Abstract

Despite considerable interest in viral evolution, at least among virologists, viruses are rarely considered from the same evolutionary vantage point as other organisms. Early work of necessity emphasized phenotype and phenotypic variation (and therefore arguably was more oriented towards the broader biological and ecological perspectives). More recent work (essentially since the development of molecular evolution in the 1960's but beginning earlier) has concentrated on genotypic variation, with less clarity about the significance of such variations. Other aspects of evolutionary theory, especially considerations of natural selection and of evolutionary constraints, have not widely been applied to viruses, and an evolutionary framework for virology has long been lacking. This becomes apparent in considering 'emerging' viruses, which have often been treated on an ad hoc basis. It was often felt that, because previously unrecognized viruses are involved, mechanisms of viral emergence must mirror the unpredictability of mutations in the viral genome. However, most examples of viral emergence are independent of mutation, at least initially, and are often pre-existing viruses in changed circumstances ('viral traffic'). This conclusion also readily follows from ordinary Darwinian premises, which would require that, like other living species, 'new' organisms are descended only from existing species. In this respect, from a Darwinian perspective, viruses would appear to resemble other organisms