Biological Diversity
Classifying life
Taxonomy orders life as Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species; binomial names capitalize the genus, lowercase the species, and are italicized. The three domains are Bacteria, Archaea, and Eukarya. Viruses are not in the tree of life - they are obligate intracellular parasites with no independent metabolism, a standard trap.
Major groups and their hallmarks
Bacteria and archaea are prokaryotic; archaea are often extremophiles with distinct membrane lipids. Protists are mostly unicellular eukaryotes and form a paraphyletic grouping. Fungi are heterotrophic absorbers with chitin walls - not plants. Plants have cellulose walls and photosynthesize: bryophytes (no vascular tissue), ferns (vascular, no seeds), gymnosperms (seeds, cones), angiosperms (flowers, fruit). Alternation of generations alternates haploid gametophyte and diploid sporophyte. Animals are multicellular heterotrophs; invertebrate phyla (sponges, cnidarians, mollusks, annelids, arthropods, echinoderms) dominate, with chordates including vertebrates.
Phylogenetics
Cladograms show shared derived characters (synapomorphies); a clade includes an ancestor and all descendants. Trap: similarity alone is not relationship - convergent evolution produces analogous structures, while homology reflects common ancestry. Molecular data often revise morphology-based trees.
Worked example
In a cladogram where fur is a derived trait shared by mammals but absent in an outgroup, fur marks the branch leading to mammals. Rule of thumb: shared derived traits define groups; shared ancestral traits do not.