Variation

In the syllabus and the exam

Variation refers to the differences in characteristics found between individuals of the same species, and the syllabus requires candidates to classify any given example correctly as continuous or discontinuous variation before explaining its cause. Candidates should link variation to its underlying sources: genetic variation, arising mainly from the random combination of alleles during meiosis and fertilisation and from mutation, and environmental variation, arising from external factors such as diet, exercise, sunlight exposure or climate acting on an organism during its lifetime. A key exam skill is judging, for a given characteristic, whether it is caused mainly by genes, mainly by environment, or by both interacting together, for example explaining that while maximum possible height is set genetically, an individual's actual height also depends on childhood nutrition, so height shows both genetic and environmental influence. A common error is assuming all variation is genetic and therefore automatically inheritable; environmentally caused variation, such as a suntan or a scar, is not passed on to offspring because it does not change the alleles present in reproductive cells. Candidates should also understand that variation is the essential raw material acted upon by natural selection, since without differences between individuals, a population could not evolve or adapt to a changing environment over time, connecting this chapter forward to later material on mutation. Candidates should further note that a twin study, comparing identical and non-identical twins raised in similar or different environments, is a classic method used to separate the relative contribution of genetic and environmental factors to a given characteristic.

Example

Human height, which ranges smoothly from short to tall across a population, and human ABO blood group, which falls into only four distinct categories, are both examples of variation but of two different types.

Related terms

Continuous variationDiscontinuous variationMutation

Source:SRC-DSKP-EN