Panbiogeography: Tracking the History of Life

Books: Biogeography

Panbiogeography : Tracking the History of Life (Oxford Biogeography Series No 11) by John R. Grehan, Michael J. Heads, Robin C. Craw

Long after the Middle Ages a certain Equivalence of knighthood and a Doctor’s degree was generally Acknowledged…For the history of Civilization the perennial dream of a Sublime life has the value of a very Important reality.

Huizinga, J. – The Waning of the Middle Ages, 1924 – quoted on the front page of Space, Time, Form: The Biological Synthesis, by Leon Croizat, 1962.

Like righteous knights, the authors of Panbiogeography are on a quest to save the discipline of panbiogeography: to bring it out from relative obscurity into the modern world of biogeography. Christened by Leon Croizat in the 1958, panbiogeography uses the geographic distribution of all biota, from plants to insects to vertebrates, to create hypotheses for historical biogeographic patterns. By a system of “tracks”, “nodes”, “main massings”, and “baselines”, Croizat and his followers mapped disjunct distributions and used correlations to define historical ranges. Largely due to his abject rejection of dispersal as a mechanism for vicariant patterns of biota, Croizat’s theories were discounted as extreme (Cox, 1999). The main tool of panbiogeographers is the “track”, a line drawn on a map that links localities of a taxa. The “track” represents a hypothesis of previous geographic connection. Although Panbiogeography is full of many examples of tracks drawn on maps, some issues are left unclear. For instance, what taxonomic level should tracks connect? The taxa selected in this book appear arbitrary; they use both taxa with similar generic and familiar relations in order to draw tracks. Furthermore, how discrete is the geographic region described by the points at the end of a track? The scale of tracks vary from continental to discrete local scales. On the other hand, the authors illustrate that the track may function well as a heuristic device, simply drawing a line on a map may represent the possible relationship between biota between two geographic areas. Grehan and Craw, as proponents of this concept appear to believe that these tracks and especially those tracks of many taxa over-layed, called “generalized tracts”, represent vicariant events, rather than simply dispersal of individuals and subsequent speciation. Although this issue was contentious in the 1970’s (Dipersalists vs. Vicariants), this book fails to bridge the gap between these two views. Instead, while claiming to recognize the importance of both forces, it simply reiterates the vicariant viewpoint.

Grehan and Craw attempt to revitalize the discipline of panbiogeography by incorporating cladistical analysis. The book is full of case studies of suggestive corroboration of phylogenetic systematics with previous hypotheses postulated by Croizat and earlier vicariant biogeographers. Although these examples span various phyla, they generally emphasize morphological systematics. In order to support the claims of vicariant biogeographic patterns, the book would benefit from additional examples of recent molecular systematic findings.

Throughout the book, many examples of faunal and floral disjunctions are correlated with geological (i.e. tectonic) patterns. It is postulated that geological events have caused vicariant distributions. Although compelling in the sense that this represents a mechanism for the patterns of disjunct biota, inferring that this is the only mechanism is nonsensical. Dispersal may function as an equally important factor in distribution. Even so, the emphasis on locality, on where an animal is found, is one of the main strengths of this book: on some level, systematics should take geography in to account in order to determine phylogeny.

Panbiogeography is written purportedly to assist in the understanding of this discipline. Even so, the language is opaque and appears purposefully inaccessible. Furthermore, the authors could improve understanding by clearly defining and illustrating the concepts and methodology in the beginning of the book; it is in the 5th chapter that the authors approach the methodology of this approach.

True to the very nature of panbiogeography, Panbiogeography gives examples of disjunct distributions for varied species from cotton, to starlings, to weevils. While this a data appears to be well researched, the conclusions drawn are not evident. Conclusions are made from correlations. While hypotheses are clearly stated, they are not tested, only corroborated. Comparative evidence alone is insufficient to evaluate a hypothesis. While this style often broaches upon a rant, these case studies are not uninteresting. Demonstrating that dissimilar fauna and flora have similar vicariant distributions, is inherently interesting. Unfortunately, it is difficult to disentangle any bias from selection of fauna and flora.

Finally, while Grehan and Craw imply that they encourage others to use panbiogeography, they have not made this easy. The greatest disappointment in this book is that the authors do not explicitly present their methodology. The maps, adorned with crossing lines and circled regions, appear arbitrary constructions instead of the well-honed result of careful and discerning research. One is left with the impression that one needs faith in order to use this method – much like the mythic knights of the middle ages on a religious quest to obtain the holy grail.

Cox,B. 1998. From generalized tracks to ocean basins – how useful is Panbiogeography? J. Biogeog., 25:813-828.