> * Or, is it possible that the lack of mutations (presuming that this is very accurate) indicates an evolutionary pressure to maintain functionality that is se

> * Or, is it possible that the lack of mutations (presuming that this is very accurate) indicates an evolutionary pressure to maintain functionality that is sensitive to failure due to even minor mutations or deviations in functionality? Yes -- A lack of mutations on the site is a strong hint that it is functionally important. If virions with mutations at this site were equally fit, you'd expect more variation.