Cambridge O Level Biology · Syllabus 5090 · Inheritance
Variation
What is Variation?
Differences between individuals of the same species. Variation may be caused by differences in the genes an individual inherited, by differences in the environment the individual has experienced, or by the two acting together, and most measurable features in living organisms are influenced by both. Variation is the raw material on which natural selection and artificial selection act: without differences between individuals there is nothing for selection to select.
This definition is part of the Inheritance chapter in Cambridge O Level Biology.
Common mistakes with Variation
- “Continuous variation is controlled only by genes.” It is usually influenced by many genes and by the environment, and that combination is what produces a smooth range rather than distinct groups. Write “Continuous variation is usually caused by many genes acting together with the environment.”
- “Discontinuous variation cannot be inherited.” The reverse: the required examples are controlled mainly by genes and are inherited very reliably. Your blood group was fixed at fertilisation. Write “Discontinuous variation in these examples is controlled mainly by genes, with little environmental effect.”
- “All the differences you can see are inherited.” Scars, tans, muscle built by training and a plant kept small by shade are all genuine variation and none of them is passed on. Write “This variation is environmental, so it is not inherited.”
- “All mutations are harmful.” Most are neutral, some are harmful, and occasionally one is beneficial in a particular environment. Without that occasional useful one there would be no new variation at all. Write “Mutations may be harmful, neutral or beneficial.”
- “The organism mutated because it needed to.” Mutation is random with respect to need. Variation arises first; the environment then selects among it. Write “The mutation occurred at random, and the environment then favoured the individuals carrying it.”
Questions students ask about Variation
Did the antibiotic cause the bacteria to become resistant?
No. The resistance allele arose earlier, by random mutation, in bacteria that had never encountered the drug. The antibiotic then acted as a selection pressure: it killed the bacteria that lacked the allele, leaving the resistant ones to survive and reproduce. The proportion of resistant bacteria in the population rose. The antibiotic selected variation that already existed — it did not create it.
Is a mutation always harmful?
No. Most mutations are neutral, having no detectable effect. Some are harmful. Occasionally one is beneficial in a particular environment, and those are the source of the new variation that natural selection can act on. Whether a mutation is harmful or helpful can also depend on the environment: the same allele may be a disadvantage in one place and an advantage in another.