Historically, organisms were grouped into five kingdoms based on observable physical features and cell types: Prokaryotae, Protoctista, Fungi, Plantae, and Animalia.
Prokaryotae (Monera) includes all organisms with prokaryotic cells (no nucleus), such as bacteria, which are typically smaller than .
Fungi are eukaryotic saprotrophs that possess cell walls made of chitin and reproduce via spores; they can be unicellular or multicellular (hyphae).
Plantae are multicellular autotrophs with cellulose cell walls and chloroplasts for photosynthesis, while Animalia are multicellular heterotrophs that lack cell walls entirely.
Modern classification uses the Three Domain System (Archaea, Bacteria, and Eukarya), which sits above the kingdom level and is based on molecular phylogeny.
Molecular phylogeny involves analyzing similarities and differences in molecules like DNA, RNA, and proteins to determine how closely related organisms are.
This approach revealed that the original Kingdom Prokaryotae actually contained two distinct groups—Archaea and Bacteria—that are as different from each other as they are from eukaryotes.
Organisms with more similar molecular sequences share a more recent common ancestor, providing a more accurate map of evolutionary history than physical appearance alone.
| Feature | Archaea | Bacteria | Eukarya |
|---|---|---|---|
| Cell Type | Prokaryotic | Prokaryotic | Eukaryotic |
| Cell Wall | No peptidoglycan | Contains peptidoglycan | Cellulose/Chitin (if present) |
| Membrane Lipids | Ether-linked | Ester-linked | Ester-linked |
| Ribosomes | 70S (eukaryote-like) | 70S | 80S (cytosol) |
| Histones | Present | Absent | Present |
Species Concept: A group of organisms that can interbreed to produce fertile offspring. This is the fundamental unit of classification.
Phylogeny vs. Taxonomy: Taxonomy is the naming and grouping of organisms, whereas phylogeny is the study of the actual evolutionary relationships between them.
Formatting is Critical: Always check that you have capitalized the Genus and lowercased the species name in binomials. If the question asks you to write a name, underline it to show you know the convention.
Hierarchy Mnemonics: Use a mnemonic like "Dear King Philip Came Over For Good Soup" to remember the order of ranks (Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species).
Evidence Evaluation: If asked why a classification changed, focus on technological advances like DNA sequencing and molecular analysis of RNA genes, which provide more objective data than physical traits.
Common Pitfall: Do not confuse 'Species Richness' (number of species) with 'Classification' (grouping of species). Ensure you use the term 'Taxon' when referring to a specific group in the hierarchy.
