Classification of organisms

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Classification of organisms 





1. Meaning of the terms species and speciation

Species

A species is a group of organisms that:

  • share similar morphological, physiological and genetic characteristics,

  • can interbreed to produce fertile offspring, and

  • are reproductively isolated from other such groups.

Speciation

Speciation is the process by which new species evolve from existing ones.
It usually occurs when populations become reproductively isolated due to:

  • geographical barriers (allopatric speciation),

  • ecological or behavioural differences (sympatric speciation),

  • genetic changes over time that prevent interbreeding.

2. Classification of organisms into the three domains

Based on molecular and genetic analysis, organisms are classified into three domains:

Archaea

  • Prokaryotic (no nucleus)

  • Often live in extreme environments (e.g., hot springs, salty lakes)

  • Cell walls without peptidoglycan

  • Membrane lipids and ribosomal RNA very different from bacteria

Bacteria

  • Prokaryotic

  • Found in almost all environments

  • Cell walls with peptidoglycan

  • Wide range of metabolic types (photosynthetic, heterotrophic, etc.)

Eukarya

  • Eukaryotic cells (with a nucleus and membrane-bound organelles)

  • Includes four major kingdoms: Protoctista, Fungi, Plantae, Animalia

3. Classification within the domain Eukarya (Taxonomic hierarchy)

Organisms are classified according to the hierarchy:

Kingdom → Phylum → Class → Order → Family → Genus → Species

Example (human classification):

  • Kingdom: Animalia

  • Phylum: Chordata

  • Class: Mammalia

  • Order: Primates

  • Family: Hominidae

  • Genus: Homo

  • Species: Homo sapiens

4. Characteristic features of the five kingdoms

Monera (all prokaryotes — sometimes split into Archaea & Bacteria)

  • Unicellular

  • No nucleus

  • Asexual reproduction

  • Cell wall present (peptidoglycan in bacteria)

Protoctista (Protists)

  • Mostly unicellular, some multicellular

  • Eukaryotic

  • Very diverse: includes algae, protozoa

  • May be autotrophic or heterotrophic

Fungi

  • Eukaryotic, mostly multicellular (yeast is unicellular)

  • Heterotrophic (absorption)

  • Cell walls made of chitin

  • Reproduce by spores

  • No photosynthesis

Plantae

  • Multicellular

  • Eukaryotic

  • Autotrophic (photosynthetic, containing chlorophyll)

  • Cell walls made of cellulose

  • Store carbohydrates as starch

Animalia

  • Multicellular

  • Eukaryotic

  • Heterotrophic (ingestion)

  • No cell walls

  • Highly specialised tissues and systems

5. How viruses are classified

Viruses are not placed in any of the five kingdoms because they are non-living particles.
They are classified based on:

  • Genetic material: DNA or RNA

  • Strandedness: single-stranded or double-stranded

  • Shape of capsid: helical, icosahedral, complex

  • Presence/absence of an envelope

  • Type of host: bacteriophages, plant viruses, animal viruses

6. Define ecosystem and niche

Ecosystem

An ecosystem is a community of organisms (biotic components) interacting with the physical environment (abiotic components).

Niche

A niche is the role an organism plays in an ecosystem, including:

  • how it obtains food,

  • its interactions with other organisms,

  • its habitat,

  • its environmental requirements.

“No two species can occupy the same niche indefinitely” (competitive exclusion principle).

7. Levels at which biodiversity can be assessed

Biodiversity can be measured at three levels:

  1. Genetic diversity – variety of genes/alleles within a species

  2. Species diversity – number of species and their relative abundances

  3. Ecosystem diversity – variety of different habitats or ecosystems in an area

8. Importance of random sampling in determining biodiversity

Random sampling ensures:

  • unbiased data (avoids favouring certain areas)

  • representative results

  • valid comparisons between regions or time periods

  • reliable estimates of species abundance and distribution

It reduces human subjectivity and increases scientific accuracy.

9. Methods to assess distribution and abundance of organisms

1. Quadrat sampling

Used for plants or slow-moving animals.

  • Quadrat placed randomly or systematically

  • Count number of individuals per quadrat

  • Calculate mean density, percentage cover, or frequency

2. Transects

Used to measure change along an environmental gradient.

  • Line transect: record species touching a line

  • Belt transect: place quadrats along the line continuously or at intervals

3. Capture–mark–recapture (for mobile animals)

Steps:

  1. Capture animals

  2. Mark harmlessly

  3. Release

  4. Recapture later
    Using Lincoln Index:
    Population size = (n1 × n2) / m2
    n1 = number caught first
    n2 = number caught second
    m2 = number recaptured with marks

4. Kick sampling (for aquatic organisms)

  • Kick stream bed

  • Net downstream catches dislodged organisms

  • Used for assessing water quality (indicator species)

5. Sweep netting, pitfall traps, light traps

Used for insects and small animals.

6. Point counts and camera traps

Used for birds or elusive mammals.

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