Ameasure of how commonly a particular allele occurs in a population is known as the

allele frequency

Explanation:

Allele frequency describes how often an allele appears in a population. This measure is usually expressed in percentage. Allele frequency can describe the variation that exist within the population. It can be calculated as the number of alleles divided with the total number of gene copies in that population.

Change in allele frequencies over a certain short period of time is defined as microevolution. Mechanisms of evolution that can change allele frequencies through time are mutations, natural selection, genetic drift and gene flow (migrations).

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explanation:

a = executive branch; b = judicial branch