Abstract
This chapter argues that the vast majority of genetic changes or mutations fixed by RNA viruses are essentially neutral or nearly neutral in character. In molecular evolution one of the remarkable observations has been the uniformity of the molecular clock. An analysis of proteins derived from complete potyvirus genomes, positive-stranded RNA viruses, yielded highly significant linear relationships. These analyses indicate that viral protein diversification is essentially a smooth process, the major parameter being the nature of the protein more than the ecological niche it finds itself in. Synonymous changes are invariably more frequent than nonsynonymous changes. Positive selection exploits a small proportion of genetic variants, while functional sequence space is sufficiently dense, allowing viable solutions to be found. Although evolution has connotations of change, what has always counted is natural selection or adaptation. It is the only force for the genesis of a novel replicon.