High-fidelity record plays back vertebrates' change

Robert Carroll has been one of the more prolific students of Al Romer. Romer was the doyen of vertebrate palaeontology from the early 1930s until his death in 1973. His reputation was established most firmly through the production of several editions of his classic Vertebrate Paleontology, as well as the more broadly aimed class textbooks, The Vertebrate Body and The Vertebrate Story and, for my specialist interests, the brilliant but rather intense Osteology of the Reptiles.

Patterns and Processes of Vertebrate Evolution is a very different approach to the subject of vertebrate history and evolution compared with Vertebrate Paleontology. Rather than tackling the taxa involved in a classically (and logically) hierarchical fashion, Patterns and Processes represents an approach to the subject that is closer in spirit to the more synthetic works of G. G. Simpson, one of Romer's contemporaries. Emphasis is placed on the interdisciplinary nature of modern palaeo-biological research, encompassing genetics, developmental biology, population dynamics, ecology, functional morphology and, of course, the fossil record. It also resonates with the pattern-and-process approach that has been at the heart of the writings of Stephen Jay Gould, Niles Eldredge and others over the past 20 years.

What Carroll sets out to do is to study the process of evolution by reference to one coherent group, the vertebrates. They are clearly recognisable as a natural group (clade), have a fairly passable fossil record and have been the subject of considerable research in the laboratory and in the field. But the evolutionary research that is carried out on the group as a whole falls into two camps: those who study vertebrate fossils and those who study extant vertebrates.

This brings us to a central issue being addressed by this book. Darwin assumed, quite logically, that the process of evolution that could be applied through the principle of natural selection as it affects populations and species in the present could, with equal force, be applied to the history of life on earth: a direct biological parallel to Lyell's geological uniformitarianism of the 1830s. The fossil history of the vertebrates (and other groups) appears to argue against such uniformity of process and has pushed some palaeontologists towards the advocacy of what I term trans-Darwinian models of evolution. For example Eldredge and Gould's punctuated equilibrium model has been advocated to account for large-scale evolutionary change; this has been further refined by consideration of the importance of mass extinctions in re-setting the general "plot" of life's history on earth. Carroll conjures up a slightly contrived but apposite analogy between these added textures to Darwinian models of evolution and uniformitarianism's original explanatory power.

Throughout this book, Carroll grapples with the palaeontological (fossil) and neontological (living) perspectives on evolution. The first two chapters outline the origin and development of evolutionary thought from Darwin through the eyes of neontologists and palaeontologists - comparing their perspectives on the general modus of evolution. The bottom line for evolutionary biology is that the punctuational model (arguably, the palaeo-biologist's perspective) claims that natural selection is not significant as a mechanism for controlling the pattern and rate of evolution at the level of species.

From this starting point Carroll goes on to examine our understanding of evolution from the study of modern populations, which tends to contradict the punctuationist view, and from the fossil record (but with emphasis on the limits to our understanding of the fossil record) as a "high fidelity" record of the timing and circumstance of historical change. This makes interesting, and to some extent sobering reading, influenced as many of us have been by the exuberance of the Gouldian "view of life".

The remainder of the book examines the better-documented parts of the vertebrates' fossil record for patterns that might be illustrative of evolutionary processes. Examples from the relatively recent (and therefore richest) part of the fossil record tend to support Darwinian models of evolution through natural selection. Darwin is not completely vindicated, however, for rates of morphological change or stasis are far more variable, and factors not considered by Darwin, including continental drift and mass extinctions, are seen to account for patterns of evolution beyond a strictly Darwinian model. The application of phylogenetic systematics also is shown to offer a far more objective way of analysing the pattern of large-scale evolutionary change, and it garners support for a punctuational model of change over longer times: in essence short periods of intense cladogenesis followed by extended periods of relative stability. The genetic control of development, genetic factors limiting or constraining evolutionary change, the physical forces that can effect evolution locally and globally and selected evolutionary transitions among vertebrates, all fall under Carroll's remit.

This is an intellectual tour de force, albeit limited almost exclusively to vertebrates. The perspectives taken on Darwinian and punctuational change are in large part shown to be a consequence of the practitioners looking down opposite ends of the same pair of binoculars - magnifying the present or bringing the past into focus. Undoubtedly palaeo-biology has enriched evolutionary studies and its theoretical basis. Carroll's overview is welcome, well organised, affordable in paperback format and will prove a very useful backdrop to undergraduate courses on general evolution or vertebrate history.

David Norman is director, Sedgwick Museum, University of Cambridge.