genetic drift

In population genetics, genetic drift (or more precisely allelic drift) is the statistical effect that results from the influence that chance has on the survival of alleles (variants of a gene). The effect may cause an allele, and the biological traits that it confers, to become more common or rare over successive generations. Whereas natural selection defines the tendency of alleles to become more common or less common in a population over time as a result of the alleles' affects on adaptive and reproductive success, genetic drift is the fundamental tendency of an allele distribution in a population to statistically alter over time due to random events. Genetic drift is one of the primary mechanisms of biological evolution.
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Evolutionary change over generations due to random events in small populations (not to be mixed with sampling error due to a small sample size). It only operates in the absence of selective forces. Link to a lecture on genetic drift and a simulation.