Chapter 1: Biological Classification

Introduction

Biological classification is the scientific procedure of arranging organisms into hierarchical groups and subgroups based on similarities and differences in their characteristics. This systematic organization helps in understanding evolutionary relationships and biological diversity.

Historical Development of Classification Systems

Aristotle’s Classification

• First to classify organisms into plants and animals
• Further classified animals based on habitat (air, water, land)

Linnaeus’ System of Classification

• Carolus Linnaeus (1707-1778) developed the binomial nomenclature system
• Published “Systema Naturae” in 1735
• Classified organisms into two kingdoms: Plantae and Animalia
• Introduced hierarchy: Kingdom → Class → Order → Genus → Species

Whittaker’s Five Kingdom Classification (1969)

• R.H. Whittaker proposed a five-kingdom system
• Kingdoms: Monera, Protista, Fungi, Plantae, and Animalia
• Based on cell structure, mode of nutrition, and reproduction

Carl Woese’s Three-Domain System (1990)

• Based on molecular studies and genetic relationships
• Domains: Archaea, Bacteria, and Eukarya
• Revolutionized understanding of microbial life and evolution

Kingdoms in Modern Classification

Kingdom Monera

• Prokaryotic, unicellular organisms
• No membrane-bound organelles
• Includes bacteria and cyanobacteria
• Examples: Escherichia coli, Nostoc, Anabaena

Archaebacteria

• Ancient bacteria found in extreme environments
• Three types:
  • Methanogens (produce methane in anaerobic conditions)
  • Halophiles (salt-loving organisms)
  • Thermoacidophiles (thrive in hot, acidic environments)

Eubacteria

• True bacteria with peptidoglycan cell walls
• Found in diverse environments
• Includes pathogenic and beneficial species
• Classified based on shape: cocci (spherical), bacilli (rod-shaped), spirilla (spiral)

Kingdom Protista

• Eukaryotic organisms with membrane-bound organelles
• Mostly unicellular, some colonial or multicellular
• Diverse group including algae, protozoans, and slime molds
• Examples: Amoeba, Paramecium, Euglena, Chlorella

Kingdom Fungi

• Eukaryotic, heterotrophic organisms
• Cell walls contain chitin
• Absorptive nutrition through hyphae
• Reproduce by spores
• Examples: Mushrooms, yeasts, molds, rusts, smuts

Kingdom Plantae

• Eukaryotic, multicellular organisms
• Autotrophic (photosynthetic)
• Cellulose cell walls
• Non-motile
• Examples: Mosses, ferns, conifers, flowering plants

Kingdom Animalia

• Eukaryotic, multicellular organisms
• Heterotrophic by ingestion
• No cell wall
• Capable of movement at some stage of life
• Examples: Sponges, worms, insects, fishes, amphibians, reptiles, birds, mammals

Viruses, Viroids, and Lichens

Viruses

• Non-cellular entities
• Consist of genetic material (DNA or RNA) enclosed in a protein coat
• Obligate parasites that replicate only inside host cells
• Structure includes capsid (protein coat) and sometimes an envelope
• Examples: TMV (Tobacco Mosaic Virus), HIV, Influenza virus

Viroids

• Smaller than viruses
• Consists of naked RNA without protein coat
• Plant pathogens causing diseases
• Example: Potato spindle tuber disease

Lichens

• Symbiotic association between algae/cyanobacteria and fungi
• Algal component (phycobiont) provides food through photosynthesis
• Fungal component (mycobiont) provides protection and absorbs minerals
• Act as pollution indicators
• Examples: Parmelia, Cladonia

Taxonomic Hierarchy

The hierarchical arrangement of taxonomic categories in descending order:

• Domain
• Kingdom
• Phylum (in animals) or Division (in plants)
• Class
• Order
• Family
• Genus
• Species

Binomial Nomenclature

• Scientific naming system introduced by Linnaeus
• Each species has a two-part name: genus name + species epithet
• Written in Latin or latinized form and italicized
• First letter of genus name is capitalized
• Example: Homo sapiens (humans), Mangifera indica (mango)

Taxonomic Aids

• Herbaria: Collections of preserved plant specimens
• Botanical gardens: Living collections of plants
• Museums: Preserved specimens of animals
• Zoological parks: Living collections of animals
• Keys: Analytical tools for identification of species
• Taxonomic literature: Books, monographs on classification