Abstract
Hosts and pathogens impose coevolutionary pressure on each other as pathogens strive to establish themselves and hosts seek to suppress infection. RNA interference (RNAi) is a mechanism by which cells repress viruses and transposable elements, thereby serving as a form of immune defense. Previous studies have shown that antiviral RNAi genes evolve extraordinarily quickly in the fruit fly Drosophila melanogaster, suggesting that they may adaptively coevolve with viruses and transposable elements. An article by Palmer and colleagues extends this observation to nematodes and multiple insects. Their article can be combined with this Primer to demonstrate the use of comparative genomics and molecular evolutionary analyses in the measurement of natural selection.