 RNA interference genes , as components of the antiviral innate immune system in many eukaryotes , are involved in an evolutionary arms race with viral genes . Some viruses have evolved mechanisms for suppressing the RNAi response in their host cells , particularly for plant viruses . Studies of evolutionary rates in Drosophila have shown that genes in the RNAi pathway are subject to strong directional selection and are among the fastest @-@ evolving genes in the Drosophila genome . 
