The value of dN/dS ratio (or ω, the ratio of non-synonymous to synonymous distances) for each group was analyzed with the program codeml from PAML version 4 (Yang, 2007). A likelihood ratio test (LRT) was used to detect variation in ω among sites between M0 vs. M3 and M7 vs. M8, respectively. The χ2 statistics with degrees of freedom equal to the differences between parameters is twice the log likelihood difference between the two models in the LRT (3 for M0/M3 tests and 2 for M7/M8 tests; Mondragon-Palomino and Gaut, 2005). If the LRT was statistically significant and the evolution of genes below groups, then sites are assumed to be under positive selection pressure through Bayes methods (Yang et al., 2005).