foot-and-mouth, swine flu), human (smallpox, AIDS).

Viroids:

  • Single-stranded RNA, no protein coat, smaller than viruses.
  • Cause diseases like potato spindle tuber (discovered by T.O. Diener, 1971).

Prions:

  • Proteinaceous infectious particles causing neurological diseases (e.g., mad cow disease, Creutzfeldt-Jakob disease).

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