Molecular Panbiogeography of the Tropics by Michael J. Heads . Berkeley: University of California Press (Species and Systematics), 2012. 576 pp. ISBN 9780520271968. $75; £52.
Article first published online: 17 DEC 2012
© 2012 The Linnean Society of London
Biological Journal of the Linnean Society
Volume 108, Issue 1, pages 236–239, January 2013
How to Cite
James, P. J. (2013), Molecular Panbiogeography of the Tropics by Michael J. Heads . Berkeley: University of California Press (Species and Systematics), 2012. 576 pp. ISBN 9780520271968. $75; £52. Biological Journal of the Linnean Society, 108: 236–239. doi: 10.1111/j.1095-8312.2012.02014.x
- Issue published online: 17 DEC 2012
- Article first published online: 17 DEC 2012
The seventeenth century saw a revolution in Natural Philosophy embodied in the epithet ‘New’. Kepler's Astronomia Nuova was published in 1609, Bacon's Novum Organum in 1620 and Galileo's controversial ‘… due Nuove scienza’ in 1638. The ‘New Learning’, based on the Experimental Method, changed the way in which Nature was perceived and investigated. Four centuries later, several other revolutions were in the making. Alfred Wegener's theory of Continental Drift was beginning to be taken seriously, also the works of Willi Hennig, Leon Croizat and Karl Popper. Nowadays the new buzz word is ‘Molecular’, transforming biology from descriptive to analytical. In 1965 Zuckerkandl and Pauling's paper, ‘Molecules as Documents of Evolutionary History’, was published. The molecules in question were proteins and nucleic acids, which, collectively, they called ‘Semantides’ or ‘information bearers’. In the half-century since this new source of knowledge was tapped, the molecular approach has become all pervasive, sweeping traditional disciplines before it. To be respectable everything must be prefixed by the epithet ‘Molecular’, including Michael Heads' monumental work on Molecular Panbiogeography, using molecular data as his analytical tool.
Heads justifies his title, stating that the method employed in his study of the distribution of living organisms is based on ‘… spatial variation in DNA, …’, and says this form of analysis has revealed a ‘molecular/geographic structure … which is one of the most exciting developments in molecular biology’ with ‘far-reaching implications for evolutionary studies in general’. However, he makes it clear that he is not going to deal with the techniques of DNA sequencing per se nor with the statistical intricacies of their cladistic analysis; Popper and Hennig are ‘taken as read’ and Heads explicitly adopts the ‘Method of Multiple Working Hypotheses’ so carrying Popper's ‘Conjectures and Refutations’ into the realm of biogeography and evolution. Early in the book (pp. 2–3), we are warned of the dangers of cramming all the ‘facts’ into a single, favourite hypothesis without rigorously testing their fit; a healthy scepticism. One of Heads' targets in this rigorous testing is the ‘Centre of Origin, Dispersal, Adaptation’ (CODA) model, which he sees as having ‘… sent genetics and biogeography on a century-long detour’ (p. 454) and he spends many of his 500 pages subjecting it to a Popperian broadside. Indeed, he suggests that by showing the CODA model to be mostly a ‘factoid’, the traditional problems of biogeography evaporate.
To underline this sceptical approach, Heads often adopts a catechismic style, querying assumptions and posing searching questions such as ‘Does natural selection lead to speciation?’ (p. 47). Answering this is another of the book's main themes, reflecting Charles Darwin's own concern with the significance of geographical distribution and its key role in providing evidence for the origin of species, prompted by Charles Lyell's ideas on the links between geology and biogeography. Heads follows Darwin around the world in eight chapters focusing on biogeographical case studies of well-documented plant and animal taxa, but unlike Darwin he has the advantage of those semantides, phylogenetic systematics and plate tectonics.
Thus Heads invites us aboard a vessel which could well be named ‘Herald of Vicariance’, for an east to west circumnavigation ‘Transect’ of the tropics in which we disembark to explore issues on those huge remnants of Wegener's Gondwana; South America, Africa and India. The Herald is powered by a PCR machine while the shades of Charles Darwin himself, Alfred Russel Wallace, and Philip Sclater take turns at the helm, steering along Leon Croizat's ‘tracks’ and ‘baselines’. In the bows we find a ghostly Edward Forbes taking soundings in an attempt to find any trace of those lost continents and sunken land bridges which the extensionists ‘invented’ to explain the infamous disjunct distributions while Darwin, watching from the bridge, mutters ‘poor Forbes’. The passengers (the readers), are better informed, as the book opens with two masterly chapters covering two of the three elements of panbiogeography; evolution in space and time. These chapters constitute both a summary of the present state of biogeography and a manifesto of the author's intentions: ‘The theme of this book is the distribution of plants and animals and how it developed’. The subject is approached using the methods of panbiogeography, which originated with the charismatic Leon Croizat in the late 50s and early 60s when the dominance of the ‘Modern Synthesis’ and the ‘New York’ school of ‘dispersalism’ were complete. He was a self-styled ‘non-serious scientist’ challenging these ideas in a confrontational manner in what could be described as a work of literature. Furthermore, to call Darwin, ‘… poor as a thinker’ only compounded Croizat's heresies, ensuring that his works were either ridiculed or ignored. Moreover, Croizat was often perceived as a proponent of Wegener at a time when the ‘Dispersalists’ rejected Continental Drift. In truth, although he did invoke tectonics as part of his explanation of geographical distribution, his ‘Horstian’ distribution hypothesis was far removed from the Wegenerian version of ‘Plate’ Tectonics. This ‘Horstian’ distribution, with its ‘microcontinents’ colliding and exchanging biota, was more akin to what we now call ‘Terrane Aggregation’. Significantly, we find no mention of Croizat's term in this book but Heads makes several sotto voce reassertions of his allegiance to Croizat. He is the master of the dramatic, throw-away line, but you must read the book carefully in order to appreciate this. Fifty years on from Croizat's work, his profound and independent thinking has flowered again in the soil of molecular panbiogeography, resulting in some luscious fruit including the present book.
Heads' book is not a history of biogeography. For that, and for some context, you must read Heads et al., Panbiogeography: Tracking the history of life (1999) or Chapter 1 of Cox and Moore's 8th edition of Biogeography: an ecological and evolutionary approach (2010) plus, of course, Janet Browne's enduring classic, ‘The Secular Ark’ and David Quammen's truly awesome, 700 page, The Song of the Dodo. Heads does, perforce, enter into a dialogue with history addressing that old debate between dispersal and vicariance. The very same, under different names, that Darwin, Wallace, Forbes, Hooker and others agonised over. Heads, however, has new tools at his disposal, DNA and plate tectonics, and he devotes much of the book to discussing how the two should be used in concert. He illustrates this by using a series of examples to ask how we can be certain that the nodes of a cladogram correspond with tectonic events. This question is of prime importance since the answer constitutes the acid test to discriminate between dispersal and vicariance. Another critical factor in this discrimination is the age and location of fossils.
Ever since Croizat's day, panbiogeography has been criticised for ignoring the fossil record (p. 69). Heads naturally refutes this accusation, emphasising the crucial role of fossil evidence whilst warning not to take it at ‘face value’. This leads to a discussion of how molecular and fossil clocks are to be calibrated and synchronised. This, Heads sees as absolutely critical since it is the timing of geographical barrier formation, vicariance/tectonic events, in relation to speciation which distinguishes between dispersal and vicariance and goes some way towards determining whether speciation is allopatric or sympatric.
The voyage of the ‘Herald’ is an extended exploration of these questions and relationships. It is, at times, a stormy passage and in its wake it leaves the disjecta membra of many refuted conjectures; basal clades as ancestral, the CODA model, sympatric speciation, cladogram branch lengths as divergence indicators, and many others including those Pleistocene Amazonia refugia; nothing is spared. Even the Indian/Eurasian collision, the closure of the Tethys Ocean and the nature of magma plume ‘hotspots’, which we non-geologists thought were all sorted out, turn out to be rather more complex and debatable than the ‘official version’ would have us believe! In particular the ‘hot spots’ of the Pacific plate are seen to be of great significance and Heads devotes three chapters to the islands of the Central Pacific as perfect test cases for the resolution of the ‘Dispersalist versus Vicariance debate’. His study of ‘weedy’ ecology of organisms endemic to ‘the ring of fire’ makes fascinating reading. By questioning intra-plate geology in the Pacific he introduces reasonable doubt as to the validity of dispersalist explanations for biogeographic anomalies.
I am reminded of Darwin's refusal of Wallace's request that he write a text on geographical distribution, commenting that he had nothing to add above and beyond the two chapters in the ‘Origin’ and that he did not suppose that ‘any man could master so comprehensive a subject as it now has become’. I know how he felt and that was in 1866. Darwin never visited Hawaii, but he realised that such a place would be a natural laboratory for biogeographers and these islands feature again in Heads' critique of the ‘Adaptationist Paradigm’ (pp. 388–9). Following in the footsteps of Gould, Lowentin and, more recently, those of Michael Lynch and S. M. Scheiner, Heads draws our attention to the extraordinary Hawaiian Biota with its seemingly gratuitous morphological diversity. He singles out the Euphorbiaceous tree, Chamaesyce forbesii which has stomata simultaneously on both adaxial and abaxial leaf surfaces as well as on each surface singly. All its neighbours, in an identical habitat, flourish with the ‘normal’, abaxial stomata (and what on earth is a C4 shrub doing in a rain forest anyway?).
J. T. Gulick raised the same questions 150 years earlier with his work on the Hawaiian land snails (Achatinella spp.), which had a different species on every island and in every valley. Since these habitats were identical climatologically, edaphically and biotically, why the diversity and what price natural selection and adaptation? Gulick's answer was ‘isolation’ but these issues remain controversial. As Heads points out (p. 437), Darwin, himself co-founder of the concept, became less and less of a panselectionist and isolationist and yet the ‘New Synthesis’ was wedded to these very concepts and to the CODA model. Heads emphasises just how many biogeographical paradoxes resolve themselves once these unions are dissolved.
Whilst Heads is at pains to test dispersalist ideas to breaking point, and it is very clear that he is no supporter of torpid pregnant primates clinging to storm-tossed logs, he nevertheless concedes a limited role for dispersal in what he calls Dispersal Vicariance Analysis, (DIVA). After all, biogeography is no stranger to what has been called ‘sweepstake’ dispersal. Had Wallace caught that direct boat from Singapore to Macasser would he ever have set foot on either Lombok, Bali or the Aru Islands? The real problem with ‘sweepstake’, long-distance dispersal is, and always has been, the paradox of stochastic events producing the congruent biogeographical distribution of so many unrelated taxa. In this respect it is perhaps a pity that P. L. Sclater, proposer of the ‘Lost Continent’ of ‘Lemuria’ and the instigator of a biological research station on Hawaii, makes only a fleeting appearance. Ironically, whilst himself an ‘extensionist’, Sclater was responsible for converting Wallace from being a ‘card-carrying extensionist’ to being a committed ‘dispersalist’, Sclater, however, receives but one passing mention (p. 6) in which he is bracketed with Croizat as a promoter of the vicariance model long before the term was coined. Sclater's six biogeographic regions were quickly and widely adopted after his 1857 paper. His land-based regions came to dominate biogeographical thinking and, in modified form, they still do. Those of Croizat, on the other hand were, controversially, based on ocean basins. In fact, Croizat did answer Sclater's call to establish ‘… primary ontological divisions of the earth's surface …’ and his chosen method was to evolve into none other than panbiogeography in which, to quote Heads, ‘… earth and its living layer have always evolved as one’ (p. 454). Heads sees land-based regions as part of the discredited CODA model and of the ‘Modern Synthesis’. If, he says, ‘… phylogeny is traced far enough’ their distribution is seen to be centred around Croizat's ocean basins. So, modern molecular panbiogeography has shifted Sclater's ‘ontological’ regions from land to sea.
Nothing, it seems is simple and even the word Panbiogeography is a word that has taken on a different meaning without anybody noticing. The result is that, in their Panbiogeography: Tracking the History of Life, Craw, Grehan and Heads (1999) felt obliged to point out that ‘Anyone attempting to provide a sketch of panbiogeography has to contend with a curious diversity of portraits on display in the literature’. They cite interpretations as ranging from ‘faddish’, merely old wine in new bottles, to ‘a distinct evolutionary metatheory’, and exciting new synthesis. Take your pick, it seems. In any case the word is used both as a discipline and a method, both product and process. Biogeography seems to have acquired more than its fair share of these metanyms and the reason is of course that all the relevant burning questions were framed in a pre-Mendel-Wegener-Watson and Crick and even Darwin world. The words are still with us, their original meanings are not. We owe an immense debt of gratitude to Heads for navigating the ‘Herald’ through these treacherous semantic reefs.
For Heads, panbiogeography is a broad church encompassing both method and metatheory. His injunction not to accept received wisdom without subjecting it to a varifocal scrutiny, echoes Croizat's confession that he was ‘… absolutely impervious to authority …’ The combination of Cladistics and Plate Tectonics has, at last, provided the tool kit for differentiating between dispersal and vicariance, a task which Joseph Hooker saw as impossible. His dilemma was that both models were ‘Panchrestons’, since either could be made to explain everything; neither, actually, explained anything! How he would have enjoyed this cruise on the ‘Herald’!
It is important to re-emphasise that Heads is not asking us to accept vicariance over dispersalism, but to use the new tools with an open mind. He sternly warns us that, ‘… whether or not an explanation is pervasive in a scientific community is – or should be – irrelevant to whether or not it is scientifically plausible’ (p. 434).
Heads' last Chapter is in many ways the most thought provoking and challenging. The voyage is over, we reflect on what we have discovered and draw up the new charts. As Darwin famously wrote at the beginning of his last chapter, ‘As this whole volume is one long argument, it may be convenient to the reader to have the leading facts and inferences briefly recapitulated’. Heads does precisely this by using Croizat's third panbiogeographical element, ‘Form’. Received wisdom holds that the results of ‘Form Making’, both phylogenetically and ontogenetically, are organisms adapted to their environment. Heads disagrees and we are invited to draw a parallel between the CODA model as a biogeographical explanation and the random mutation-natural selection model of evolution. Both are stochastic processes and fatally flawed according to Heads. There follows a summary and a discussion of gene regulatory networks and of a present understanding of those ever-more complex genotype-phenotype relationships. All the strands of this mighty text are thus drawn together and the result is that ‘Mt. Improbable’ of that arch-panselectionist, Richard Dawkins, becomes ‘Mt. Probable’ because of ‘… intrinsic bias in mutation as a cause of non-randomness’. Speciation is inevitable, not improbable and function does not determine form, QED, and back to the Transcendentalists but with molecular underpinnings.
Noah's Ark has long been relegated to the nursery and Darwin disposed of multiple special creations by asking why, if the Almighty were omnipotent, were bats the only mammals He saw fit to have placed on oceanic islands? More secular approaches were needed in order to understand ‘Darwin's Chessboard’. Dispersalism was perhaps the only viable proposition, given the pre-Wegenerian stable geology. Humboldt's ‘Ignorabimus’ nihilism was the road to nowhere and we have now passed through Willis' ‘Age and Area’ (1922) and MacArthur and Wilson's formidably mathematical ‘Island Biogeography’ (1967). Whether or not molecular Panbiogeography will prove to be the definitive approach is of course impossible to say, but I cannot do better than to sum up Heads' book with the words Darwin used to praise Wallace's work on geographical distribution in 1876. This is a ‘… grand and memorable work, which will last for years as the foundation of all future treatises’.