Biological Classification Basis

Life on Earth started about 3.7 billion years ago. Our planet hosts an incredible diversity of life, from tiny microbes to enormous whales.

What is Biological Classification?

Biological Classification, or Taxonomy, is a scientific method for arranging living beings into hierarchical groups based on their similarities and differences. This process helps organize both known and yet-to-be-discovered organisms systematically.

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Key Categories in Biological Classification

Organisms are sorted into various groups as follows:

• Kingdom
• Phylum (for animals) / Division (for plants)
• Class
• Order
• Family
• Genus
• Species

Historical Background

Aristotle, known as the 'Father of Biological Classification', was the first to use group names to study animals based on their living environments: water, air, and land. Following him, Carolus Linnaeus, a Swedish botanist, introduced the Binomial Nomenclature system in his 1735 book "Systema Naturae." This method names species based on their genus and specific characteristics.

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Charles Darwin's "Origin of Species" later provided a robust framework for categorizing organisms based on evolutionary relationships.

Levels of Biological Classification

Here are the different taxonomic levels, organized from highest to lowest:

1. Domain: Highest level, divided into Archaea, Bacteria, and Eukarya.
2. Kingdom: Includes five main kingdoms—Plantae, Animalia, Fungi, Protista, and Monera.
3. Phylum/Division: Groups organisms based on genetic and morphological similarities.
4. Class: A division within phyla characterized by common features.
5. Order: A classification below class, grouping families.
6. Family: Groups genera that share many characteristics.
7. Genus: Helps identify and define species.
8. Species: The most specific classification, describing organisms that can interbreed.

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Types of Classification Systems

• Two-Kingdom System: Initially, all organisms were classified into Plantae and Animalia.
• Three-Kingdom System: Introduced by Ernst Haeckel, adding Protista to include certain unique organisms.
• Four-Kingdom System: Added Monera for bacteria, proposed by Copeland.
• Five-Kingdom System: Distinguished between prokaryotic (bacteria) and eukaryotic (plants and animals) cells, outlined by Robert Whittaker.
• Six-Kingdom System: Devised by Carl Woese, splitting Monera into Archaebacteria and Eubacteria.

Importance of Biological Classification

Classifying organisms offers insights into the evolutionary links between different species and helps in the study of biology by providing a common language understood by scientists worldwide. It simplifies research and enhances our understanding of life's diversity and the relationships between living things.

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1. Introduction

• Biological classification is the scientific arrangement of living organisms into groups and categories based on similarities and differences.

• Helps in identifying, naming, and studying organisms systematically.

• Main goals:

  • To organize vast diversity of life.

  • To establish evolutionary relationships.

  • To make study and communication easier.

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2. Need for Classification

• Large number of species (over 1.7 million identified).

• To avoid confusion due to local names.

• To understand relationships among organisms.

• To trace evolutionary history.

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3. Early Classification Systems

a) Aristotle’s Classification

• Classified animals into with red blood and without red blood.

• Plants classified as herbs, shrubs, trees.

• Very simple, but not scientific.

b) Two-Kingdom Classification (Linnaeus, 1758)

• Two kingdoms: Plantae and Animalia.

• Limitations:

  • No place for prokaryotes, fungi, and unicellular organisms.

  • Could not distinguish between autotrophic and heterotrophic organisms.

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4. Modern Systems of Classification

a) Three-Kingdom System (Haeckel, 1866)

• Added Protista to Plantae and Animalia.

• Protista included unicellular organisms like algae, protozoa.

b) Four-Kingdom System (Copeland, 1956)

• Kingdoms: Monera, Protista, Plantae, Animalia.

• Distinguished prokaryotes (Monera) from eukaryotes.

c) Five-Kingdom System (R.H. Whittaker, 1969)

• Based on cell structure, body organization, mode of nutrition, reproduction, and phylogenetic relationships.

• Five kingdoms:

  1. Monera – Prokaryotic, unicellular (bacteria, cyanobacteria).

  2. Protista – Eukaryotic, unicellular (protozoa, unicellular algae).

  3. Fungi – Eukaryotic, heterotrophic, cell wall of chitin (yeast, mushrooms).

  4. Plantae – Multicellular, autotrophic (algae, bryophytes, gymnosperms, angiosperms).

  5. Animalia – Multicellular, heterotrophic, no cell wall (sponges to humans).

d) Six-Kingdom System

• Some scientists further divided Monera into:

  • Archaebacteria (extremophiles).

  • Eubacteria (true bacteria).

e) Three-Domain System (Carl Woese, 1990)

• Based on rRNA sequencing.

• Domains: Archaea, Bacteria, Eukarya.

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5. Kingdom-Wise Overview

Kingdom Monera

• Prokaryotic, unicellular.

• Examples: Bacteria, Cyanobacteria.

• Modes of nutrition: autotrophic (photosynthetic/chemosynthetic), heterotrophic.

• Reproduction: binary fission, conjugation.

Kingdom Protista

• Eukaryotic, unicellular.

• Examples: Amoeba, Paramecium, Euglena.

• Modes of nutrition: autotrophic (algae), heterotrophic (protozoa), mixotrophic.

Kingdom Fungi

• Eukaryotic, multicellular (except yeast).

• Heterotrophic (saprophytic/parasitic).

• Reproduction: spores, sexual and asexual methods.

• Examples: Mushroom, Penicillium, Yeast.

Kingdom Plantae

• Multicellular, eukaryotic, autotrophic (photosynthetic).

• Cell wall of cellulose.

• Show alternation of generations.

Kingdom Animalia

• Multicellular, eukaryotic, heterotrophic.

• No cell wall.

• Highly organized organ systems.

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6. Viruses, Viroids & Lichens

• Viruses – Non-cellular, obligate parasites, have DNA/RNA surrounded by protein coat.

• Viroids – Infectious agents smaller than viruses, consisting only of RNA.

• Lichens – Symbiotic association between algae (autotroph) and fungi (heterotroph).

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7. Basis of Modern Classification

• Cell type: prokaryotic/eukaryotic.

• Cell wall composition.

• Body organization: unicellular/multicellular.

• Nutrition: autotrophic/heterotrophic.

• Phylogeny (evolutionary relationships).

• Molecular evidence (DNA, RNA, proteins).

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8. Key Facts

• Linnaeus – Father of Taxonomy.

• Whittaker – Five-Kingdom classification.

• Woese – Three-domain classification.

• Viruses are acellular and placed at the boundary of living and non-living.