Ever since the inception of the Modern Synthesis (the combination of Darwin’s Theory of Evolution by means of Natural Selection and the laws of genetic inheritance by Gregor Mendel) the role of natural selection shaping the variation observed in populations has represented one of the most attractive topics in Biology. However, evolution is driven by other additional forces, most notably mutation (the most important source of genetic variation). In this instant classic, Professor Masatoshi Nei examines the different mechanisms by which mutation contributes to evolution. Prof. Nei’s research has constituted a constant inspiration for our work at Chromevol, so we are specially proud that our own research on the evolution of multigene families is cited in this book.