Geological hierarchies and biogeographic congruence in the Caribbean

If it is agreed that an understanding of biohistory in some ways is tied to an understanding of geohistory, then one might also agree that what is needed is a precise means of specifying how a given biohistory is explicitly tied to a particular geohistory. The constraint in this type of analysis is the branching diagram, or cladogram, that permits a precise comparison of geographic area cladograms to demonstrate congruence between the cladistic message from biology with the cladistic message from geology. A proposal for identifying these cladistic constraints is given using a comparison of several different historical geologies of the Caribbean region as an example that contrasts with the constraints used by previous biogeographies in which an a priori notion of process, e.g., dispersal or extinction, was used to direct the outcome of biogeographic analysis. Whether espousing dispersalism (Darlington, 1957) or vicariism (Croizat, 1958, 1962), biologists have always assumed that the distributions of organisms in some way reflect the nature of the world's geologic history. It was, thus, implied that an understanding of biohistory is tied to an understanding of geohistory. In the DarwinianDarlingtonian tradition the relevant geohistory was assumed to be one of stabilism necessitating an interpretation of biohistory as one of active or passive dispersal. In other words, so long as the continents stood still something had to move to account for the occurrence of closely related organisms spanning large water gaps. Thus were invoked temporary land bridges suitable for crossing ocean barriers, birds with feet and feathers to which seeds and small animals would adhere during their transoceanic flights, and menageries supported on floating debris that was spewed into the ocean's currents from river mouths and deltas. The difficulties inherent in these scenarios of a haphazard biohistory are apparent in Darlington's (1957) hypothesis that ostariophysans arose in central Asia and made their risky way along ephemeral freshwater routes to the southern continents without leaving a trace of these great migrations. But the hypothesis of a steadfast geography and a dancing biota was deemphasized after the theory of plate tectonics was elaborated. Since most biologists had a penchant for believing that geologists had a special hold on the truth, the acceptance of continental drift caused some of these biologists to shift into reverse strategy of proposing that it was the geography that moved while the organisms got carried about to their new longitudes and latitudes. Other biologists (Darlington, 1965; McDowall, 1971; Briggs, 1984) tried and still try to rescue the past by agreeing that the geography did in fact move but that the timing of these great events was wrong in relation to the ages of the biotas. Such attitudes might invoke the ages of fossils to show that all the taxa are too young to have been influenced by the geographic cataclysms. This view involves two assumptions, both wrong at some level: 1) that fossils can tell us how old a taxon is and 2) that the ages of the geologic events have been correctly assigned. The first assumption is wrong because fossils give a minimum rather than maximum age of a taxon, and the second assumption is put into question by recent age reassignments. For example, parts of the Caribbean which were originally supposed to have been moving along a transform fault at the moderate rate of 2 cm per year (Kellogg & Bonini, 1982) are now believed to be moving at the brisker pace of 4 cm per year (Sykes et al., 1982; Wadge & Burke, 1983), thus doubling the rate of motion and halving the ages of the events associated with translocation. But these relations leave us still at the mercy of general supposition when what is needed is some precise means of specifying that this biohistory is tied explicitly to that geohistory. This still unrealized need for precision is supplied by another conceptual revolution that, in terms of time spans involved in human intellectual history, might be said to have occurred more I For comments during the work and review of the typescript I thank Drs. Kevin Burke, Arnold Kluge, Lynne Parenti, Norman Platnick, Edward Robinson, Peter Tolson, Lynn Sykes, and, especially, Gareth Nelson. 2 Department of Ichthyology, American Museum of Natural History, New York, New York 10024. ANN. MISSOURI BOT. GARD. 7 2: 6 3 6-6 5 9. 19 8 5. This content downloaded from 207.46. 13.64 on Sat, 03 Sep 2016 04:52:04 UTC All use subject to http://about.jstor. org/terms 1985] ROSEN-GEOLOGICAL HIERARCHIES 637 or less simultaneously in systematics and biogeography. This revolution has been called cladistics, the science of character analysis and the use of branching diagrams. Its premises permit precise comparisons between biological and geological systems (e.g., Rosen, 1978). The general objective of cladistics is to discover congruence between the two that constrains historical explanation. In the Darwinian tradition in biogeography it is dispersalthat was the constraining concept, requiring an interpretation of the history of life in space contrary to what the data of life suggested. Leon Croizat (1958, 1962) was one of the pioneers who recognized that the biological data tell their own story, which can be at odds with a stabilist geology. Now that stabilist geology has been rejected in favor of a concept of mobilism, some biogeographers (Nelson & Platnick, 1981; Wiley, 1981) accept that biology has an independent story to tell about the history of the world. It is this independence of biological from geological data that makes the comparison of the two so interesting because it is hard to imagine how congruence between the two could be the result of anything but a causal history in which geology acts as the independent variable providing opportunities for change in the dependent biological world. The comparison becomes especially interesting if there is a congruence among geohistories based on different approaches to the geographic problem, and if there is a congruence among cladistic relations of different taxa with respect to the same geographic areas. The specific questions are: 1) do the members of different monophyletic groups of organisms have the same relations to each other with respect to geographic regions in which they are endemic and is their congruence with respect to these areas non-random; and 2) does this non-random congruence of different groups of organisms correspond to a branching diagram that represents part of the history of some geographic region? The constraint in these comparisons is the branching diagram rather than a process assumed to be of causal importance. In 1976 and 1978 I suggested a history for the Caribbean land and water that, at a rather general level, is consistent with the present distributions of plants and animals in the region including the Antilles, Central America, northern South America, and the southeastern and southwestern United States. The 1976 proposal, which was based largely on a descriptive history by Malfait and Dinkleman (1972) and Tedford (1974), was questioned by Pregill (1981) whose claim was that there is no geologic evidence warranting such a proposal. This claim was critically evaluated recently by Hedges (1982) who disagreed with Pregill on the grounds that ample geologic data had been available for some time in support of the Malfait and Dinkleman-Tedford theory (he cited 13 literature sources). Since the time of Hedges' reply to Pregill, I became aware of several other accounts of Caribbean history based on a variety of geologic data sources (Birnie, 1977: movement along major fault zones; Pindell & Dewey, 1982: plate contour matching and paleomagmatic data; Kellogg & Bonini, 1982: gravity data, seismic profiles, radiometric data, and earthquake data; Sykes et al., 1982: slip vectors of shallow earthquakes and other seismic data; Wadge & Burke, 1983: reconstructing plate motions by closing the Cayman Trough along its bounding transform faults). It is apparent, therefore, that Pregill was operating with a different set of constraints, namely, that the biota is recently distributed by means of dispersal. Hence, no number of geologic accounts or amount of data would be expected fundamentally to alter his position. My constraint is the cladogram and how it describes relationships of taxa and areas. What I propose is to divide the historical geology of the Caribbean into the minimum number of time periods in which different geologic theories agree on the geographic contacts between different areas and the severing of those contacts. In this way I have covered four main periods spanning 165 million years.