Testing for signatures of convergent evolution
Northern and southern populations of each species were analysed for genetic patterns driven by adaptation to some unmeasured factor related to latitude, by searching for genes with abnormally high patterns of F­ST. Using the top candidate approach (Yeaman et al. , 2016) 73 genes had extreme values of F­ST in threespines compared with 65 genes in tubesnouts (Table S4; Fig. 3A). None of the top candidates were directly shared between these species, but a pair of candidate genes encoding proteins in the forkhead box family were detected (Ts: foxo3b ; Tu: foxb2 ; Table S4). No signatures of high FST could be detected in ninespines because too many scores were close to FST = 1 to identify meaningful outliers. Additionally, comparing all species, only three \({\overset{\overline{}}{H}}_{E}\) scores overlapped in the upper 95% of the distribution (Fig. 3A). The Null-W test identified five possible signatures of convergent evolution between threespines and tubesnouts (Fig. 4B), but after adjusting for false discoveries these signatures lost significance (Table S5). The Null-W test did not identify any forkhead box genes as candidates.