Partitioning genetic variation between population structure and
climate
The percentage of variation explained by the joint effect of climate and
population structure was calculated by subtracting their independent
effects from the total variation explained. We performed a partial
redundancy analysis (pRDA) to separate the effect of climate from that
of population structure, calculating the genetic variation explained by
each climate variable by conditioning its effect on the other variables.
The significance of the models was tested using ANOVA with 1,000
permutations. To identify the climate variables that contributed most to
adaptive variation, we performed pRDA on the set of loci identified as
outliers. Finally, to identify the climate variable that was most
strongly associated with variation at each outlier locus, we extracted
loci scores from the separate pRDA for each climate variable and
normalized these scores to a mean of zero and a unit variance. We then
considered the climate variable with the highest absolute value to be
the one with the strongest influence on that locus.