WNV genetic diversity in Culex mosquitoes from suburban Chicago, 2005–2007. Genetic diversity is expressed as nucleotide diversity (Nei 1987), or mean proportion of nucleotide differences among sequences within each year, corrected for multiple substitutions using a model of molecular evolution derived from a hierarchical likelihood ratio test approach implemented using the computer program Modeltest, v. 3.7 (Posada & Crandall 2001). Lines and small error bars indicate means and standard errors, respectively. Boxes indicate third quartiles, and large error bars indicate ranges. The dashed line is a least squares regression line indicating the rate of increase of viral genetic diversity over time (0.94 × 10−3 yr−1, R2 = 0.32, t = 36.8, p < 0.01). Grey lines are least-squares regression lines derived from 1000 bootstrap replicates of the data; solid lines indicate 95% CI around the nucleotide diversity estimation. Sample sizes for each year are indicated in parentheses.