Fig. 4.

Gene expression variation within and among populations indicating different patterns of evolutionary divergence. Plotted are the log of within- and among-population variation for gene expression. The ratios of these values are often used in statistical tests (e.g., ANOVA). Under neutral drift, within-population variation is correlated with among population variation (open circles). For other genes, different forms of selection overwhelm the general patterns indicated by drift and reject this null model. Genes under directional selection (pink circles) were identified as divergent along a habitat temperature gradient after correcting for variance due to phylogeny (phylogenetic generalized least squares method), and have higher variation among populations than within. Genes most influenced by stabilizing selection (yellow circles) have lower variation both within and among populations than most genes (F test), and genes under balancing selection (blue circles) have higher variation within than among populations (inverted F test).