Diversity of Life

Naming organisms and arranging taxa

Classification groups organisms so that similarities and relationships can be studied systematically. Taxonomy includes identification, nomenclature and classification. Systematics also considers evolutionary relationships. A taxon is a named group at any rank, while a taxonomic category is the rank itself.

Binomial nomenclature uses a genus name followed by a specific epithet. The genus begins with a capital letter and the epithet with a lowercase letter; both are italicised in print. In handwritten text they are conventionally underlined separately. The species name includes both words, rather than the epithet alone.

• A common sequence is species → genus → family → order → class → phylum/division → kingdom.
• The biological species concept emphasises interbreeding and reproductive isolation, but has limits for fossils and asexual organisms.

Read a scientific name correctly

In Mangifera indica, Mangifera is the genus and indica is the specific epithet. The complete binomial identifies the species. As you move from species toward kingdom, groups generally include more organisms and fewer shared detailed characteristics.

Major classification frameworks

Classification systems change as evidence changes. Whittaker’s five-kingdom system uses cell type, organisation, nutrition, reproduction and relationships to distinguish Monera, Protista, Fungi, Plantae and Animalia. A three-domain framework separates Bacteria, Archaea and Eukarya using deeper cellular and molecular differences.

Use the framework specified in the question. For example, “Monera” belongs to the five-kingdom scheme, while Bacteria and Archaea are distinct domains in the three-domain scheme. These systems should not be mixed as if their categories were identical.

• Cyanobacteria are photosynthetic prokaryotes, not eukaryotic algae.
• Archaea differ from bacteria in important molecular and cell-envelope features. Not all archaea live in extreme environments.

Protists, fungi and non-cellular agents

Protists include diverse eukaryotic organisms such as diatoms, dinoflagellates, euglenoids and protozoan groups in conventional school-level descriptions. Their modes of movement and nutrition vary. Diatoms have silica-rich walls, while euglenoids can show different nutritional modes depending on conditions.

Fungi obtain nutrients by absorption after extracellular digestion. Many form hyphae that collectively make a mycelium; yeasts are commonly unicellular. Reproduction can involve vegetative methods and asexual or sexual spores. Lichens associate a fungal partner with a photosynthetic partner, while mycorrhizae associate fungi with plant roots.

• Viruses are acellular and require host machinery for multiplication. Their genetic material is DNA or RNA, depending on the virus.
• Viroids are small infectious RNA agents without a protein coat. Prions are infectious protein forms.
• Do not classify a virus as a prokaryotic cell: it is not a cell.

Plant groups and the transition to land

Compare plants using vascular tissue, seeds, flowers and the dependence of fertilisation on external water. Bryophytes lack the vascular organisation of higher plants and have a conspicuous gametophyte. Pteridophytes possess vascular tissue but reproduce by spores. Gymnosperms and angiosperms are seed plants.

Gymnosperm ovules and seeds are not enclosed within an ovary. Angiosperms produce flowers, and their ovules develop within ovaries that can form fruits. Alternation of generations describes the succession of haploid gametophyte and diploid sporophyte stages; their relative prominence varies among groups.

• Algae include diverse mostly aquatic photosynthetic organisms and do not all share one life-cycle pattern.
• Pollen and seeds reduce dependence on free external water compared with the motile-sperm fertilisation of many non-seed plants.

The features used to compare animals

Animal classification uses several independent traits: level of organisation, symmetry, germ layers, body cavity, segmentation and presence of a notochord. Diploblastic animals have two primary germ layers; triploblastic animals have three. A true coelom is lined by mesoderm, unlike a pseudocoelom.

Sponges show cellular-level organisation and a water canal system. Cnidarians have cnidocytes. Flatworms are acoelomate, roundworms pseudocoelomate, and annelids show metameric segmentation. Arthropods have jointed appendages and a chitinous exoskeleton. Molluscs typically have a mantle and muscular foot. Echinoderms possess a water vascular system.

• Adult echinoderms usually show radial symmetry, while their larvae are bilateral.
• Chordates possess a notochord, dorsal hollow nerve cord, pharyngeal slits and a post-anal tail at some stage.
• The notochord and vertebral column are not identical structures. Vertebrates are a subgroup of chordates.

Using characters to solve classification questions

Begin with the most diagnostic feature instead of matching a familiar name. If an animal has jointed appendages and an exoskeleton, arthropod characters are more informative than its habitat. If a plant has vascular tissue but no seeds, compare pteridophytes rather than bryophytes.

Avoid treating one character as sufficient in every case. A useful identification key combines features in a sequence of alternatives. Homologous similarities reflect shared ancestry more directly than superficially similar adaptations that evolved independently.

Reason through a specimen

A specimen is a multicellular animal with bilateral symmetry, three germ layers, a true coelom and obvious repeated body segments, but no jointed appendages. These observations support an annelid identification more strongly than an arthropod one. Confirm using additional structures and the choices provided.