Introduction

In the introduction to his classical textbook Molecular Evolutionary Genetics Nei (1987) says: ‘in the study of evolution there are two major problems. One is to clarify the evolutionary histories of various organisms, and the other is to understand the mechanisms of evolution’. These two problems have been traditionally addressed by paleontologists (and taxonomists) and by population geneticists, respectively. The introduction of molecular techniques, first protein electrophoresis and sequencing, later restriction analysis and sequencing of nucleic acids, has removed the boundary between these two aspects of evolutionary studies. Molecular techniques have allowed the analysis of a large and representative sample of viral genomes, and make viruses especially suitable for molecular evolutionary studies. In fact, a large amount of information on viral evolution has been collected during the last 20 years.

As a perusal of the contents of this book will clearly show, virologists have been concerned mainly with the evolutionary histories of viral species or groups as analysed by different procedures of phylogenetic reconstruction. This may be related perhaps to the traditional interest of virologists (particularly animal virologists) in following the track of epidemic developments.

Phylogenetic analyses, in addition to clarifying evolutionary histories, can also serve other purposes, from the simplest graphic representation of genetic distance (Dopazo et al., 1988; Air et al., 1990) to the testing of different evolutionary models (Gojobori, Moriyama & Kimura, 1990; Fitch et al., 1991). This approach can, and we dare say should, be complemented with a population genetics’ view of viral evolution, ranging from the quantitative description of genetic variation within and between populations to ascertain what mechanisms are responsible for the observed genetic structure of these populations.