Pairs of taxa around the central Texas suture zone (top). Pairs in the red box are expected to be the same species (negative control); pairs in the green box are well-separated, different species (positive control); pairs in the yellow box include possible species or subspecies, but experts disagree on their status (test set). Specimens are shown in pairs of counterparts: eastern is above and western is below, and species names are given below specimens. Dorsal side of each specimen is shown except when marked with "V" between the antennae, which indicates that ventral surface is shown. Quantification of the reproductive barrier and genomic criteria for defining species (bottom). (A) Fixation index (FST) of Z-linked proteins and index of gene flow (IGF) calculated on Z-linked genes clearly distinguish pairs of different species from conspecific pairs, while the same measurements computed on all nuclear genes present in RNA-seq (upper right corner) dataset do not separate them as well. (B) Divergence (percent of different nucleotides) in COI barcode and nuclear genes performs poorly in evaluating whether two taxa are different species.