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February 2, 1998
Rapporteur: Ranessa L. Cooper
Biogeography explains the geographic distribution and diversity of taxa and can be divided into two major subsets: proximate and ultimate. Proximate biogeography incorporates ecological constants, dispersal capabilities, and the maintenance of diversity, while ultimate biogeography was first presumed to be the result of dispersal and contraction events based on the classical views of G.G. Simpson and E. Mayr. These early arguments focused on areas of endemism, centers of origin, and location of old/new taxa as the important aspects of biogeography.
In the late 1960s, Croizat established the field of panbiogeography. Panbiogeography involves mapping the distribution of closely related taxa and joining the distributions with lines on the map. Croizats work inspired many cladists, including Don Rosen. In 1978, Rosen examined the biogeography of the fishes Xiphophorus and Heterandia by applying cladistic methods using geographic areas in place of taxa. He proposed vicariance as the main mode of disjunct distributions. This school of thought combined with others led to the pioneering of cladistic biogeography.
Hovenkamp (1997)* defines cladistic biogeography as involving the construction of phylogenetic trees and then substituting the areas of distribution for taxa onto the trees; divergences are then ordered in a temporal fashion. One then determines which groupings are supported by more that one node. Supported nodes are then inferred to be SVEs with a temporal order. However, there are some existing problems w/ cladistic biogeography such as: 1) if vicariance is the main cause, then all area cladograms should be consistent, 2) hypocrisy exists in cladistic biogeography if ignoring ecology and/or basing vicariance on plate tectonics, and 3) later work suggests that all areas may not be equivalent.
Hovenkamp clearly distinguishes Earth history biogeography and taxon history biogeography, and he questions several assumptions. Does one use areas as taxa, the evolution of areas, or taxa as characters? Actually, none of the above, as he has devised a new method that looks for vicariance events and their relative sequences. There are four parts to his methodology: 1) replace taxa with geologic data and put onto a cladogram, 2) infer the order of events by looking at the number of each node, 3) extract the data into vicariance events that agree with one another based on nodal support, and 4) map the supported vicariance events back onto the relative sequences of the original cladogram.
Interesting to note, in Hovenkamps first example (pp. 72-3), Borneo has two nodes of support as seen in Table 1; however, Table 3 indicates only one node of support. Keith chose to redo the biogeographical analysis using both nodes of support for Borneo separating from the Philippines. Keiths revised version supports the tectonic history of SE Asia as described by Dr. Doug Craig following the lecture. Moreover, Hovenkamp does not clarify his reasoning for excluding this element from the analysis.
The lecture was concluded by identifying some of the limitations of the usage of vicariance events followed by the questioning of their importance in biogeographical analysis.
*Hovenkamp, P. 1997. Vicariance events, not areas, should be used
analysis. Cladistics . 13: 67-79.
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Discussants: Gavin Hanke and Jan Jekielek
The discussion of the focal paper covered several topics including questions concerning specific examples in the paper, the meaning of vicariance, and the use of fossil evidence in biogeographical analysis; these topics served as the highlights of the discussion.
Some felt that Hovenkamp did not clearly present Example 1 in his paper. In fact, the discussion began with the presentation of a map of the Borneo region to illustrate and clarify the misunderstanding. By looking at a fish distribution of three families overlaid on this map, it was suggested that Hovenkamp had misrepresented the tectonics of the region. In addition, many agreed that by placing an actual map in the paper would have made the concepts easier to understand as no map as such was presented.
The rainforest example from the focal paper was addressed as one questioned how Hovenkamp broke up the Amazon region into quarters to get the bases in his analysis. At that time in history, the Amazon area was mostly grassland with some parts lacking grasses; the periphery of the area consisted of both diverse flora and fauna. These distinct regions were a result of climate variation. This illustrated that vicariance events can be climatic in nature.
Hovenkamps fern example was revisited after noting that dispersal is a pertinent factor in analyzing biogeography. The distribution of the fern species may be due to dispersal of spores rather than vicariance. Furthermore, which taxa should be used to illustrate vicariance events? Which taxonomic level is most appropriate to use in these analyses? It was decided that this is dependent on the variability of the organism being examined, including the types of dispersal patterns exhibited as well as the level of dispersability.
Differing views of the meaning of vicariance were discussed and disputed. What construes a vicariance event knowing that a time and spatial scale is important to consider? Some say vicariance is the separation of gene flow into two separated populations; the building of a dam is a vicariance event in this instance. Others were in accord with this idea but felt that more evidence was necessary such as geography. Still yet, some believed that vicariance is a physical separation that precedes speciation; not all speciation is a result of vicariance events. Sometimes vicariance is not the explanation for speciation; for example, Hawaii and the Galapagos Islands were formed from volcanic activity. Radiation then independently occurred at these localities. The question is currently debatable: Are vicariance events and speciation events synonymous?
How does fossil evidence fit into the analysis of biogeography using vicariance? The use of fossil taxa should add to rather than confuse the issue of vicariance events. It was noted that fossil evidence was key in linking S. America and Africa. In another example, the most primitive termites are found in Australia, but this does not necessarily mean that these primitive creatures are endemic to Australia or even evolved there. In fact, these primitive termites have a fossil record in N. America! This same scenario can be applied to anteater/fossils and platypus/fossils to verify the importance of the fossil record and correlations with geographic distributions.
Does anything useful come out of the focal paper? Some say no. Some commented that the information can be quite useful and should be used in conjunction with geological data. Perhaps, Hovenkamps technique should be applied when other information is not available. It was concluded that patterns of vicariance events are hypotheses as are cladograms; both are evaluated by testing largely observational hypotheses.
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