Boxplots showing the median values (thick horizontal black lines) and quartiles for π (a & b) and θ
(c & d) estimates at synonymous and non-synonymous sites. Open dots indicate outlying estimates. The red lines with asterisks indicate significant differences between clusters based on a paired t-test (p < 0.05). Note that for both synonymous π and θw the y-axis is on a Log10 scale.