Genetic drift is random and doesn't decrease the genetic diversity of a species. If anything it would increase the diversity since the genetic changes are not the same throughout the population.

How does genetic drift affect genetic variation?

Genetic drift involves the loss of alleles from a population by chance. Random fluctuations in allele frequencies in small populations reduce genetic variation, leading to increased homozygosity and loss of evolutionary adaptability to change.

Does genetic drift reduce genetic variation in a population?

Random forces lead to genetic drift

Sometimes, there can be random fluctuations in the numbers of alleles in a population. These changes in relative allele frequency, called genetic drift, can either increase or decrease by chance over time. ... Both possibilities decrease the genetic diversity of a population.