2.7. Calculation of hybrid index values

For the calculation of hybrid index (HI) values, two westernmost populations of “nominally” ssp. helenitis (nos. 1, 2) and two easterly populations of ssp. salisburgensis (nos. 22, 24) were chosen to represent each taxon, based on three criteria: (a) sufficiently large sample size (n ≥ 7), (b) essentially nonadmixed ancestry, with mean q‐values ≥0.90 (range 0.90–1.0) for either parental cluster, based on the tess results, and (c) fixation or at least predominance (frequency >90%) of diagnostic achene types (Table ​(Table1).1). These four populations served as “parental” types to train the program introgress 1.1 (Gompert & Buerkle, 2010), using the “prepare.data” function for dominant markers. Maximum likelihood estimates of individual HI values were then calculated (function “est.h”) and averaged for each population using the fragment (“allele”) frequency differentials (δ) of the samples representing each “parent”. All estimates were scaled between 1 (“helenitis”) and 0 (“salisburgensis”), with intermediate values indicating genetically admixed individuals. For comparison, we also calculated HI values from the neutral AFLP data set with outlier loci excluded.