📖 In-Depth Theory
The Linnaean Classification System
CLASSIFICATION is the organisation of living things into groups based on their similarities and differences.
CARLUS LINNAEUS (18th century) developed the binomial system of naming organisms:
Every organism has a two-part Latin name:
1. GENUS (capitalised) — a group of closely related species
2. SPECIES (lowercase) — organisms of the same species can interbreed to produce fertile offspring
EXAMPLES:
Homo sapiens — genus Homo, species sapiens (modern humans)
Felis catus — domestic cat
Panthera leo — lion
HIERARCHY OF GROUPS (largest to smallest):
Kingdom → Phylum → Class → Order → Family → Genus → Species
Memory: 'King Philip Came Over For Good Soup'
TRADITIONAL KINGDOMS:
Animalia, Plantae, Fungi, Protista, Prokaryota (bacteria)
The Three-Domain System
CARL WOESE proposed the THREE-DOMAIN SYSTEM in 1977 based on analysis of ribosomal RNA (rRNA).
This replaced the five-kingdom system with three domains:
1. ARCHAEA — primitive prokaryotes, often found in extreme environments (hot springs, salt lakes).
DNA differs significantly from bacteria despite looking similar under a microscope.
2. BACTERIA — true bacteria, the most numerous organisms on Earth.
Cells without membrane-bound nuclei; different cell wall chemistry from Archaea.
3. EUKARYA — all organisms with membrane-bound nuclei.
Includes all animals, plants, fungi, and protists.
WHY THREE DOMAINS?
RNA sequencing showed Archaea are more closely related to Eukaryotes than to Bacteria — despite looking like bacteria under the microscope.
This illustrates that observable characteristics can be misleading — molecular evidence is more reliable.
Evolutionary Trees
EVOLUTIONARY TREES (phylogenetic trees) show the evolutionary relationships between organisms.
How to read an evolutionary tree:
Branching points (NODES) = common ancestors.
The LENGTH of branches may represent evolutionary time.
Organisms that share a more RECENT common ancestor are more closely related.
EVIDENCE used to build evolutionary trees:
Anatomical similarities — homologous structures.
Fossil record — order of appearance of species.
DNA and RNA sequences — the more similar the sequences, the more closely related.
Protein similarities — amino acid sequences in key proteins (e.g. cytochrome c).
IMPORTANCE:
Evolutionary trees help us understand how species are related.
Can identify the most likely common ancestor of a group.
Help in medicine — understanding which organisms are closely related to pathogens.
LINNAEAN vs EVOLUTIONARY:
Linnaeus classified by appearance/structure (morphology).
Modern classification uses both morphology AND molecular evidence (DNA, RNA).
Molecular evidence has revised some traditional classifications.
⚠️ Common Mistake
A species is defined by the ability to interbreed and produce FERTILE offspring — not just by looking similar. Horses and donkeys can breed but produce sterile mules → they are different species. The three-domain system has Archaea as a SEPARATE domain from Bacteria — they are not the same despite both being prokaryotes.