This chapter briefly outlines the lines of evidence of virus evolution that are discussed in this book; hopefully it augments the information given in the Table of Contents by indicating where those lines anastomose. Evolution is the process whereby the population of an organism changes genetically over a period of time. It occurs when genetic variation within a population, combined with selection from among the mixture of genotypes, results in change. Viruses, especially those with RNA genomes, sometimes evolve very rapidly and so their evolution can be studied as it occurs. By contrast, all cellular organisms evolve more slowly and so their evolution is deduced by comparing extant forms and, for some, the fossils of their ancestors. No fossils of viruses are known.

Pre-molecular evidence of virus evolution

It has been realized for a long time that pathogens evolve. The fact that new epidemics of disease appear (Chapter 3), and that some viral diseases, such as measles and smallpox, induce life-long immunity in those individuals that survive, whereas others, such as the common cold or influenza, do not, all indicate that some viruses change. Of course, such differences influence choice of control strategies because stable viruses are amenable to control by vaccination, whereas rapidly changing viruses are not. Another type of evidence of virus evolution is the fact that avirulent strains of some viruses can be selected for use as vaccines, usually by growing them in unusual hosts, or under unusual conditions.

One of the first deliberate attempts to study natural virus evolution was the classic study of myxoma leporipoxvirus when it was liberated in Australia to attempt to control European rabbits (Chapter 2).