Even when natural selection is not operating, the gene frequencies may change a little from the previous generation just by chance. This can happen because the genes that form a new generation are a random sample from the parental generation.
Random sampling occurs whenever a smaller number of successful individuals (or gametes) are sampled from a larger pool of potential survivers and the fitnesses of the genotypes are the same. Random sampling works at every stage as a new generation grows up but it starts at conception.
In every species, each individual produces many more gametes than will ever fertilize, or be fertilized, to form new organisms. The successful gametes which do form offspring are a sample from the many gametes that the parents produce. Provided the parent is a heterozygote, such as Aa, it will then produce a large number of gametes, of which approximately one half will be A and the other half a.
If that parent produces 10 offspring, it is most likely that five will inherit an A gene and five a. But because the gametes that formed the offspring were sampled from a much larger pool of gametes, it is possible that the proportions would be something else. Perhaps six inherited A and only four a, or three A and seven a.
Random sampling can have important evolutionary effects such as genetic drift and the founder effect.
In what sense is the sampling of gametes random?