The considerable genetic heterogeneity of field strains of bluetongue virus (BTV) occurs as a consequence of both genetic drift and shift. Comparison of strains of BTV from the People's Republic of China and North America showed that viruses from the two regions were readily distinguished by sequence analysis of their S10 (which encodes the NS3/3A proteins) but not their L2 (which encodes the serotype-specific VP2 protein) genes. Subsequent laboratory studies showed that individual BTV genes evolve through a combination of genetic drift coupled with founder effect in vector insects. This model explains the diversification of BTV gene segments within each region, and can be extrapolated to explain diversification of BTV into distinct topotypes worldwide.