Glossary: Biodiversity

This chapter's terms in context

Chapter 23 covers four terms describing how the diversity of living things is organised, named and measured: species, classification, binomial nomenclature and biodiversity. A species is the basic unit of biological classification, traditionally defined as a group of organisms that share similar characteristics and can interbreed with one another to produce fertile offspring; this reproductive criterion is what makes a species a biologically meaningful category rather than an arbitrary label, and it is also why a mule, produced from a horse and a donkey, is not itself considered a separate species, mules are typically infertile, so the two parent species remain genetically and reproductively distinct.

Classification is the systematic process of grouping organisms into a hierarchy of categories based on their shared characteristics, moving from the broadest group down to the most specific: kingdom, phylum, class, order, family, genus and species, with organisms in the same lower-level group (such as the same genus) sharing more characteristics in common than organisms grouped together only at a higher level (such as the same kingdom). A frequent exam error is stating the taxonomic hierarchy in the wrong order, or skipping a level, so committing the full sequence to memory in the correct direction, from most inclusive to most specific, is a foundational skill this chapter depends on.

Binomial nomenclature is the standardised, two-part naming system used to give every species a single, universally recognised scientific name, consisting of the genus name (capitalised) followed by the species name (lowercase), both conventionally written in italics or underlined by hand, for example, Homo sapiens for humans. This system, introduced to avoid the confusion of using different common names for the same organism in different languages or regions, is precise specifically because the genus name places the organism within a wider group of closely related species, while the species name distinguishes it from every other species within that same genus. A common exam error is capitalising the species name or failing to italicise (or underline) the whole binomial name, both of which are treated as formatting errors even when the organism identified is otherwise correct.

Biodiversity is the total variety of living organisms found in a particular area, and it is measured and discussed at three distinct levels that a candidate is expected to distinguish clearly: genetic diversity, the variation in genes within a single species (such as the different coat colours within one species of dog); species diversity, the variety of different species present within a habitat or ecosystem; and ecosystem diversity, the variety of different habitats or ecosystems present within a larger region. A frequent exam error is discussing biodiversity only at the species level, when a full answer often requires recognising that a reduction at one level, such as a loss of genetic diversity within a single crop species, can threaten the long-term stability of the level above it, such as an entire agricultural ecosystem.

Paper 2 questions on this chapter frequently supply a simplified classification key or an unfamiliar set of organisms and ask a candidate to place them into groups based on shared and differing characteristics, testing the underlying logic of classification directly rather than requiring the memorisation of any particular organism's full taxonomic position.

Paper 3 investigations in this chapter sometimes ask a candidate to construct a simple dichotomous key for classifying a small set of unfamiliar specimens based on visible characteristics, and full marks depend on each question in the key offering exactly two mutually exclusive choices that lead progressively closer to a single named organism, rather than a vague, overlapping description that could apply to more than one specimen at once.