The afrikaners are an example of a founder effect that occurs when a new colony is started by a few members of the original population and the random decrease in genetic variation is an example of

genetic drift

Explanation:

When a small number of individuals from a larger population establish a new population another area resulting in a loss of genetic variation then this is termed as the founder effect. Since there is a loss of genetic variation this leads to a change. The frequency of the alleles in the existing population and this is a result of random sampling and hence the founder effect is a type of genetic drift. In the example given in the question, it has been stated that the Afrikaners are an example of a founder effect and since there is a loss of genetic variation this shows that there is a change in the allele frequency and hence, this is an example of genetic drift.