Biological Classification and Diversity of Life
Core concepts
Taxonomy organizes life into domains, kingdoms, phyla, classes, orders, families, genera, and species; binomial names are italicized (Genus species). The three domains are Bacteria, Archaea, and Eukarya. Prokaryotes lack a nucleus; archaea differ biochemically from bacteria. Eukaryotic diversity includes protists (mostly single-celled, not a single clade), fungi (heterotrophic absorbers with chitin walls), plants (photosynthetic, cell walls, alternation of generations), and animals (multicellular heterotrophs, most with tissues). Phylogenetics reconstructs evolutionary relationships using shared derived characters (synapomorphies); a clade is a monophyletic group containing an ancestor and all descendants. Viruses are acellular obligate parasites and are not classified as living organisms by most schemes. Major evolutionary transitions include colonization of land, vascular tissue, seeds, flowers, and amniotic eggs.
Traps
Protist is a convenience grouping, not a clade. Fungi are not plants. Archaea are not bacteria. A shared ancestral trait does not define a clade; only shared derived traits do. Viruses are not killed by antibiotics.
Worked example
On a cladogram, a trait appearing on the branch leading to a group is derived for that group. If birds and crocodiles share a more recent common ancestor than either does with lizards, birds and crocodiles form a clade with that ancestor-even though birds fly and crocodiles do not. Rule of thumb: classify by evolutionary history, not by habitat or appearance alone.