Classification and evolutionary trees
Biology · GCSE · The School
Linnaeus organised life into nested groups
Carl Linnaeus classified organisms into a hierarchy: kingdom, phylum, class, order, family, genus, species. Each level is narrower than the one before. A species' two-part scientific name comes straight from this system — genus then species, such as Homo sapiens. It is the same name in every language, avoiding the confusion of common names.
Modern classification adds genetic evidence
Linnaeus classified organisms by observable features alone. Modern classification adds DNA sequencing. Comparing how similar two species' genetic material is gives a direct measure of how closely related they are. This has moved some organisms into different groups than their appearance alone once suggested.
A worked example: three domains
DNA evidence split organisms into three DOMAINS above kingdom level. Archaea: single-celled, no nucleus, often in extreme environments. Bacteria: also single-celled and without a nucleus, but genetically distinct from Archaea. Eukaryota: cells WITH a nucleus, including all plants, animals, fungi and protists. Archaea and Bacteria look similar under a microscope. DNA evidence is what revealed how genetically different they really are.
Reading an evolutionary tree
An evolutionary tree branches from a shared starting point at the base. Two species with a branch point CLOSER to the tips diverged more recently from each other. That makes them more closely related than two species whose branches only meet near the base. Read the LENGTH along a branch for relatedness, not its position on the page.
A worked tree-reading question
A tree shows four species. A and B branch from a point near the tips. C branches off earlier. D branches earliest of all, near the base. A and B are the most closely related pair — their branches diverged from each other most recently. D is the most distantly related to the others, having diverged earliest. C sits in between: more closely related to A and B than D is, but less closely related to them than they are to each other.
Extinction is data too
The fossil record provides evidence for the tree. It shows which organisms existed at which point in time, and it can reveal species that no longer exist at all. Extinction happens when an environment changes faster than a species can adapt, when a new predator or disease arrives, or through a sudden catastrophic event. A species with low variation is more vulnerable to all three.