Mutations are random. You can't make a gene mutate in the wild.

When a gene mutation does occur, it is still largely unrelated random factors that will determine if the individual with that mutation survives to pass on the gene. Only once that mutation has spread to a large enough portion of the population, will statistical tends become significant.

If an established gene is not harmful enough to survival and mating, then diffusion will sustain it in the population. Random mating means that any sufficiently established gene will reach an equilibrium between selective pressure reducing its prevalence and diffusion bringing all alleles into equality.