NSC 10 - Chapter 2: Biosystematics
Systematics
Definition: branch of biology dealing with identification, naming and classifying living organisms.
Systematics means to put together; term first used by Carolus Linnaeus in Systema Naturae.
Purpose: systematic placing of organisms into groups (taxa) based on relationships.
Taxonomy and Phylogeny
Augustin-Pyramus de Candolle coined the word 'Taxonomy'.
Species is the basic unit of classification.
Species are grouped into more inclusive taxa; 7 main categories used: hierarchical ranks: Kingdom, Phylum/Division, Class, Order, Family, Genus, Species.
Phylogeny: evolutionary history of a taxon.
Phylogenetic classification: classification based on evolution.
Artificial classification: not explicitly based on evolutionary relationships.
History of Classification
Aristotle (3rd–4th century BC): plants vs animals.
Theophrastus (370–282 BC): first attempt to classify by form and habitat (not medicinal value).
Hippocrates (460–377 BC): Father of Medicine; listed organisms with medicinal value.
Pliny the Elder (AD 23–79): Historia Naturalis; first artificial system.
John Ray: introduced the term species.
Carolus Linnaeus (1707–1778): Father of Taxonomy; Binomial Nomenclature; described many species in Species Plantarum and Systema Naturae.
Two Kingdom System of Classification
Linnaeus (1758) divided life into two kingdoms:
Kingdom Plantae: includes bacteria, plants and fungi.
Kingdom Animalia: includes protozoa and metazoans.
Kingdom Plantae vs Kingdom Animalia (key contrasts):
Plants: non-motile; autotrophic; photosynthesis; cellulose cell walls; starch as reserve; growth points unlimited.
Animals: motile; heterotrophic; no photosynthesis; glycogen reserve; no cell wall.
Note: This system is outdated and limited (see page on limitations).
Limitations of Two Kingdom System
Organisms showing plant- and animal-like traits (e.g., nonliving Euglena).
Fungi are heterotrophic and lack chlorophyll, not fitting neatly as plants.
Bacteria do not fit well into either kingdom.
Virus debate: whether living or nonliving remains unsettled.
Overall: Arbitrary and artificial scheme.
The Five Kingdom System of Classification
Whittaker (1969) proposed 5 kingdoms based on phylogenetic relationships and four criteria:
Complexity of cell structure (prokaryote to eukaryote).
Mode of nutrition (autotrophs vs heterotrophs).
Body organization (unicellular vs multicellular).
Phylogenetic/evolutionary relationships.
Five kingdoms: Monera, Protista, Fungi, Plantae, Animalia.
Five Kingdom Classification
Kingdom Monera: prokaryotes (mycoplasma, bacteria, actinomycetes, cyanobacteria).
Kingdom Protista: eukaryotic, mostly unicellular; many photosynthetic; some parasitic; includes organisms bearing cilia/flagella; Euglena is mixotrophic (autotrophic in light, heterotrophic in dark).
Kingdom Fungi: unicellular or multicellular; heterotrophic; hyphae; chitin cell walls.
Kingdom Plantae: multicellular; cellulose cell walls; mostly autotrophic with chlorophyll; some heterotrophs (parasites like Cuscuta); some insectivory in plants (e.g., Nepenthes, Drosera).
Kingdom Animalia: multicellular; heterotrophic; motile; muscles and nerves; some parasites.
Kingdom Monera (Detailed Traits)
Prokaryotic, no true nucleus, no membrane-bound organelles.
Shapes: cocci, bacilli, spirilla, etc.
Nutrition: autotrophic or heterotrophic; some photosynthetic; some chemosynthetic.
Nitrogen fixation: bacteria like Rhizobium, Azotobacter, Clostridium convert N₂ to ammonia.
Diversity: includes Archaebacteria (extreme environments).
Kingdom Protista (Detailed Traits)
Eukaryotic and mostly unicellular, aquatic.
Locomotion: cilia/flagella in many forms.
Nutrition: many photosynthetic; some holozoic; some mixotrophic.
Phytoplankton vs Zooplankton: plants-like vs animal-like protists.
Euglena example: mixotrophic; border line between plants and animals.
Kingdom Fungi (Detailed Traits)
Mostly multicellular (some unicellular, e.g., yeast).
Nutrition: absorptive heterotrophs; lack chlorophyll.
Hyphae form the mycelium; cell wall made of chitin.
Kingdom Plantae (Detailed Traits)
Multicellular; cellulose cell walls; chlorophyll.
Nutrition: autotrophic via photosynthesis; some heterotrophic exceptions (parasites like Cuscuta).
Kingdom Animalia (Detailed Traits)
Multicellular; heterotrophic; mobility and nervous systems.
Complex tissue and organ systems; parasites present.
Merits of the Five Kingdom Classification
Reveals phylogenetic relationships.
Based on cell structure complexity (prokaryotic vs eukaryotic).
Reflects body organization (unicellular vs multicellular).
Reflects modes of nutrition (autotrophic vs heterotrophic).
Demerits of the Five Kingdom Classification
Chlamydomonas and Chlorella are placed in Plantae but are unicellular and should be Protista.
Animal protozoans are not grouped with animals in Protista.
Some animal-like protists show differing nutrition modes.
Yeasts (unicellular eukaryotes) are not consistently placed in Protista.
Major Differences Among the Five Kingdoms
Cell type: Monera (prokaryotic); Protista, Fungi, Plantae, Animalia (eukaryotic).
Organization: Monera and Protista mostly unicellular; Fungi, Plantae, Animalia mostly multicellular.
Cell wall: Monera present; Protista variable (some with cell wall); Fungi (chitin); Plantae (cellulose); Animalia (absent).
Nutrition: Monera (photoautotrophy/chemoautotrophy/heterotrophy); Protista (phototrophic/heterotrophic/chemoautotrophic); Fungi (absorptive heterotrophy); Plantae (autotrophy); Animalia (ingestive heterotrophy).
Mobility: Monera (motile/nonmotile); Protista (motile/nonmotile); Fungi (mostly nonmotile); Plantae (nonmotile); Animalia (motile).
Difficulties in Classification
By the 1970s, molecular biologists realized prokaryotes comprise two distinct groups.
Domain-based system proposed by Woese, Kandler, and Wheelis:
Domain Eucarya: all eukaryotes.
Domain Bacteria: most familiar prokaryotes.
Domain Archaea: formerly archaebacteria; prokaryotes in extreme conditions.
This led to the three-domain system, refining the previous five-kingdom approach.