By chance one allele increases in some lines, the other allele in others Generation 4. One population fixes for bw 75 allele Generation 6. Average gene frequency, over all populations, stays same. The three questions. Yes, it changes allele frequencies.
Random changes are likely to destroy adaptation.
Generally removes variation. Eventually one allele or the other will drift to fixation. Populations may be effectively smaller than they seem. Populations may be initiated by a few migrants, and chance plays a major role in what genes they carry. That affects the entire future of the population.