Scientists and philosophers have long debated the trajectory of evolution. Some of the questions they consider include: is there a predictable direction for evolution, and if there is, what is the pathway? Are there evolutionary dead ends?
Viruses are excellent subjects for the study of evolution: they have short generation times, high yields of offspring, and prodigious levels of mutation, recombination, and reassortment. Furthermore, selection pressures can be readily applied in the laboratory, and may be often be identified in nature.
When studying evolution of viruses, it is important to avoid judging outcomes as ‘good’ or ‘bad’. Anthropormorphic assessments of virus evolution come naturally to humans, but concluding that viruses become ‘better adapted’ to their hosts, for example, fails to recognize the main goal of evolution: survival. Or, in the case of the non-living viruses, existence.
Evolution does not move a viral genome from simple to complex, or along a trajectory aimed at perfection. Change comes about by eliminating those viruses that are not well adapted for the current conditions, not by building something that will fare better tomorrow.