Allele frequencies tend to change from one generation to the next purely as a result of random sampling error. We can specify a probability distribution for the allele frequency in the next generation, but we cannot specify the numerical value exactly. There is no systematic bias to the change in allele frequency, i.e., allele frequencies are as likely to increase from one generation to the next as to decrease. Populations will eventually fix for one of the alleles that is initially present unless mutation or migration introduces new alleles. The time to common ancestry for all alleles within a population is directly proportional to population size, approximately .
Natural selection introduces a systematic bias in allele frequency changes.
Alleles favored by natural selection tend to increase in frequency. Because there is a random component to allele frequency change when genetic drift is involved, however, we can't say for sure that a selectively favored allele will increase in frequency. In fact, we can say that there's a chance that a selectively favored allele won't increase in frequency.