A scientific truth that is set in stone

While in manuscript this book was titled Evolution Explained , a title preferred by its author, Ernst Mayr, who suggests that it is "very questionable whether the term 'evolutionary theory' should be used any longer. That evolution has occurred and takes place all the time is a fact so overwhelmingly established that it has become irrational to call it a theory." Evolution, he says, is "the name of a process in nature the occurrence of which can be documented by mountains of evidence that nobody has been able to refute... discovered over the last 140 years" (in other words, since Charles Darwin's seminal work on the origin of species).

Mayr notes that although some details of evolution are vigorously debated by scientists - for example, punctuated vs gradual timing in evolving forms; the relative importance of sources of inherited variation, for example DNA base-pair mutation, duplications, chromosomal rearrangements; whether group selection occurs - the conclusion must be that evolution is a fact. One might as well believe the flat-earth theory as deny the truth of evolution, Mayr says. Evolution is one of the concepts that are undeniable to reasonable people who are not religious believers, like the sphericity of our planet and the heliocentricity of the solar system.

What Evolution Is is the work of a master who, as the author of hundreds of primary scientific articles and two dozen books, participated mightily in the establishment of modern views of evolutionary change. In language accessible to anyone with even a passing interest in evolution, this book summarises the work of literally hundreds of scientists over more than a century. "Evolution is not the most important subject in the world because I study it," Mayr's Harvard colleague and friend George Gaylord Simpson used to quip, "I study evolution because it is the most important subject."

This book echoes Simpson's sentiment by reviewing a fact made clear in the many writings of Mayr: no single set of measurements, no singular observation, no one field of science can "prove the fact of biological evolution".

Intrinsically, evolution is a complex idea with many components. Each can be separately demonstrated; the central contribution of Darwin was to string the components together to make a comprehensible and comprehensive view of the history and current behaviour of living matter. The main parts of the Darwinian argument are: the irrefutable and measurable tendency of all populations of organisms to grow and reproduce at enormous rates, and the unsustainability of this population growth that inevitably leads to relative survival and reproduction of descendants and therefore to natural selection. As Mayr puts it, "Natural selection is really a process of elimination." Furthermore, in any population, whether bacteria, bananas, bodonid protists, birds or Brachylophus , the presence of demonstrable inherited variation on which selection acts can be shown. Over hundreds to millions of generations, evolutionary sequences can be documented by direct observation of fossils in the sedimentary rock record.

Mayr dedicates the book to the naturalists from whom we have learned so much, "from Aristotle to the present". His erudition and comprehensiveness are as impressive as they are unusual in any modern scientific text, especially one so succinct as this. Mayr claims his book is meant to engage three kinds of readers: members of the literate public who want to know about the evolution of life and how it has occurred but can find only highly technical or polemical accounts; those who accept the concept of evolution but doubt that the Darwinian modes of change can really explain it; and those creationists who "want to know about the current paradigm of evolutionary science if for no other reason than to be able to better argue against it".

To this end, he includes handy "boxes" in which the kernels of complex arguments are presented. Some two dozen of the most commonly asked questions about evolution appear with concise answers, in addition to an 11-page glossary with terms such as Baldwin effect, flora and panmictic. There are also ten pages of references but only to comprehensive and comprehensible books. To me, the book is pitched more to the general reader than to first-year college students.

Mayr expects to convert no one, of course, but hopes to fit the bill of a concise, well-organised, reader-friendly account of evolution, which he thinks is not currently available. So well does he do this that his new book is probably the best reference for anyone wishing to know the status of ideas in this wide-ranging scientific field. The concept that all life on earth shares common descent from earlier ancestors is explained and documented with evidence from many fields of science. The breadth and integrated nature of the narrative is reminiscent of comments I have heard in Mayr's own lectures. As he points out, all scientists, if they are to write research articles in their chosen field, must of necessity be specialists - yet no one who identifies himself as an evolutionist can possibly be only a specialist. This is because evolutionary thought and analysis encompass so many superficially unrelated fields, which require critical evaluation by the evolutionist.

It is true that nearly all the examples of speciation, variation, geographical isolation, development and embryology are from zoological sources. They tend to be drawn from the vast literature on terrestrial vertebrates, with which Mayr is exceedingly familiar. Although the amount of space in the book dedicated to, for example, biochemistry, hox genes and their role in development, molecular biology, prokaryote evolution, protist diversity, and polyploidy in plants is small, these topics are covered. I have inevitable quibbles with several of the definitions (such as biota, entropy, recombination and symbiosis) and I note omissions from the glossary of terms encountered in the text (such as fitness, prokaryote), and some missing references. This only reflects the difference between the best academic publishing and the more hurried nature of publications for commercial purposes. Yet Mayr's useful and balanced narrative of what Jared Diamond in his preface calls "the most profound and powerful idea to have been conceived in the last two centuries", written by one of its most competent and prolific practitioners, does just what it set out to do: explain what evolution is.

Lynn Margulis is professor in the department of geosciences, University of Massachusetts, Amherst, United States.