When there is a change in the frequency of genes in a small population due to chance event as a result of which a few alleles disappear from the population, is known as genetic drift.
In the given situation, the population size is small i.e. 25 and 40 respectively and in both these populations, it can be seen that one of the population only has allele A whereas the other has allele a, and because of this it can be concluded that this situation must have arisen due to genetic drift.
The populations got established due to random sampling and then the progenies inherited only the alleles of the organism that reproduced and thus, the frequency of the other allele decreased and this is merely a chance event.
