CHROMOSOMES AND SPECIES FORMATION

Abstract

Ten years ago, Science published a provocative paper by Michael White called Models of Speciation. Somewhat earlier, John Maynard Smith had produced his extraordinary article Sympatric Speciation. These two short papers broke new intellectual ground in evolutionary theory. The extent of the revolution they started, however, cannot fully be appreciated until one follows White, chapter by chapter, through the present book. Throughout, he maintains a consistent viewpoint on the cladistic process; it places . . less emphasis on geographic isolation (allopatry) as an absolute precondition for and attributes . . a much larger role to structural chromosomal rearrangements. He also perceives an important role for sympatric speciation. The promotion of these views is documented by coverage in depth of a large recent literature, most of it not previously examined critically. White's accurate scholarship, combined with a deftness of argument, forces the reader to consider these unconventional hypotheses very seriously. One does not have to believe all of it to realize nevertheless that this book is the first major contribution to original thinking in this field since Mayr's 1963 classic, Animal Species and Evolution. Speciation is a word which is only just beginning to appear in dictionaries. In Webster, it is defined as the process whereby species are formed. White, I think, would be uncomfortable with this definition because a major theme of his book is to stress the evidence that speciation can occur in different ways. This treatment implies that we must deal with a number of processes, not a single one. Investigation of cladistic events as opposed to phyletic (anagenetic) ones requires a different perspective from that normally assumed in classical population genetics. The statistical and mathematical comfort of the Hardy-Weinberg equilibrium in large populations has to be abandoned in favor of the vague realization that nearly everywhere in nature we are faced with data suggesting the partial or indeed complete sundering of gene pools. If we are to deal realistically with cladogenesis we must seek to delineate each genetic and environmental factor which may promote isolation. The most important devices are clearly those which operate at the very