‘Classical’ has connotations of ‘ancient’, but for this book the word relates to studies on virus evolution dating from the time before the sequencing of viral genomes became a way of life for evolutionary virologists. Classical ideas on the origins of viruses were pure speculation and are not worth pursuing further; it was suggested that viruses had arisen either from host cell components in some unknown way, or that some of the more complex viruses may have evolved by progressive degeneration along the sequence bacterium, mycoplasma, rickettsia, chlamydia, poxvirus. Since the focus of this book is the role of molecular biology in understanding viral evolution, I have selected as examples the only two viruses that were discussed in an evolutionary context in books on animal virology published before the emergence of recombinant DNA technology in the early 1970s, namely influenza A virus and myxoma virus (Burnet, 1955, 1960; Andrewes, 1967; Fenner et al., 1974).

Early studies on the evolution of influenza A virus

Here I describe our knowledge of the evolution of influenza viruses as it appeared in 1975, when the only methods of study were serology and peptide mapping of selected proteins. Chapter 34 takes over from that time and tells how molecular biological methods have expanded our understanding of the evolution of these very interesting viruses.

Structure of the virion

First, a little background is needed. There are three species of influenza virus, A, B, and C, each of which can produce respiratory disease in humans.