What causes genetic drift allele frequency?

Answer 1

Genetic drift is the spread of specific random variations throughout the gene pool in the absence of specific selection pressures. There's always random variation in the population, but there aren't always changes in the environment for the population to adapt to. So natural selection, in stead of moving the population towards adaptation, might select from that random variation to move 'sideways', as it were, to a state that's equally well-adapted to the environment as what came before, but different. As random variation may produce many variants that are, more or less, equally well-adapted to their environment, the direction of evolution that results is more or less random.

Answer 2

Genetic drift is caused by random sampling effects that occur when a small population reproduces, leading to random changes in allele frequencies over generations. This can be exacerbated in small populations or during founder events when a subset of individuals establishes a new population with limited genetic diversity.