Natural Selection and Genetic Drift

This lesson covers natural selection and genetic drift — the two main mechanisms that change allele frequencies in populations — as required by the Edexcel A-Level Biology specification (9BI0, Topic 8).

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Allele Frequency

The allele frequency is the proportion of a particular allele in a population's gene pool. For a gene with two alleles (A and a):

• Frequency of allele A = p
• Frequency of allele a = q
• p + q = 1

Changes in allele frequency over time are the basis of evolution.

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Natural Selection

Natural selection is the process by which organisms with heritable traits that give them a survival or reproductive advantage in a particular environment are more likely to pass those alleles to the next generation. Over time, this changes allele frequencies in the population.

Requirements for Natural Selection