IGCSE Biology / Natural selection and evolution
How natural selection works
Edexcel International GCSE Biology (4BI1), chapter 19: Natural selection and evolution, specification point 3.38. Free lesson.
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Natural selection is the process by which inherited alleles that help individuals survive and reproduce become more common in a population over generations. Knowing the five steps, and explaining that variation comes before selection, helps you describe evolution accurately.
The five steps of natural selection
Natural selection happens when some inherited differences give individuals an advantage in a particular environment. The process has five linked steps, in this order:
Where the variation comes from
Variation must exist before selection can act. Mutation creates new alleles, while meiosis and fertilisation produce new combinations of alleles already present. Mutation is a change in DNA. It is the only source of genuinely new alleles. Meiosis shuffles existing alleles through crossing over and independent assortment, making gametes with different combinations. Random fertilisation means any gamete can join with any other, creating further combinations.
Selection favours, it does not create
The variation is already present when the environment changes. Selection then favours the variants that happen to be better suited. It cannot create a useful characteristic just because it is needed. For example, a random mutation may give some bacteria resistance before an antibiotic is used. When the antibiotic is applied, resistant bacteria are more likely to survive and reproduce. Their resistance allele then becomes more common.
Populations evolve over generations
Evolution happens when allele frequencies in a population change over many generations. Individuals do not evolve during their lifetimes. Natural selection changes how common inherited variants are in the population. Describe which individuals survive and reproduce, and which alleles they pass on. Then explain how the advantageous alleles become more common over generations.