To investigate whether positive selection drove the evolution of the vertebrate IFITM gene family, we calculated the nonsynonymous substitution (dN) and synonymous substitution (dS) distances [33] between each pair of the sequences from the three clades. To exclude false signals caused by recombination, we first evaluated the effect of gene conversion using GENECONV [34] . Gene conversions were found in some species which are under species-specific duplication including dog, cow, horse, etc. (Table  S2 ). Those sequences were removed from our datasets for subsequent analyses.