During the last decade and a half, studies of evolution and variation have been revolutionized by the introduction of the methods and concepts of molecular genetics. We can now construct reliable phylogenetic trees, even when fossil records are missing, by compara- tive studies of protein or mRNA sequences. If, in addition, paleon- tological information is available, we can estimate the rate at which genes are substituted in the species in the course of evolution. Through the application of electrophoretic methods, it has become possible to study intraspecific variation in molecular terms. We...