Making evolution clear

Mayr, the long-time grand master of modern evolutionary biology, has prepared a surprisingly short book that is both erudite and concentrated. He presents clear statements and explanations of virtually every aspect of the evolution of life on earth. What an accomplishment! He deals with the vast literature in a series of twelve intensely written chapters. Evolution, which is simply descent with genetic change, is quickly accepted as a fact that he proceeds to defend with great skill and thoroughness. The author states that this book is written for ‘‘ . . . anyone, biologist or not, who simply wants to know more about evolution’’. The general reader, however, will soon find it necessary to have had considerable prior exposure to the many intricacies of biological processes. These range from mating systems in local populations to the detailed nature of the genetical coding system and what happens to it when it is forced, by wholly natural forces, through the amazing egg/sperm bottleneck in each generation. Over the years, Mayr’s primary area of expertise has been with natural populations, encouraging the perceptive observer to draw conclusions based directly on natural-history observations. In this, he follows very closely in the steps of Darwin. Thus, the strongest parts of this book deal with his enthusiastic endorsement of Darwinian natural selection in all its aspects, as the primary cause of evolutionary change and the building of specific adaptedness in populations. Few professional biologists will disagree with this presentation. A major ‘‘natural-history’’ contribution of Mayr is his longterm advocacy of ‘‘population thinking’’. Mayr is a ‘‘panselectionist’’, championing the powerful idea that the object of selection is ultimately the sexually produced individual. Indeed, each such individual is monitored by selection from the time that it originates as a fertilized egg carrying a unique genetic code. After braving the many challenges that natural selection poses to survival, some small proportion may finally reach sexual maturity. This is when the code-bearer faces the new game of sexual selection, which is perhaps the hardest test of all. Mayr’s point stresses the realization that the causes of evolutionary change can be profitably analyzed at the level of the small, local interbreeding population of present-day organisms. This viewof thegrowing point of evolutionary change has had a wide influence among modern experimental population geneticists. There are, of course, the dissenters, who hold that to producean innovative character, selectionmust be carried out within large out-breeding populations, which hold maximum genetic variability. In biology, the 20th was the century of genetics. This fact imposes daunting difficulties for the popular writer in the 21st century. The influence of this hugely documented, quantitative,molecular, statistical and highly technicalmodern science has reached into every corner of biology, including medicine. The discovery in 1953 of the precise nature of the hereditary material as codedmolecular information that goes from parent to offspring, seized the imagination of biologists. Molecular genetics was born as an exciting new research adventure that required novel modes of investigation. Curiously, the science developed quite separately from the study of evolution. Nowhere has the influence of genetics been greater than in evolutionary biology. It has fallen to traditional evolutionists, like Mayr, to connect Darwinian natural selection with genetics. In this book, genetical properties of organisms relevant to evolution are dealt with in a complex thirty-two page chapter called ‘‘Variational Evolution’’. The account goes from one technically difficult genetic topic to another, barely getting to the central molecular aspects. Mayr tackles this difficult task boldly but I feel that the job is too big for a book intended for a broad audience. Indeed, for example, it has never been possible to successfully present nuclear physics to a general reader. Biology has now reached the stage that to really understand it requires extensive specialized furniture for the mind. There is no easy road. In discussing the various patterns of descent with change, Mayr has longagomadeuphismindwhere the evidence leads on many topics and he is not hesitant to make declamatory statements of his view. An example is his conclusion that the individual, not the gene, is themajor object of natural selection. I happen to agree with him on this, but there are certain contexts, as in clonal selection, where single-gene selection can be cogently argued. A long-term interest of Mayr’s has been the mode of origin of new species. In the 1940s, he advocated that species arise through gradual change in geographically partly separated populations that slowly, through selection, have acquired reproductive isolation from one another. Mayr has always made ‘‘isolation’’ an integral part of his definition of the species. Dissenters from this view look at the evolutionary process as a continuous accumulation of genetic differences over time in a gradual manner. This process is considered to proceed in a manner unrelated to any final human-conceived, idealized