The K=2 conundrum

Assessments of population genetic structure have become an increasing focus as they can provide valuable insight into patterns of migration and gene flow. STRUCTURE, the most highly cited of several clustering-based methods, was developed to provide robust estimates without the need for populations to be determined a priori. STRUCTURE introduces the problem of selecting the optimal number of clusters and as a result the ΔK method was proposed to assist in the identification of the ‘true’ number of clusters. In our review of 1,264 studies using STRUCTURE to explore population subdivision, studies that used ΔK were more likely to identify K=2 (54%, 443/822) than studies that did not use ΔK (21%, 82/386). A troubling finding was that very few studies performed the hierarchical analysis recommended by the authors of both ΔK and STRUCTURE to fully explore population subdivision. Furthermore, extensions of earlier simulations indicate that, with a representative number of markers, ΔK frequent...