Glossary: Variation

This chapter's terms in context

Chapter 27 covers four terms describing the differences that exist between individuals of the same species: variation, continuous variation, discontinuous variation and mutation. Variation is the term for the differences in characteristics that exist between individuals of the same species, arising from genetic causes, environmental causes, or a combination of both, and it is the raw material on which natural selection, and consequently evolution, can act; without variation, every individual in a population would be identical, and no individual would have any advantage over another when conditions in the environment changed.

Continuous variation describes a characteristic that shows a complete, unbroken range of values between two extremes, without any distinct or separate categories, human height, body mass and skin colour are the standard SPM examples, since a large population sample would show every value across a smooth range rather than falling neatly into a small number of separate groups. Continuous variation is typically controlled by the combined, additive effect of many genes acting together (described as polygenic inheritance) and is often further influenced by environmental factors, such as diet affecting height, which is exactly why a graph of continuous variation across a population typically produces a smooth bell-shaped (normal) distribution curve rather than a small number of separate bars.

Discontinuous variation, in contrast, describes a characteristic that falls into a small number of distinct, separate categories with no intermediate values possible, human blood group (A, B, AB or O) and the ability to roll one's tongue are the standard SPM examples, since an individual falls clearly into one category and never into a value between two categories. Discontinuous variation is typically controlled by one gene or a very small number of genes, and it is usually not significantly influenced by the environment, which is exactly why a graph of discontinuous variation across a population produces a small number of separate bars rather than a smooth continuous curve. A frequent exam error is misclassifying a characteristic, describing blood group as continuous, or height as discontinuous, so a reliable test is to ask whether intermediate values between the categories are possible (continuous) or impossible (discontinuous).

Mutation is a sudden, random change in the structure or amount of DNA (or in the number or structure of chromosomes) that was not inherited from either parent, and it is the ultimate original source of all new alleles and all new genetic variation within a species, since no entirely new allele can otherwise be created by sexual reproduction alone, which only reshuffles alleles that already exist. A mutation can be caused spontaneously by errors during DNA replication, or its rate can be significantly increased by exposure to a mutagen such as ultraviolet radiation, ionising radiation, or certain chemicals; most mutations are either harmful or have no noticeable effect on an organism, but a small number can occasionally be beneficial, providing new variation on which natural selection can act. A common exam error is describing every mutation as harmful or as causing a visible physical deformity, when the syllabus specifically requires recognising that a mutation's effect can be harmful, neutral or, occasionally, beneficial, depending on the specific change involved and the environment the organism lives in.

Paper 2 questions on this chapter frequently supply an unfamiliar characteristic with a brief description or a sample data set and ask a candidate to classify it as continuous or discontinuous variation with justification, and full marks depend on stating the specific reasoning, the presence or absence of intermediate values, and the shape of the resulting graph, rather than simply naming the category without support.