Systematics: The term “systematics” is derived from the word “systema,” meaning the systematic arrangement of organisms. It considers the evolutionary relationships among organisms.
Plant systematics, in particular, deals with the interrelations between plants and their evolutionary descent. Systematics studies biological diversity and organizes this information into a classification system. Organisms are classified based on similarities, relationships, and evolutionary descent. The phylogenetic taxonomy shows relationships between different organisms and their lineage.
Similarities among individuals suggest they may have evolved from a common ancestor. Closely related organisms are grouped together, sharing a common gene pool. Organisms are categorised into different taxonomic categories according to the similarities and specific features. The different taxonomic categories in their hierarchical order are:
Kingdom
Phylum
Class
Order
Family
Genus
Species
As one moves from species to kingdom, the number of common characteristics decreases. Species contain organisms with fundamental similarities, while members of the same kingdom share fewer features.
Basic terminology of systematics
1.Taxonomy: The science of classification, identification, and naming of organisms. It follows the binomial nomenclature system established by Carl Linnaeus.
2. Classification: The arrangement of organisms into hierarchical categories based on similarities and differences.
3. Nomenclature: The system of assigning scientific names to organisms, governed by:
International Code of Zoological Nomenclature (ICZN)
International Code of Nomenclature for Algae, Fungi, and Plants (ICN)
4. Phylogeny: The evolutionary history of an organism or group, depicted using phylogenetic trees.
5. Important Terms in Systematics
Anagenesis: Evolutionary change through time.
Analogous: Similarity due to convergent evolution rather than common ancestry.
Apomorphy: A unique derived character.
Clade: A monophyletic group descended from a single ancestral species.
Cladistics: Classification based on common ancestry.
Homologous: Similarity due to shared ancestry.
Paraphyletic: A group excluding one or more descendants of a common ancestor.
Polyphyletic: A group that does not include the most recent common ancestor of all members.
Taxon (Taxa): A named classification group.
Theories of biological classification
The classification system used today originates from the hierarchical scheme devised by Carl Linnaeus (1707-1778). In the 10th edition of Systema Naturae (1758), he listed animals and grouped them based on similarity. His system consists of seven major categories (kingdom, phylum, class, order, family, genus, species) and introduced binomial nomenclature (genus and species names). While Linnaean classification remains foundational, taxonomists have refined the system due to new discoveries and data. Modern classifications emphasize evolutionary relationships.
Evolutionary Systematics vs. Cladistics: Evolutionary systematics (Gradistic taxonomy): Reconstructs phylogenetic trees by considering both branching and divergence.
Cladistics: Groups organisms based on shared derived characters. It includes all descendants of an ancestor within taxonomic groups.
Molecular Taxonomy: Uses DNA sequencing, hybridization, and other molecular techniques for classification.
Historical Approaches to Classification
Aristotle: First attempted classification using morphological traits. He divided animals into Enaima (with red blood) and Anaima (without red blood).
Jean-Baptiste Lamarck: First to classify animals into invertebrates and vertebrates.
Linnaeus: Introduced the two-kingdom classification system.
Types of Classification Systems
1. Two-Kingdom Classification (Linnaeus, 1758)
Kingdom Plantae: Cell wall present, autotrophic, starch as reserve food, no nervous or excretory system.
Kingdom Animalia: No cell wall, heterotrophic, glycogen as reserve food, muscular and nervous systems present.
2. Three-Kingdom Classification (Ernst Haeckel, 1866)
Added Kingdom Protista for unicellular organisms.
Limitations: Did not separate prokaryotes from eukaryotes.
3. Four-Kingdom Classification (Copeland, 1956)
Added Kingdom Monera for prokaryotic bacteria.
Fungi remained within Plantae.
4. Five-Kingdom Classification (R.H. Whittaker, 1969)
Based on cell structure, nutrition, reproduction, and phylogeny:
Monera: Prokaryotic bacteria.
Protista: Unicellular eukaryotes.
Fungi: Heterotrophic, chitin cell wall.
Plantae: Autotrophic, cellulose cell wall.
Animalia: Heterotrophic, no cell wall.
5. Six-Kingdom Classification (Carl Woese, 1990)
Introduced the Three-Domain System, dividing Monera into Archaea and Bacteria:
Domains and Their Kingdoms
Domain Archaea: Prokaryotic, unique lipid membranes, includes methanogens and halophiles.
Domain Bacteria: Prokaryotic, peptidoglycan cell walls, includes cyanobacteria.
Domain Eukarya: Eukaryotic, includes:
Protista
Plantae
Fungi
Animalia
Biological classification has evolved from Aristotle’s simple morphological classification to Linnaeus’ binomial nomenclature and modern molecular systematics. Advances in DNA sequencing, cladistics, and computational phylogenetics have improved classification accuracy, making it more reflective of evolutionary history.