Diversity of Living Organisms

The One Thing

Biological classification organizes the diversity of life by comparing cellular structure, body organization, nutrition, reproduction, and evolutionary relationships. Scientific naming and hierarchical classification allow organisms to be identified, compared, studied, and conserved systematically.

Reactions, Processes and Experiments

Key Terms

• Biodiversity: The variety of living organisms, including diversity within species, between species, and among ecosystems.
• Taxonomy: The science of identifying, naming, describing, and classifying organisms.
• Systematics: The study of organismal diversity and evolutionary relationships, including classification based on common ancestry.
• Species: A group of similar organisms that can generally interbreed under natural conditions and produce fertile offspring.
• Binomial nomenclature: The two-word scientific naming system in which the first word is the genus and the second word is the specific epithet, for example, Homo sapiens.
• Taxonomic hierarchy: The arrangement of organisms in ranked groups: species, genus, family, order, class, phylum or division, kingdom, and domain.
• Taxon: Any named unit of classification, such as a species, genus, family, or kingdom.
• Identification: The process of recognizing an organism and assigning it to a known group.
• Nomenclature: The system of assigning scientific names according to internationally accepted rules.
• Monera: A traditional kingdom containing prokaryotic organisms such as bacteria and cyanobacteria; its members lack a true nucleus and membrane-bound organelles.
• Protista: A kingdom of mainly unicellular eukaryotes, including Amoeba, Paramecium, Euglena, and many unicellular algae.
• Fungi: Eukaryotic, mostly multicellular, non-photosynthetic organisms that obtain food by absorption and usually possess chitinous cell walls.
• Plantae: Multicellular, generally photosynthetic eukaryotes with cellulose cell walls and mostly fixed modes of life.
• Animalia: Multicellular, ingestively heterotrophic eukaryotes whose cells lack cell walls and are usually capable of movement.
• Prokaryote: An organism whose cell lacks a membrane-bound nucleus and other membrane-bound organelles.
• Eukaryote: An organism whose cells contain a membrane-bound nucleus and membrane-bound organelles.
• Autotroph: An organism that produces organic food from inorganic substances, commonly through photosynthesis or chemosynthesis.
• Heterotroph: An organism that obtains organic nutrients from other organisms or their products.
• Lichen: A stable association between a fungus and a photosynthetic partner, usually an alga or cyanobacterium, in which both partners benefit.
• Virus: An acellular infectious particle containing genetic material within a protein coat that reproduces only inside a living host cell.
• Viroid: A very small infectious agent made only of a short circular RNA molecule without a protein coat.
• Dichotomous key: An identification tool that presents pairs of contrasting statements and leads to the correct identity of an organism.
• Phylogeny: The evolutionary history and relationships of an organism or group of organisms.

Easily Confused

• Taxonomy and systematics: Taxonomy focuses on identifying, naming, describing, and classifying organisms, whereas systematics also examines evolutionary relationships and common ancestry.
• Prokaryotes and eukaryotes: Prokaryotes lack a membrane-bound nucleus and membrane-bound organelles, whereas eukaryotes possess them.
• Bacteria and Archaea: Both are prokaryotic, but they differ in cell membrane structure, cell wall composition, and genetic machinery; archaea often inhabit extreme environments.
• Autotrophs and heterotrophs: Autotrophs produce organic food from inorganic substances, whereas heterotrophs obtain organic nutrients from other organisms or their products.
• Fungi and plants: Fungi are non-photosynthetic and absorb nutrients, usually possessing chitinous cell walls; plants are generally photosynthetic and have cellulose cell walls.
• Bryophytes and pteridophytes: Bryophytes lack well-developed vascular tissues, whereas pteridophytes possess vascular tissues but reproduce by spores and do not produce seeds.
• Gymnosperms and angiosperms: Gymnosperms bear naked seeds, whereas angiosperms produce flowers and seeds enclosed within fruits.
• Viruses and viroids: Viruses contain genetic material within a protein coat, whereas viroids consist only of a short circular RNA molecule.
• Common names and scientific names: Common names vary among languages and regions, whereas a species name is universal under binomial nomenclature.
• Classification and identification: Classification places organisms into groups, whereas identification recognizes an organism and assigns it to a known group.
• Cellular organisms and acellular forms: Cellular organisms possess cellular organization, whereas viruses, viroids, and prions lack all the characteristics of independent cellular life.
• Natural classification and superficial classification: Natural classification reflects similarities, differences, and evolutionary relationships, whereas classification based only on convenient or superficial traits has less biological meaning.

What Gets Asked

• Define key terms, such as biodiversity, taxonomy, systematics, species, phylogeny, prokaryote, eukaryote, lichen, virus, and viroid. Marks are lost when taxonomy is confused with systematics or when a virus is described as a cellular organism.
• State or compare classification systems, especially Whittaker’s five-kingdom system proposed in 1969 and Woese’s three-domain system proposed in 1990. The relevant names, dates, and groups must be matched correctly.
• Write and interpret taxonomic ranks and binomial names. Questions may require the order from species to domain, or the correct formatting of Homo sapiens. Marks are lost by reversing genus and specific epithet, using incorrect capitalization, or omitting italics or separate underlining.
• Compare major kingdoms and groups using cell type, nutrition, body organization, reproduction, and structural features. Specific distinctions include Monera versus Protista, fungi versus plants, bryophytes versus pteridophytes, and gymnosperms versus angiosperms.
• Explain the borderline status of viruses, viroids, and prions. The essential distinction is that they lack independent cellular organization and metabolism, although viruses show biological activity inside living host cells.
• Apply taxonomic aids, including a dichotomous key, field guides, herbariums, botanical gardens, museums, zoological parks, and preserved specimens. The key error is treating identification as classification rather than as the process of assigning an organism to a known group.