A random, directionless change in allele frequencies that occurs...

Genetic drift refers to the random, directionless change in allele frequencies that occurs by chance, particularly in small, isolated populations. Unlike natural selection, which is a directional process based on fitness, genetic drift is entirely driven by stochastic events.

Gene migration involves the transfer of genetic variation from one population to another.

Natural selection is the differential survival and reproduction of individuals due to differences in phenotype.

Saltation refers to a sudden and large mutational change from one generation to the next.

Answer: Genetic drift