We performed hierarchical population-clustering analyses using the maximum likelihood approach of ADMIXTURE (Alexander et al. 2009). We used five replicates to evaluate up to six discrete populations per analysis, and subsequently determined the K value with the lowest cross-validation error.
Analyses were performed on the full data set, and subsets that consisted of either the Northern range or the Southern range.
Our work was performed using scripts that helped automate analyses, which can be found here: https://github.com/dportik/Phylo_Wrapper_Scripts/tree/master/Admixture
The output files with the extension "_cross_validation_averages.txt" were then used in the associated R script to create plots to determine the best K value.