book reviews A popular evolution An authoritative introduction to the history of life from a respected theorist. What Evolution Is by Ernst Mayr Basic Books: 2001. 336 pp. $26 (US); Weidenfeld & Nicolson: 2002. £14.99 Mark Ridley Ernst Mayr is a living legend. He was born in Germany in 1904, and began research in ornithology with a field trip to pre-pacified PRESS UNIV. K. SNIBBE/HARVARD New Guinea in 1928. After moving to the United States he was soon making classic contributions to the theory of evolution. Mayr is best known for his ideas on what bio- logical species are and on how new species originate. On both topics, his views are radically different from Darwin’s, but Mayr’s views prevail (if not universally) today. They are a turning point in the history of biology. Hands of time: Ernst Mayr has spent a lifetime explaining the mechanisms of evolutionary change. Species are populations of interbreeding organisms, argues Mayr — they are real But what is more astonishing is how up-to- biological species concept. For example: “G. genetic units, not artificial sets defined by date he is. The history of life is a fast-moving G. Simpson’s so-called evolutionary species phenetic attributes, or aggregate similarity. research topic at present, mainly because concept contains undefinable criteria and This biological species concept linked sys- of molecular research. Mayr is known to is useless in praxis.” Again, for experts this tematics with population genetics and was have slapped down a molecular geneticist or is a sharp judgement, to be argued with. But a key piece in the ‘modern synthesis’ of two in his time, and I wondered whether he beginners will have more difficulty because evolutionary biology. Mayr’s theory of would appreciate the new work. He does. Mayr forgets to tell us what Simpson’s species allopatric speciation argues that new species He explains, straightforwardly and without concept actually is. arise when an ancestral species is subdivided ifs and buts, that molecular phylogenetics Mayr’s intended readers may sometimes into a number of geographically separate is where it’s at, and summarizes the main have problems when he debates without populations. These populations then evolve findings. He seems to be thinking about the explaining that there is a debate, or debates independently and may over time accumu- latest research like any graduate student. without explaining his opponent’s position. late so many genetic differences that they He also argues as if he were in an egali- But his views all seem to me to be arguable can no longer interbreed if they do meet. tarian graduate seminar. The book bristles within the range of modern biological Mayr’s ideas were gathered together in with provocative opinions, some of which thought, and are entirely in order in a popu- his 1942 book Systematics and the Origin only professional readers will understand. lar introductory book. The only exception of Species, which is among the maybe five For instance, I detect a good-humoured is what he has to say on neutral evolution. (for fans) or ten (for scoffers) most famous argument with Mayr’s Harvard colleague I’d rate Motoo Kimura’s neutral theory as books ever written on evolution. He has Stephen Jay Gould in the reiterated stress on among the biggest, if not the biggest, contri- also written standard works on ornithology, gradualism. One paragraph bangs the point bution to evolutionary biology in the past texts on systematics, a huge book on the home, repeating the word ‘gradual’ seven 35 years. Mayr dismisses it in a short para- history of biology, and has contributed ideas times in five sentences. graph, as if it were an elementary conceptual on the philosophy of science that are actively Elsewhere, Mayr discusses adaptation, mistake. He does not make his objection discussed in the literature. and asserts that “it is irrelevant for the classi- completely clear. However, he objects to the Now he was written a popular book about fication of a trait as an adaptation whether it definition of evolution as changes in gene evolution, What Evolution Is. He says he had had the adaptive quality from the beginning, frequency. Neutrality is a property of genes, three kinds of reader in mind: general read- like the external skeleton of an arthropod, or and neutral drift produces changes in gene ers who simply want to know more about acquired it by a change of function, like the frequency. For Mayr, neutral evolution is evolution; people who accept evolution but swimming paddle of a dolphin”. Experts will impossible because the sorts of change pro- are sceptical about the darwinian explana- suspect that Mayr is dealing with Gould and duced by neutral drift are not real evolution. tion for it; and creationists — not in the hope Elisabeth Vrba’s concept of exaptation here, The book has a few dense sentences, using of “converting” them, but for them to find but Mayr keeps quiet about that. A beginner concepts that are either not explained or not out more about what they are up against. would not know that Mayr is taking sides explained until later. And it is occasionally Accordingly, Mayr describes the history rather than introducing an uncontroversial repetitive, with topics being explained more of life, including a full chapter on human point. But Mayr does support Gould on than once in much the same words in more origins, and explains how the history is another subject: the Cambrian explosion. than one place. A duplicative transposition reconstructed. He also explains the mecha- He suggests that the phenetic differences across 70 pages of a section on gene duplica- nisms of evolutionary change — such topics between organisms were greater then and tion is the most glaring example. But readers as natural selection and speciation. Mayr that evolution may have been embryologi- will indulge these small problems. Almost shows off his methuselan skills in the effort- cally more anarchic at that time. all popular books by scientists have some less way he sketches the history of the science, Mayr briskly and characteristically dis- abstruse material. The only alternative is and its conceptual context on the large scale. poses of the alternatives to his preferred to read the work of a science journalist, but NATURE | VOL 417 | 16 MAY 2002 | www.nature.com © 2002 Macmillan Magazines Ltd 223 book reviews then you miss out on a mind of Mayr’s calibre and influence. Mayr has now published a major ornithological monograph and a popular book on evolution in the past year. I sense he is only just getting up to speed. My next SARKASSIAN/CSIRO J. request is for an autobiography. ■ Mark Ridley is in the Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK. Tuning in to radio galaxies The Physics of Extragalactic Radio Sources by David S. De Young University of Chicago Press: 2002. 450 pp. $45 Radio star: the Parkes Radio Telescope near Alectown in Australia detects extragalactic signals. K. I. Kellermann infrared through the optical and ultraviolet that leads to a better understanding of the After the Second World War, scientists, espe- to the X-ray parts of the electromagnetic physics involved. In other cases, the reader is cially in England and Australia, turned their spectrum. Huge radio-telescope arrays show referred to the appropriate literature. In the wartime radar equipment towards the skies remarkably fine detail in the synchrotron later chapters De Young applies the theory to to follow up on the pre-war discovery by Karl clouds, revealing thin jets that carry the rela- both extended and compact radio sources, Jansky and Grote Reber of radio emission tivistic electron gas from active galactic nuclei relativistic jets and AGN, including a discus- from the Milky Way. Although hampered at (AGN) out to distant radio lobes, as much as sion of unified radio-source models which first by poor angular resolution and limited a million light years or more away from the attempt to explain the range of observed sensitivity, they were able to detect powerful central engine. Global radio-interferometer properties as the result of the orientation of sources of non-thermal radio emission from observations using angular resolutions up the relativistic jet or an obscuring torus several distant galaxies. During the 1950s to 100 times better than that of the Hubble surrounding the central engine. He discusses and 1960s, powerful interferometric and Space Telescope show directly the ejection the nature of the host galaxy or associated aperture-synthesis techniques were devel- of relativistic plasma from the central engine quasar and includes a review of their optical oped. When combined with theoretical with apparent velocities in excess of the and X-ray properties. advances, primarily in the Soviet Union, they speed of light. This is an illusion caused by There are few original drawings. Most of showed that the observed radio emission was the finite signal-propagation time from a the illustrations are taken from the literature, due to synchrotron radiation emitted from relativistically moving source of radiation. but are sometimes out of date in this rapidly extensive clouds of ultrarelativistic electrons However, the new observations may have moving field and often have inadequate cap- moving in weak magnetic fields far removed raised more questions than they answer. tions or explanation. There are a few errors, from the parent galaxy. The energy required The source of energy is still unclear, although some of which seem to have been introduced in the form of relativistic particles and it is widely assumed to be associated with by the editors. Nevertheless, the book will be magnetic fields appeared to be enormous.