QST - FST: Comparison of
quantitative trait differentiation to neutral genetic expectation
In support of our second hypothesis, we found evidence of divergent
selection (QST > FST)
driving phenotype differences in most traits (Table 2, Fig. 4). Overall,
the average QST value across all traits and all gardens
(0.48) was above the FST confidence interval
(0.144-0.205), consistent with directional selection shaping trait
differences and local adaptation across these populations. Phenology
traits exhibited some of the highest levels of population
differentiation, with mean QST values for bud flush
between 0.53-0.90 and bud set between 0.42-0.79. In all but one case
(bud set in Agua Fria), the confidence intervals for these
QST values lie above the confidence interval for
FST. Divergent selection on specific leaf area was also
apparent in the mid and cold gardens of Agua Fria and Canyonlands, but
not detectable in the hot Yuma garden. In contrast to this relative
consistency in results for leaf traits, traits related to tree
performance showed more variation in QST -
FST comparisons across gardens. Tree height showed
significant divergent selection (QST >
FST) when measured at the hottest common garden in Yuma,
Arizona, but was not statistically different from FST in
the two cooler gardens of Agua Fria and Canyonlands. We found evidence
of stabilizing selection for basal trunk diameter in the cold
Canyonlands garden, where the QST value (0.03) fell
below the FST confidence interval. However, this trait
was indistinguishable from the neutral expectation of
FST in the warm and mid gardens.