Criterion 2 considers the gene flow between populations, measured by FST. A high degree of differentiation between two populations among genes that provide no adaptive advantage to either population (known as "neutral markers") implies a lack of gene flow, showing that random drift has occurred in isolation from other populations. Very few migrants per generation are needed to prevent strong differentiation of neutral markers. Even a single migrant per generation may be enough for neutral markers to show gene flow between populations, making it difficult to differentiate the populations through neutral markers.