Final answer:
Gradual changes in allele frequency in a population, also known as genetic drift, are influenced by chance events along with natural selection, gene flow, and other factors.
Step-by-step explanation:
The gradual change in allele frequency across space is known as genetic drift. This change can occur due to random events that affect which individuals survive and reproduce. Other mechanisms that can lead to allele frequency changes include natural selection, gene flow, mutation, nonrandom mating, and environmental variances. In population genetics, evolution is defined as a change in the frequency of an allele within a population. For example, using the ABO blood type system, the frequency of one allele, like IA, is calculated by dividing the number of copies of that allele by all the copies of the ABO gene in the population. A change in frequency over time indicates that evolution is occurring within that population.