Biological Unity and Diversity
This domain covers classification, phylogeny, and the survey of major groups from prokaryotes through vertebrates, emphasizing shared biochemistry and the structural adaptations that mark major transitions.
Classification and Phylogeny
- Linnaean hierarchy: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species; binomials are italicized with the genus capitalized.
- Three domains: Bacteria, Archaea, Eukarya. Comparative rRNA reshaped classification and showed that Archaea are closer to Eukarya than to Bacteria.
- Cladistics groups organisms by shared derived characters (synapomorphies). Monophyletic groups are valid clades; paraphyletic and polyphyletic groupings are not.
Survey Highlights
- Prokaryotes lack a nucleus and have circular DNA; bacterial walls contain peptidoglycan. Viruses are acellular and outside the three domains.
- Protists are a paraphyletic collection of mostly unicellular eukaryotes. Fungi have chitin walls and absorb nutrients after external digestion.
- Plants show alternation of generations, with land adaptations building in order: cuticle and stomata, vascular tissue, seeds, flowers and fruit.
- Animals are grouped by symmetry, germ layers, body cavity, and protostome versus deuterostome development.
Worked Example
A cladogram places a bony fish, a lizard, and a dolphin. A student groups fish and dolphin together because both are aquatic. Reasoning: aquatic habitat is analogous, produced by convergent evolution, not a shared derived character. Mammary glands place the dolphin closer to the lizard than to the fish. Grouping all "fish" together creates a paraphyletic taxon because tetrapods descend from a bony-fish ancestor.
Traps
- Treating similarity as relatedness without checking homology versus analogy.
- Placing a dolphin with fish instead of mammals.
- Forgetting that viruses are not cellular and not part of the three domains.
- Assuming "invertebrate" or "protist" names a clade - both are non-monophyletic groupings.