Cambridge O Level Biology · Syllabus 5090 · Inheritance
Natural Selection
What is Natural Selection?
The process by which individuals whose inherited features make them better suited to their environment survive and reproduce more successfully than others, so that the alleles responsible become more common in the population over generations. Natural selection requires inherited variation within the population, the production of more offspring than can survive, and competition for limited resources. It acts on variation that already exists rather than creating it, and the outcome is measured in reproductive success rather than in survival alone.
This definition is part of the Inheritance chapter in Cambridge O Level Biology.
Natural Selection in context
Inheritance is the transmission of genetic information from one generation to the next. A gene is a length of DNA that codes for a protein; an allele is one of the alternative forms that gene can take. The order of the bases A, T, C and G along a strand of DNA sets the order of amino acids in a protein, which sets the protein’s shape, which sets what the protein can do — and that is how a molecule you cannot see decides a feature you can. Because gametes carry one allele of each gene, a cross between two known genotypes has a predictable outcome: Tt × Tt is expected to give 3 tall : 1 short, and Tt × tt is expected to give 1 tall : 1 short. Those are probabilities, not guarantees. Mutation makes new alleles at random, meiosis and fertilisation shuffle them into new combinations, and selection — by the environment in natural selection, by people in artificial selection — changes how common each allele becomes over generations. That change is evolution.
Common mistakes with Natural Selection
- “Natural selection is survival of the strongest.” Strength is usually irrelevant, and survival is only half of it — what matters is reproducing. Write “Individuals with inherited features that suit the environment survive and reproduce more successfully.”
Questions students ask about Natural Selection
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.