Important mechanism of extinction of beneficial alleles.
Genetic drift is particularly important as a means of extinction of beneficial alleles in sexually reproducing populations, or populations that otherwise are subject to substantial amounts of gene exchange (and thus not clonal). Note that in clonal populations periodic selection can be an important cause of extinction of beneficial alleles as well.
The result of extinction of all alleles but one, as found at a specific locus, is equivalent to the fixation of the remaining allele. With genetic drift, contrasting natural selection, there is a relatively high likelihood that this remaining allele will be a detrimental allele. Thus, extinction of all "beneficial" alleles – as found at a given locus in an otherwise viable population – is equivalent to fixation of a detrimental allele.
Note further that genetic drift does not necessarily lead to the fixation of detrimental alleles but instead simply cannot distinguish between detrimental, beneficial, or indeed neutral alleles when effecting the fixation, or simply change in frequency of a a given allele.
In terms of genetic drift, see: impact of population size, impact on allele frequency, impact on genetic variation, and impact on allele fixation.