Plenary10: Biogeography on the eve of the twenty-first century: Towards an epistemology of biogeography¹

Department of Ornithology, American Museum of Natural History, Central Park West at 79th Street, New York, NY, 10024-5192, USA, fax 212 769 5759

Vuilleumier, F. 1999. Biogeography on the eve of the twenty-first century: Towards an epistemology of biogeography. In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban. Ostrich 70 (1): 89–103.

‘No one knows what . . . changes in our image of science the future may bring. The best one can do under the circumstances is to try to present an outline of the kind of science that prevails in our time, at the end of the twentieth century’ (Mayr 1997: 26).

To reconstruct the spatio-temporal development of life on earth biogeographers have traditionally used several units of analysis and various kinds of data. The species has long been a fundamental unit, and a map of a species’ distribution a basic datum. Some recent workers reject the biological species concept and advocate a phylogenetic one instead. Just as the definition of species may be changing, new technology and new mapping techniques are enlarging the concept of map. Through area cladograms, cladistics has modified the way taxonomic and area relationships are considered. Biogeography at the end of the 20th century thus is a conglomerate of old and new concepts, and of classic and revised interpretations. However, a literature review shows that, whether they use older trends or newer approaches, and irrespective of the school of thought they adhere to, biogeographers try to answer three hierarchically interrelated questions: (1) What are the patterns of geographical distribution? (2) What processes explain these patterns? and (3) How are these processes controlled? Pattern recognition is indispensable for any further work, process identification is concerned with proximate explanation, and process control seeks ultimate causation and may lead to theoretical considerations.

In this essay I explore the epistemology of biogeography, a topic that remains poorly understood despite numerous publications (Ball 1975, 1980; Vuilleumier 1975, 1978, 1985; Keast 1977, 1991; Nelson 1978; Mayr 1982a, 1997; Browne 1983; Larson 1986; Blondel 1987; Llorente & Espinosa 1991; Andersson 1996; see also the paper on the epistemology of systematic biology by Frost & Kluge 1994). The present paper, therefore, has three goals: (1) to describe the nature of biogeographic knowledge, (2) to investigate how biogeographic knowledge is acquired, and (3) to suggest a new theory of biogeography. To carry out these tasks, I first review aspects of the history and philosophy of biogeography, and then analyse critically some current practices in this discipline.

Biogeography, the science of the distribution of life on earth in time and in space, is about 250 years old, as the thinking and concepts of some of its brilliant early practitioners, like Alexander von Humboldt (1769–1859; Humboldt 1807) and Augustin-Pyramus de Candolle (1778–1841; Pilet 1971; Candolle 1820), can be traced back to Linnaeus (1707–1778). According to Candolle (1820) ‘the first ideas about botanical geography really originated with Linnaeus’ because he was the first ‘who carefully distinguished habitations, that is, the countries in which plants grow, and stations, that is, the special nature of the localities in which they usually occur.’ As Candolle (1820: 383) explained: ‘The study of stations is, so to speak, topography, and that of habitations, botanical geography.’ Habitation, a term that is no longer employed, thus referred to a regional scale of perception, and station to a local scale. The term station, denoting a sampling area at a local scale, is used today in landscape ecology (Long 1974) and in biogeography (Blondel & Choisy 1983; Vuilleumier 1993).

If Linnaeus was the grandfather of plant geography, then Buffon (George-Louis Leclerc, Comte de Buffon, 1707–1788; Bertin et al. 1952) was the grandfather of zoogeography (Nelson 1978; Mayr 1982a; Browne 1983; Larson 1986). In his Histoire naturelle, générale et particulière Buffon (1761) found that the taxonomic differences between Old World and New World quadrupeds were at odds with the then prevailing view, namely that different areas of the globe under similar conditions should host the same taxa.

At the same time as Buffon, Zimmermann (1777, 1778–1783) raised arguments against the views of both Linnaeus and Buffon, and suggested that historical causes could help explain certain distribution patterns (Bodenheimer 1955; Mayr 1982a; Browne 1983; Larson 1986). Other important early zoogeographers include Illiger (1812–1813), who reviewed avian distributions, Latreille (1817), who described the biogeography of insects and spiders, Swainson (1835), who arranged taxa and faunas into regions, and Schmarda (1853), author of a general synthesis nearly a quarter of a century before Wallace (1876).

Twentieth century biogeography has been influenced mostly by the two men who, in 1858, published simultaneously the first modern views on the mechanism of evolutionary change, Charles Darwin (1809–1882; de Beer 1964) and Alfred Russel Wallace (1823–1913; W. George 1964). Even though Darwin’s biogeographical views have been placed at the base of all subsequent work (Darlington 1959; Ghiselin 1969; Mayr 1982a: 448), I believe that it is Wallace whose thought has most dominated biogeographic research, especially since the publication of The geographical distribution of animals (Wallace 1876) and Island Life (Wallace 1880).

Wallace did nothing less than establish the foundation of a science of biogeography, a foundation that has lasted well into the 20th century. Not only did he describe all the major patterns of distribution known at the time, including continental and insular patterns, disjunction patterns and dispersal pathways, but also he offered evolutionary explanations for them. The breadth and depth of Wallace’s thinking cannot be overemphasised. What I consider especially significant is that Wallace’s understanding of biogeography was based on extensive fieldwork (Quammen 1996).

Biogeography developed through several phases in the years following the appearance of Wallace’s biogeographical magna opera (Ball 1975; Vuilleumier 1975; Keast 1977; Blondel 1995). Because of methodological innovations like phylogenetic systematics and technological developments such as DNA sequencing and remote sensing, biogeography today seems to have little in common with what it was in Wallace’s time. Appearances can be deceiving, however, because modern terminology is so different from that of one hundred or more years ago. Elsewhere (Vuilleumier 1990) I listed fifteen ‘conceptual problems and approaches that interested biogeographers in the late 1880s’ and that still fascinate us today, for example vicariance, mobility of continents and oceans, and islands as model systems. Continuity of thought and methodological developments suggested to some authors that late twentieth century biogeography was a mature science that showed an increasing degree of unification (Simberloff 1983; Blondel 1987). I and others pointed out, however, that diversification in methods actually masked an epistemological difficulty.

After a penetrating analysis Browne (1983: 225) had concluded that biogeography ‘is one of those perplexing subjects where many different areas of thought meet and intermingle, where a multitude of techniques, attitudes, and styles of work are employed, where it is possible to be a biogeographer and yet remain a botanist or zoologist.’ At about the same time, I had stated that ‘Zoogeography is a heterogeneous science composed of several contrasting points of view’ (Vuilleumier 1985: 669).

Biogeography in the late 1990s still appears to be heterogeneous and composed of separate, juxtaposed, or partially overlapping disciplines, each with its own set of epistemological rules. The literature review I carried out for this essay revealed an astonishing diversity of topics subsumed under biogeography. Could this conceptual, methodological, and theoretical diversity be so great as to negate the existence of an epistemology of biogeography, hence to strip biogeography of its status as a science? Interestingly, a similar problem may exist in geography. Thus in his review of epistemology in the sciences, Piaget (1967) did not even mention geography, as pointed out by Sautter (1975: 231), who exclaimed: ‘But – if I may be permitted to paraphrase Galileo – geography does exist’. According to Raffestin & Turco (1984: 15) ‘for Piaget there is no epistemology of geography but an epistemology of each of the sciences that include geographical knowledge’. That a similar situation might be true in biogeography is suggested by two editorials in the Journal of Biogeography (Andersson 1996; Stott 1998).

Bothered by the conflict between the ‘extremely reductionist attitude’ of vicariance biogeographers, who imply that there is a natural relationship among areas or their biotas, and the view of other biogeographers, who find ‘taxon biogeography [to be] meaningful,’ Andersson (1996) stated that ‘Clearly historical biogeography has a problem with ontology.’ Specifically, Andersson (1996) wondered ‘what constitutes the explanatory basis, or what is [sic] relevant data, i.e. what facts belong in the basis of knowledge’ of biogeography. Such an uncertainty, if true, is indeed basis for worry. Andersson’s (1996) essay ends with a suggestion for improving the situation, a five-point ‘protocol for inquiries in historical biogeography’ that goes from empirical description to causal explanation ‘in terms of historical development and evolutionary theory.’

Stott (1998) suggested that if biogeography and ecology ‘are to survive into the next century as credible entities, both must experience a significant Kuhnian paradigm shift’. Stott argued that our ‘research . . . is consistently opening up to us a non-equilibrium world, in which change takes place all the time, . . . and a world in which autecology rules’. Stott believes that we need to speak a new ‘metalanguage’ (in other words a scientific code that allows one to describe scientific domains) which should be ‘more accepting of change’ and comprise ‘a new range of . . . terms like ‘ "adaptation", "migration", "movement", "opportunism", "flexibility", and "resilience".’ I do not agree with Stott that we need a new metalanguage because terms like adaptation, migration, movement, opportunism, flexibility, and resilience are already part of a biogeographer’s vocabulary and all imply the notion of change. As a biogeographer who spends several months each year in the field I consider change to be the rule in biogeography.

So it appears that Andersson queries the epistemological foundation of biogeography, and that Stott hopes for a Kuhnian revolution. In the rest of this essay I examine Andersson’s and Stott’s claims. I begin with Stott’s view that biogeography needs a Kuhnian paradigm shift, then address Andersson’s epistemological difficulty.

In Kuhn’s (1970) interpretation, progress in science is achieved during phases he called revolutions, when a paradigm shift displaces, and eventually replaces, an older, established paradigm with a new one. The successful paradigm will remain ensconced in the fabric of that science, which is called ‘normal,’ until another paradigm shift takes place. This notion of periods of normal science punctuated by revolutions during which paradigm shifts occur, derived by Kuhn mostly from an analysis of physics, was applied to other disciplines.

Whereas in 1978 and in 1995 I believed that Kuhnian-type revolutions had occurred in biogeography, I no longer hold this view. If biogeography had developed, Kuhnian style, there should have been periods of normal science interrupted by paradigm shifts leading to other periods of normal science, but not overlapping paradigms. I therefore agree with Mayr (1994, 1997) that a Kuhnian-type development does not take place in biology, and especially in evolutionary biology (of which biogeography is a component), in part because in biology ‘several paradigms may coexist simultaneously for long periods of time’ and in part because ‘active branches of biology seem to experience no periods of ''normal science'' ’ (Mayr 1994; see also Mayr 1997: 91–101). I do not believe that biogeography has developed like physics, the science that Kuhn (1970) analysed, nor do I believe that we can apply to an historical science a model of the evolution of physics. In geography, whereas authors like Harvey (1969) espoused Kuhn’s views, others like Claval (1984: 31) wrote that there cannot be a Kuhnian development when all is divergence and change, or, to quote his metaphor (my translation), ‘when there is a major earthquake every year, and then again one each time the seasons change.’ Incidentally, many authors use ‘paradigm’ in a different sense than that of Kuhn, namely to denote any hypothesis or theory, or even simply an idea, irrespective of normal science and/or paradigm shifts, for example Bermingham et al. (1992: 6624) and Avise (1996: 17–18).

Given my present view that biogeography has not had Kuhnian paradigm shifts, I disagree with Stott (1998) that this science needs such a shift. I do, however, agree with him that there is a problem in biogeography. In order to identify this problem I return to Andersson’s (1996) worry about the ontological basis of biogeography.

The answer is, of course, ‘It depends.’ What it depends on is whether biogeographers are interested in plants or in animals. If in animals, it depends on whether these creatures are marine or terrestrial. It depends also on the spatial scale studied, whether local, regional, or continental. Further, it depends on the temporal frame a biogeographer considers, whether short-term, Pleistocene, or encompassing distant epochs. Finally it depends on whether a biogeographer is interested in ecological causation (ecological biogeography) or in phylogenetic history (historical biogeography). Regardless of the actual focus, however, what biogeographers work on are species.

According to Mayr (1942, 1963, 1993) the species is the linchpin of the evolutionary edifice. Species are biological units composed of individual organisms which interact in reproductive populations that occupy a spatial range and possess a temporal frame. As species are influenced by selection pressures from their environment, from other species with which they coexist, from genetic processes within, or from all three, they evolve in time and space. New species originate mostly as a consequence of the action of barriers, extrinsic accidents of geography and of ecology (allopatric speciation, Mayr 1942, 1963; Futuyma & Mayer 1980; Vuilleumier 1980; Chesser & Zink 1994). Because of this sine qua non eco-geographical factor, the species is also the quintessential biogeographic unit.

Biogeographers work on species, whether they deal with one species at a time or whether they consider an assemblage of them (community, flora, fauna; or local biotas, see Penev 1997). Before a biogeographer can interpret distribution patterns, of whatever kind, he or she must first catalogue the species that embody those patterns (Vuilleumier & Simberloff 1980: 235–239; Blondel et al. 1984: 144–147). There can be no pattern without species. Characteristically, MacArthur (1972) had subtitled his book ‘patterns in the distribution of species.’ MacArthur (1972) also wrote that ‘to do the science of geographical ecology is to search for patterns of plant and animal life that can be put on a map.’ Biogeographers therefore study populations that they group into species, plot these species’ distributions on maps, and look for patterns.

Haffer (1992b) distinguished three species concepts: (1) morphological (species are defined on the basis of their differences); (2) biological (species are defined on the basis of the membership of their component populations in a reproductive community); and (3) historical (species are defined on the basis of the membership of their components in a phylogenetic lineage).

Of the three concepts, the one most commonly employed in avian biogeography during the second half of the twentieth century has been the biological one, which was initiated by 19th century authors like Seebohm (1882), used by early 20th century systematists like Stresemann (1919), and developed by Mayr (1942, 1963, 1993). Surveys of speciation (Keast 1961 and Ford 1974 in Australia; Hall & Moreau 1970 and Snow 1978 in Africa; Haffer 1974 and Vuilleumier 1991 in South America; Mengel 1970 and Hubbard 1973 in North America; Haffer 1977 and Vuilleumier 1977 in Eurasia) are based on the biological species concept. Founded on field and museum studies of biological species in terms of their systematic, ecological, and geographical relationships woven into faunal-level syntheses, these analyses suggest how species evolved and faunas developed in time and space at continental scales.

After phylogenetic systematics was initiated by Hennig (1950, 1966), then developed by Brundin (1966), and advocated by biogeographers like Nelson & Platnick (1981) and ornithologists like Cracraft (1981), a phylogenetic species concept entered the scene in the 1980s (Rosen 1979; Cracraft 1983; McKitrick & Zink 1988). This concept is similar to one suggested earlier by Simpson (1961; ‘evolutionary species’) but, as noticed by Avise & Ball (1990), this heritage has apparently not been acknowledged by proponents of the phylogenetic concept.

The phylogenetic species is defined as ‘the smallest diagnosable cluster of individual organisms within which there is a parental pattern of ancestry and descent’ (Cracraft 1983: 170). Applying this definition to its limit, one can reach reductio ad absurdum because biochemical technology permits one to identify minute amounts of genomic differentiation, and so could lead one to diagnose and recognise an almost infinitesimal number of ‘species’ (see Avise & Ball 1990). Modern biochemical technology might even permit the diagnosis of individual organisms as phylogenetic species. This reductionism seems to be a throwback to typological thinking or Plato’s essentialism. [Note that some biogeographers use ‘typological’ in a very different sense, namely as one of two approaches to the study of biotas, the other being ‘individualistic’ (Penev 1997).]

Many systematists and biogeographers have mentioned difficulties with the biological species concept, myself included (Vuilleumier 1976). The real problem, however, lies not with the concept but with its critics, who wish to determine whether each and every differentiated populational unit in nature is, or is not, a species. This Shakespearian dilemma is, of course, a non-problem.

Given that populations – and species, however defined – are evolving units, it follows that (a) some of these units will be in intermediate stages of the speciation process, and (b) these intermediate units will be difficult or even impossible to assign to species categories, as I pointed out long ago (Vuilleumier 1976: 50–52). That others share my view is shown by Avise (1996: 22) who wrote something similar but on the basis of a different argument. These intermediate stages have been given several names in ornithology (Vuilleumier 1976: 52–54), of which the most useful is the superspecies. This term, introduced by Mayr (1931, 1942) as a substitute for Rensch’s (1929) German term Artenkreis, was clarified by Amadon (1966). The superspecies concept allows biogeographers to deal with instances where the biological species concept is difficult to apply empirically, and does this without burdening the formal Linnaean hierarchy. Thus Haffer’s (1989, 1992a) lists of avian superspecies are an indispensable tool. I stress here that even without the superspecies concept the great majority of species are not problematic (see Mayr & Short 1970 for an analysis at a continental scale).

I thus find it strange that some evolutionists, systematists, and biogeographers cannot accept some ‘noise’ – and not even much noise – at the species level, and so condemn the biological species concept for not providing them with the universal yardstick they request. In my 1976 paper I stated that ‘The [biological] species definition is . . . only the corollary of a theory of geographic speciation: it is not a definition in the strict sense of the term, but the logical consequence of the end-point of geographical speciation. This speciation process, out of which originates the ‘definition’ of species, is in turn a model, and like every model is limited by constraints that, paradoxically, make its heuristic value more general, yet reduce its value for practical application.’ I concluded (Vuilleumier 1976: 56) that ‘our framework of systematic reference at the species level, the Linnaean binomen, is today out-of-date.’ Twenty years later Avise (1996: 22), who had not read my paper, wrote: ‘In truth, much of the current debate about species concepts may stem from a failure to recognize the epistemological impossibility of adequately ‘squeezing’ complex biological situations into simple Linnaean taxonomic summaries.’

As the phylogenetic species concept does not derive from a theory of speciation and is purely empirical (‘observational’) (Cracraft 1989), it needs no evolutionary assumptions and requires no intellectual gymnastics from its proponents other than they be able to ‘diagnose’ a biological unit in order to call it a species. If the endpoints of evolution at the species level were that simple to identify, there would no longer be any reason to study evolutionary biology. Typology redux. The end of epistemology.

Given the theoretical emptiness of the phylogenetic species concept it is not surprising that geneticists like Avise & Ball (1990) or ornithologists like Snow (1997) are uncomfortable with it. Indeed, Avise & Ball (1990) not only demonstrated the superiority of the biological concept, but in fact advocated that workers retain it. Why, then, did Avise & Ball (1990) settle for a nebulous middle ground and suggest the adoption of ‘concordance principles’ that ‘provide a compromise or composite stance between the BSC [biological species concept] and PSC [phylogenetic species concept]’? As the concordance principle is nothing more than sound taxonomic practice based on a thorough analysis of many characters, rather than on the delimitation of species by one or a few diagnosable characters, nothing is gained. Similarly, Crowe (1998) suggested a compromise approach, consilience, between biological and phylogenetic species concepts. I now return to biological species and the representation of their distribution on maps.

In his seminal paper on the biogeography of the Pleistocene Deevey (1949) stated that a map of a species’ range was the fundamental datum of biogeography. Although many biogeographers work, not on single species, but on assemblages of them (faunas), the basic unit of study remains the species. Similarly, when biogeographers make faunal comparisons, the data they use are patterns rooted at the level of species. Some recent work, describing biogeographic patterns of parts of the genome, or phylogeography (Avise et al. 1987; Avise & Ball 1991; Zink 1996), is carried out at a level below that of species, yet the species remains the basic unit, as the phylogeographic patterns are usually expressed as within-species patterns. An example is the analysis of geographical variation in mitochondrial DNA in Ammodramus maritimus in the southeastern USA (Avise & Nelson 1989). All these patterns are depicted by maps.

A map is a graphical representation of biogeographic information, whether at the species or the faunal level. Biogeographic maps come in all kinds of projections, scales, and amount of detail. Biogeographers publish maps to illustrate specific points and choose the representation that best conveys such information (see Rapoport 1982). Given their diversity, it is surprising that no one has as yet analysed the history and significance of maps in acquisition of biogeographical knowledge (see Brown 1949).

What is a map? Maps can be viewed as objects and as a concept. These two aspects have not always been distinguished by biogeographers. In A Dictionary of Geography Moore (1974) defined map as ‘The representation on a flat surface of all or part of the earth’s surface, to show physical, political, or other features, each point on the diagram corresponding to a geographical position according to a definite scale or projection.’ The definition of a biogeographic map as an object could therefore be: ‘The representation on a flat surface of all or part of the earth’s surface or subsurface, to show physical, ecological, biological, taxonomic, or other biogeographic features, singly or in combination.’

What about the biogeographic map as a concept? Things here become more complicated. Even though a map is a real entity in terms of cartography, in fact it is a representation of some other reality, which resides in the field. A map thus is an abstraction, a model, in other words a conceptual construct. To translate Pierre George’s (1970) metaphors from geography to biogeography, one can state that, whereas the presence of a breeding population P of species S at locality L in area A is a ‘visible’ object that can be studied in the field, the depiction of P of S at L in A (a map) is an abstract representation of the visible: the ‘invisible.’ Thus the quest for knowledge in biogeography (MacArthur’s [1972] search for patterns that can be put on a map) rests upon an analysis of the invisible (maps), even though maps are based on the visible.

At the limit biogeographers can forget the real world (the visible). This may be especially true when they analyse distribution patterns derived from remote sensing techniques that are, in fact, superabstractions (for example Fjeldså et al. 1997). Biogeographers manipulating on their computer screen plots of species distributions with programs that digitize biological information superimposed upon vegetational information obtained from remote sensing (the invisible compounded) are quite removed from the visible, even though, again, digitized data (Connor 1988a) and remotely sensed data (Heyland 1988) are ultimately based on the visible. A similar warning, although in different terms, was expressed by Gotelli & Graves (1966: 309), who wrote about the atlases I will mention below, and bemoaned the fact that ‘biologists who use these [kinds of data] as their primary sources are at least twice removed from the raw data.’

When they work with maps (the invisible), biogeographers must remember that biogeography is not only a spatial discipline using two-dimensional representations of the visible but also an historical science. Biogeographic maps thus possess an unexpressed temporal component because a given population of a given species at a given locality in a given area has a history. Unfortunately most published maps of a species’ distribution depict heterogeneous temporal data, as they include – and mix – both recent and older locality records (old museum label data). An exception is Andors & Vuilleumier’s (1998: 5) map of the distribution of the Patagonian flycatcher Neoxolmis rufiventris (the invisible), which is based on their exhaustive one-season field study in 1993. To obtain an appreciation of this species’ spatio-temporal distribution one would need similar maps produced from similar surveys in earlier years, but such maps do not exist. However, using Andors & Vuilleumier’s (1998) map as an exemplar, the spatio-temporal evolution of this flycatcher’s distribution can be monitored in the future.

In a similar way, the publication of regional atlases at periodic intervals will permit biogeographers to analyse the spatio-temporal development of species’ ranges, and, by extension, the dynamics of faunas, as viewed through the lens of range dynamics of all the component species of a given fauna. For example, maps in the first atlas of bird distribution in Switzerland (Schifferli et al. 1980), based on the years 1972–1976, can be compared with those in the second (Schmid et al. 1998), for the years 1993–1996. In addition, modern technology, through appropriate computer programming, can produce three-dimensional images and time-sequences of maps or three-dimensional graphs that mimic the unfolding of the spatio-temporal events that one studies. Recent advances cannot bridge the gap between visible and invisible but allow us to visualise models of spatio-temporal sequences. Therefore, one can expand the previous definition of biogeographic map to encompass events, either on a single time-plane (ahistoric) or along a space–time continuum (historic), which is the realm of historical biogeography.

However species are defined and however one maps their distributions, interpreting these distributions as patterns constitutes only one aspect of the research programme of biogeography. The reason is that species are not very old geologically speaking and that their temporal history corresponds to a small part of earth’s history. Reconstructing the longer-term history of taxa above the species level has been a task of biogeographers for a long time. This endeavour links systematic work and biogeographic work. Solid systematic work is the necessary prerequite for sound biogeographic interpretation. If taxa are wrongly allocated to categories in a systematic scheme, biogeographic speculation is useless.

Systematists do two things, which need not overlap, although some workers believe they should, namely (a) to reconstruct phylogenies and (b) to build classifications (Bock 1973; Mayr & Ashlock 1991). As human beings we need classifications, of whatever the objects, in order to communicate. We have trouble dealing with unclassified objects. As biologists we need to reconstruct the genealogy of organisms. As biogeographers we need to superimpose genealogies (time) on geography (space) because phylogenies (the genealogies of organic beings) take place against a changing spatio-temporal component. We come back to the passage from the visible to the invisible.

What the visible is (and what is accessible to us) is made up of the individual organisms that we can measure and describe, for example on the basis of museum specimens. By appropriate analysis of these measurements and descriptions (expressed in terms of characters and character states) we can diagram the phylogeny (dendrogram, phylogram, cladogram) of these creatures. This branching diagram is in essence a temporal map of their genealogy (the invisible).

As the actual phylogeny of any monophyletic group of organisms is the result of a unique sequence of evolutionary events in space–time, there can be only one true phylogeny for each such group. Although each true phylogeny is thus an historic property of the visible, as it was composed of real populations in space–time, and is in theory mappable, much of it may be inaccessible to us because it lies in the past and may not have been incorporated into the fossil record. In other words we cannot observe or measure more than a fraction of the actual objects that comprised a phylogeny, only its surviving, contemporaneous endproducts, the terminal taxa, and, in fortunate cases, a sample of fossil forebears. Thus the phylogenies we reconstruct, as closely as they may seem to us to approximate the one and only true phylogeny (the visible), can never be fully corroborated: they remain forever part of the invisible.

In order to understand the relationship between phylogeny (evolution of organisms through time) and biogeography (evolution of organisms in space), biogeographers compare phylograms and maps. In cladistic terminology, taxon cladograms are superimposed upon area cladograms. This procedure combines two representations of the invisible – phylograms and maps – into a second-order abstraction that is still further removed from the realm of the visible, in other words, specimens and their historic distributions and genealogies in time and space.

Simplified to its barest bones, cladistics is a procedure for codifying characters (observable on specimens, the visible) prior to another procedure for analysing them in order to construct a diagram of phylogeny (the invisible). As the branching evolution of organisms along phyletic (genealogic) lines is called cladogenesis (Simpson 1953), cladistic methodology makes sense nomenclaturally. Whether it also makes sense biologically, insofar as only shared-derived characters (synapomorphies in Hennig’s terminology) are claimed to contain relevant information, is debatable. Once these informative characters are identified, graphing the character states allows one to draw cladograms.

One problem with cladistics and cladists lies not in its methods or in their work, but with the fact that cladists seem to be certain that theirs is the only path to phylogeny reconstruction. Given the complexities of organic evolution (an example is the variability of avian mating systems shown by recent molecular work, Avise 1996), and hence the uncertainties facing all reconstructions in historical biology, this attitude appears arrogant.

In the 1970s, employing Hennig’s (1950, 1966) phylogenetic systematics, and espousing Léon Croizat’s (1894–1928, Craw 1984a, 1984b, Heads & Craw 1984; see Croizat 1958, 1964) generalized tracks, several workers devised a method of superimposing phylogenies upon geography, called vicariance biogeography. Impressed (as had been many others before them, and ever since Candolle 1820) by recurrent patterns of allopatry in many taxa, but disinclined to ascribe these disjunctions to dispersal, these authors explained them by vicariance, in other words by the paramount effects of geographic barriers. This could have been allopatric speciation writ large, but instead it was the beginning of an acrimonious debate.

A key paper in vicariance biogeography was signed by Léon Croizat, Gareth Nelson, and Donn Eric Rosen (Croizat et al. 1974). In one of his last publications, however, Croizat (1982) made it clear, first that he had had little to do with that paper, next that he did not approve of Hennig or his method, and third that his own method was that of generalized tracks. In other words, near his death bed Croizat disavowed his participation in vicariance biogeography. (A statistical method for testing generalized tracks was described by Connor 1988b.)

Vicariance biogeography (without Croizat) nevertheless took off (Rosen 1975; Nelson & Platnick 1981; Cracraft 1982; Humphries & Parenti 1986; Wiley 1988). The main arguments of vicariance biogeographers were that (a) dispersal was not amenable to falsification (Popper 1959, 1972), hence a non-event, but that (b) events like the separation of continents were the key processes in historical biogeography and were amenable to Popperian analysis. The problem is not really whether it is dispersal or vicariance that is ultimately responsible for this or that vicariant pattern. Vicariance biogeographers do not reject dispersal, only treat it as an ad hoc explanation. What is of interest in terms of epistemology is that vicariance biogeographers started using area cladograms.

Given a cladogram representing the phylogeny of several taxa, one introduces onto it the geographical areas where these taxa occur. The branching sequence of areas then depicts the hypothetical temporal sequence of their separation from each other and from some common ancestral area (Nelson & Platnick 1981). Clearly, the concept of area cladogram is of interest when dealing with taxa that occupy now disjunct areas that were once united as a single landmass. The archetypal such Ur-region is Gondwana and its remains, the continents and islands centered around Antarctica: Africa, South America, India, New Guinea, Australia, and New Zealand.

There are problems with the method of area cladograms. One is that taxon cladograms are not part of the visible, and hence are abstractions (the invisible), that may or may not represent the only true phylogeny (the temporal visible, but largely unavailable to direct investigation, as you will recall). An area cladogram, built upon the taxon cladogram, is thus a compounded invisible. Another problem is that geological cladograms, against which area cladograms are sometimes tested, are also part of the invisible. Finally, not one, but many taxon cladograms can be generated on the basis of a cladistic character analysis. This situation poses the problem of cladogram selection, something that cladists have gone to great lengths to solve.

A favourite solution is to adopt an a priori position, parsimony (see Nelson 1969), whereby the simplest pathway of clade subdivision is assumed to be the correct one. Computer programs like PAUP (Swofford 1993) are available to select the cladogram with the most parsimonious solution to multiple cladograms. In the late 1990s, the majority of phylogenetic analyses are carried out with PAUP or some other parsimony algorithm. A recent example is the analysis of the different taxa and populations within Troglodytes aedon (Brumfield & Capparella 1996). There is nothing wrong with this procedure, except that the philosophical and methodological uniformity is depressing. What is also discouraging is that many authors who use PAUP seem to believe that parsimony is a fact of evolution rather than one method of handling evolutionary complexity (in essence denying it).

But this view is a cop-out. After looking at the past and peering at the present again, I now believe that I can identify a science of biogeography if I ignore the quarrels, the idiocy of some attitudes, and the divisions caused by adherence to taxonomic, ecological, or other beliefs.

My current analysis of biogeography suggests the recurrence of three epistemological problems that, together, make up a science. What binds these as structures is concepts such as species and map, and methods that combine systematics with geography. Even though the concepts are not uniformly defined (indeed, differences lead to debate) and the methods are not agreed upon (and lead to conflict), they are the products of a long history. Although different thinkers at different times, interested in different topics and working under different assumptions, tried to solve – and still try to solve – a wide range of problems, their research programmes showed similarities that overrode the differences. It seems clear to me that the three interrelated, more or less sequential, epistemological questions that cut across this diversity are: (1) What are the patterns of geographic distribution?, (2) What processes explain these patterns? and (3) How are these processes controlled?

Biogeographers have searched for patterns in the past, and are diligently searching for them today. For example eight out of the fifteen articles in the January 1998 issue of the Journal of Biogeography deal with patterns, and similar proportions of articles in recent issues of the Bulletin of the Japanese Society of Biogeography and of Biogeographica are pattern-centered. But what is a ‘pattern’? The American Heritage Desk Dictionary defines pattern as ‘A combination of elements, qualities, actions, or events that form a generally regular or consistent arrangement.’

Linguistically pattern is an interesting term. There is no equivalent in German, where Schema or Muster are used but do not have the same meaning as the English pattern. Similarly there is no equivalent in Spanish or in French. Spanish-speaking biogeographers use patrón, derived from the noun designating the sketch used to cut fabric for clothes. Using patrón in a biogeographic sense thus stretches the term. And French speakers use the English word pattern which, according to the Larousse, means (my translation) a ‘simplified model of a structure, in the human sciences.’ The Oxford–Hachette French–English Dictionary uses the French word tendance for pattern (as in the English ‘statistical pattern’ or ‘weather pattern’), but I would not employ tendance to mean pattern in biogeography. Is it possible that this linguistic difference reflects a difference in the way biogeographers think? This question might well repay analysis.

Brown & Gibson (1983: 5) defined pattern as ‘nonrandom, repetitive organization.’ And further: ‘Occurrence of pattern in the natural world implies causation by a general process or processes. Science usually proceeds by the discovery of patterns, then the development of mechanistic explanations for them, and finally the rigorous testing of these theories [= mechanistic explanations] until the ones that are necessary and sufficient to account for the patterns are accepted as scientific fact.’

How do biogeographers look for patterns, how do they recognize them, and how do they describe them? How precisely repeated does ‘nonrandom repetitive organization’ have to be in order to be a pattern? The issue of how to recognize certain patterns was treated with statistical models by Simberloff & Connor (1981) and Connor & Simberloff (1983). What are some patterns in biogeography? In the following section I discuss only three of the many kinds of patterns recognized in biogeography, as this paper is not an exhaustive treatment. Other important patterns include Blondel’s insularity syndrome (Blondel et al. 1988; Blondel 1995) and Diamond’s (1975) assembly rules. For subsequent discussion of these, see Connor & Simberloff (1979) and Gotelli & Graves (1966: 169–187, and references therein). Still other possible patterns are listed by Funk & Wagner (1995) in their overview of Hawaiian island biogeography.

Whether called disjunction, allopatry, or vicariance, the knowledge that many taxa are distributed discontinuously is about as old as biogeography itself. Candolle (1820) wrote some pithy pages on the topic, as did von Hofsten (1916). Not only is disjunction common, but it is also clear that many vicariant distributions can be grouped into patterns. These patterns may involve marine organisms distributed across a land barrier (as Atlantic and Pacific taxa on either side of the Isthmus of Panamá, Jones & Hasson 1985), or terrestrial organisms on either side of marine barriers (as birds on Southwest Pacific islands).

The species-area curve is the arrangement in a bi-coordinate plot of numbers of species of a given taxon occurring together in given geographic areas, usually islands, along a gradient of surface areas. The larger the area, the greater the number of species (Arrhenius 1921; Gleason 1922; Cailleux 1953; Darlington 1957). Correlations, including causal ones, have been made between species numbers and distance from source of colonisation, as well as area as an index of ecological diversity. The analysis of the many species-area curves described in the literature by means of integro-differential equations analogous to birth and death processes in population biology is due to the genius of MacArthur & Wilson (1963, 1967). Recent developments in the study of species-area curves can be found in Blondel (1995: 156–171) and Gotelli & Graves (1996: 207–238).

In any given geographical area the ranges of many species are often found to overlap when their distributions are plotted on a geographical or ecological base map. Biogeographers call this pattern congruence and assume that such areas of distributional overlap have biogeographic significance. Areas of congruence have been called centres of origin (Wallace 1876; Darlington 1957), centres of dispersal (de Lattin 1956; Müller 1973), core areas (Fjeldså 1985), evolutionary centres (Fjeldså & Lovett 1997), or areas of endemism (Cracraft 1985a), and, in conservation biology, hot spots (Fjeldså et al. 1997).

According to Cracraft (1985a: 50) ‘Areas of endemism are defined by the congruence of species’ ranges. Usually this congruence correlates with a physiographic or climatic barrier, yet congruence is rarely perfect owing to taxa being of different ages, ecologies, or dispersal abilities. Two or more taxa may have distributions that overlap relatively little and still be assigned to the same area of endemism: in such a case, the delimitation of areas depends on the patterns described by all the taxa in an area and by the pattern of relationships and distributions of close relatives.’

How much congruence (percent, degree, ecological quality) is necessary for a particular geographical area to qualify as an area of endemism? Cracraft (1985a) seems to be saying that any congruence at all may be sufficient to identify an area of endemism. Recognizing this methodological problem, Haffer (1985) attempted to quantify congruence by plotting lines of equal species diversity, thus obtaining contour maps of species density.

After patterns have been recognized and described, and assuming that they are not a figment of their imagination, biogeographers ask what processes are responsible for them. Ironically, this task is less difficult than that of recognizing and describing patterns in the first place.

Vicariance biogeographers have usually ascribed disjunction patterns to geologically old events like continental drift (Nelson 1975; Rosen 1975). When continental splitting leads to range disjunction, one then deals with allopatric speciation. In patterns like species-area curves, variance in species numbers can be described with statistical equations (correlation, regression) and explained by short-term ecological processes like dispersal, species packing, competition, and extinction due to turnover (Simberloff 1976). Congruence of species ranges can be ascribed to processes like dispersal of species into the area in question, to species packing, or to speciation, as well as to extinction. Thus a relatively small number of potential processes can be invoked for a wide range of situations grouped into a few patterns.

Disjunction patterns can be subsumed under the rubric of geographic or allopatric speciation, a key process in evolutionary biology (Mayr 1942, 1963). And congruence patterns can be ascribed to several processes, including, again, allopatric speciation. The allopatric process is extrinsic to the organisms that undergo speciation, where a barrier appears and splits their range (Isthmus of Panamá), but both intrinsic and extrinsic in archipelagoes where a given island is initially colonized according to the founder principle (Mayr 1954). Thus, in allopatric speciation, one can distinguish dichopatric and peripatric speciation (Mayr 1982b; Cracraft 1984). Dichopatric speciation is synonymous with vicariance: a barrier separates species into fragments. Peripatric speciation starts with a dispersal event across a barrier, followed by a genetic revolution, the founder effect, after successful colonization.

Dispersal was an important process in Candolle’s (1820) early scheme of biogeography. Much more recently, various aspects of dispersal have been treated by Simpson (1949), Diamond (1970), Carlquist (1981), and Lidicker (1985). Notwithstanding the claims of vicariance biogeographers that dispersal is either of no, or of only secondary, importance in biogeography, the empirical evidence that dispersal is a fundamental process is overwhelming. In avian biogeography, two well documented examples are Bubulcus ibis (Handtke & Mauersberger 1977) and Streptopelia decaocto (Nowak 1965; Kasparek 1996). Dispersal of the kind shown by Fagus grandifolia (Bennett 1985) is called diffusion and can be modelled mathematically (Okubo 1988; Shigesada & Kawasaki 1997). Dispersal involving crossing of unsuitable barriers (as in Anolis lizards, Williams 1969) is more difficult to model and is precisely the kind that troubles vicariance biogeographers.

Species packing is the temporal ‘accumulation’ of species (species gain) in a given habitat, geographic area, or region. A key process in faunal build-up, species packing has been studied mathematically by MacArthur & Wilson (1967) and MacArthur (1970, 1972). Species packing is a composite process in that it can be due to dispersal, speciation, or both. Once faunal build-up has started, competition may be an important additional process that helps segregate species into overlapping positions along niche axes (Yoshiyama & Roughgarden 1977).

Competitive exclusion is a fundamental process in ecology and evolution (Darwin 1859; Hardin 1960). Acting usually between or among closely related species, competitive exclusion is the opposite of species packing. If one species excludes another in a given geographical area or region, then there will be species loss, not species gain. In biogeography evidence for competitive exclusion is indirect. Checkerboard patterns of presence or absence of certain species and their congeners, for instance on isolated mountain tops, are interpreted in terms of competition (Diamond 1973, 1975).

Extinction, the opposite process of speciation, represents absolute species loss. It is perhaps the most difficult process of all to analyse (Raup 1991; Lawton & May 1995). Extinction of populations, species, or other taxa can be caused by intrinsic (genetic) or extrinsic (environmental) factors. The latter can be biotic (such as predation) or abiotic (volcanism, asteroid impact, Alvarez et al. 1980). MacArthur (1972) has discussed density-dependent ecological aspects of extinction; Cracraft (1985b) has examined density-independent controls.

What regulates (controls) allopatric speciation, dispersal, species packing, competition, or extinction? These are really tough epistemological questions that require (a) repeated dialectical movements between the field (the visible) and abstracted configurations of the field (the invisible), (b) attempted causal explanation(s) of the visible through the invisible, and (c) return to the field for a confrontation (testing, falsification) of one’s predictions or schemes with the real world. Rarely can biogeographers carry out such an exhaustive research programme. Often they must be content with verbal or statistical correlations that are assumed to represent causal relationships.

Whereas it is easy to hypothesize the cause of a process, for example that vicariance is ‘caused’ by continental drift (hence is controlled ultimately by sea floor spreading) or by the pulsing of refugia (which is controlled by climatic phenomena), it is another matter to devise appropriate tests to establish cause and effect. For instance, Bermingham et al. (1992) did not really test Mengel’s (1964) model of allopatric speciation but evaluated the congruence between their results and Mengel’s. Similarly, Beven et al. (1984) did not actually test Haffer’s (1969, 1974) theory of Pleistocene refugia but discussed congruence between their patterns and his.

How does one establish causality? Mayr (1961) wrote a penetrating paper on cause and effect in biology, and expanded his views in his recent book (Mayr 1997). Testing theories is a controversial aspect of science in general, of biology or biogeography in particular. Some biogeographers (Ball 1975; Nelson & Platnick 1981) have adopted Popper’s (1959, 1972) criteria of falsification. Others, however, argue that biogeographical theories, like other historical narratives, cannot be falsified, and that falsification is not the only way to decide whether a given field constitutes a science or not, and hence whether it has, itself, a theory.

My study of the development of biogeography has led me to conclude that there is a theory of biogeography and, furthermore, that this theory has a long history. I illustrate this idea with one example.

Back in 1820 Augustin-Pyramus de Candolle (Fig. 1) classified research in plant geography under three headings: (1) study of the influence of external elements (the environment) on plants, (2) study of how environmental influences determine the composition of stations (habitats), and (3) study of the ‘habitations’ of plants (their geographical distribution). In various sections of his essay Candolle analysed each of these three topics. Of interest are his observation that some species exclude others (competitive exclusion), his remark that the ‘laws’ he established can serve to ‘direct travelers in their selection of further observations’ (my translation; theory leads to prediction), his description of latitudinal gradients of species diversity, his discussion of various modes of dispersal, his recognition of patterns in the distribution of plant taxa (‘the more or less regular disposition of species and families upon the globe’), and his identification of barriers responsible for disjunctions.

Candolle’s research programme was remarkably modern given the fragmentary nature of botanical evidence in his time. Candolle (1820) recognized patterns but lack of data prevented him from identifying processes and explaining their action: ‘it is today absolutely impossible to reduce . . . patterns to a theory’ he wrote. We have clearly come a long way since then. We have acquired much biogeographic knowledge that we can organize in a satisfactory way. However, the point is not so much that we have more information but that our theory of biogeography today is not that different from Candolle’s about 180 years ago. Now, as then, we look for patterns, describe processes that explain them, and search for factors of process control. Now, as then, patterns are based on empirical analyses of the distribution of species. In her review of the history of biogeography from Ararat to Wallace and Darwin, Browne (1983) similarly wrote about a science of patterns and a science of processes.

Given the reticular nature of this extraordinary diversity, what will this new theory of biogeography consist of? Earlier I had suggested that biogeography developed along three more or less sequential but broadly overlapping phases, which I called descriptive, analytic, and predictive (Vuilleumier 1975). Simultaneously and independently, and proceeding from a different philosophical background, Ball (1975) also identified three phases, which he named empirical (similar to the phase I called descriptive), narrative (close to my analytic one), and analytical (equivalent to what I called predictive). The congruence between Ball’s and my three phases is appealing, even if in terms of a theory of biogeography it falls short of the mark, or is even wrong, as I argue below. Two years later Keast (1977) proposed a sequence of four similar phases (descriptive, historical, ecological, and predictive), and, later still, so did Simpson (1980). Modifications of Vuilleumier’s (1975) and Ball’s (1975) classifications were adopted by Blondel (1979, 1986) in his textbooks, and are incorporated in his latest overview of biogeography (Blondel 1995). Thus my 1975 epistemological scheme may still have some heuristic value. My newer analysis of the development and state of biogeography has led me to reappraise my earlier theory of biogeography, however. The theory I describe below is thus not an historical analysis of topical research programmes, but instead cuts right across these research topics and describes their common denominators.

Given my conviction that biogeography must be rooted empirically (the visible), a new theory must begin with species and their component subunits within a given spatial arrangement, namely populations composed of individual organisms belonging to that species through reproductive bonds. If field work is carried out over a long enough period of time in a given study area (at whatever scale), say ten to fifteen years, then the empirical basis is grounded in space and time. Such a time frame may be very short in an evolutionary or biogeographic sense, yet it can give an idea of temporal variance. Species and their populations are thus the fundamental datum, Pierre George’s (1970) all-important visible. Everything other than the species or its populations, either in an upward taxonomic hierarchy (assemblages of species, or species grouped into higher taxonomic categories), or downward (subspecies), I consider part of George’s invisible.

In order to detect and describe biogeographic patterns the research programme goes from particular to general, what Sautter (1975) called research from the bottom up. Thus, investigation radiates outward from species along several potential lines, each of which represents a passage from the visible to the invisible and gives access to one or more kinds of patterns. (1) One line goes from species to subspecies or populations via genetic analysis such as mt-DNA (phylogeography). (2) A second line extends from species to faunas, faunal regions, and provinces (regional biogeography). (3) Another line reaches from species to faunas and to numerical diversity expressed as species-area curves (insular biogeography). (4) A fourth line goes from species to other taxonomic categories via taxon cladograms and area cladograms (vicariance biogeography). (5) A fifth line extends from species to faunas or species density patterns expressed as areas of congruence (areas of endemism, hot spots, or else vicariant faunas and/or areas) (refuge biogeography). None of these research constructs, of course, is entirely independent from the others: reticulation quickly obtains, and mixed patterns can be described.

Pattern description should be followed by process identification, which can be achieved empirically by pattern accumulation. One or two patterns may suggest a process, but not be necessary and sufficient to enable one to reach a conclusion about process correlation, let alone causality. More patterns, therefore, must be established for comparative analysis that can lead to a conclusion of causality by correlational relationship. Causality, then, is hypothesised on the basis of probabilitic arguments.

Processes in biogeography appear to overlap broadly with those of evolutionary biology (speciation, extinction) and of ecology (competition, species packing), and often to depend on geology and geophysics (plate tectonics). Popperian (1959, 1962) falsification of biogeographic hypotheses, although advocated by workers as distant philosophically as Connor & Simberloff (1979) and Nelson & Platnick (1981), can only rarely, if ever, be carried out in absolute terms, and is not the only criterion of scientific activity in an historical science like biogeography (O’Hara 1988; Andersson 1996). To paraphrase Popper’s ornithological metaphor, in biogeography it may take a lot of white swan watching before one finds a black one. Corroboration then, rather than falsification leading to theory rejection, is often the actual empirical theory testing procedure.

If the above appears to be a prudent approach, it probably is. After all, a good biogeographer is also a prudent one. As Moreau (1966: vii), the consummate African biogeographer, reminded us, avian distributions are often ephemeral, and rash conclusions are often wrong conclusions. Was Andersson (1996) right that historical biogeography has an ontological problem? I do not think so, as my analysis suggests that we know what facts form the basis of knowledge in biogeography. And was Stott (1998) right that there is a malaise in biogeography? In the sense that biogeographers appear rather divided along party lines (in spite of some striking unity of goals), yes he is. But he is not right that we need a Kuhnian revolution to cure the disease. Instead, what I think we need is a new Wallace, a new brilliant intellect who will galvanise the current tired fabric of biogeography into a new warp and woof. MacArthur and Wilson played such a role in the 1960s and 1970s. Who will be the catalyst in the twenty-first century?

I thank the Scientific Committee of the 22 IOC for the invitation to present this paper, the National Geographic Society and the Leonard C. Sanford Fund for financial support, and Allison V. Andors for his help during the preparation of the manuscript. I am grateful to Allison V. Andors, Ernst Mayr, Robert M. McDowall, and Daniel Simberloff, whose critical comments greatly improved the manuscript. I acknowledge the courtesy of Hervé M. Burdet of the Conservatoire et Jardin botaniques in Geneva, who generously sent me a photographic print that served to prepare Fig. 1, and for allowing me to publish it.

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