<p>A simple linear regression of the root-to-tip genentic distances against the sampling dates was performed using the Path-o-gen software. The root was determined by maximizing the coefficent of determinant R<sup>2</sup>. The vertical axis measures the genetic distances between the samples and the root while the horizontal axis scales the sampling dates (year). For subtype 1a (A), the mean evolutionary rate (the slope of regression line) is 9.05E-4 substitution/site/year and the tMRCA (the X-intercept) is located at 1941. For subtype 1b (B), the mean evolutionary rate is 4.82E-4 and the tMRCA is located at 1808.</p
