Below is another way in which area cladograms are depicted, a phylogeny of turtles based on morphology (top) and mtDNA sequences (bottom). These cladograms, show a close relationship between taxa from South America and Madagascar, and the African taxa as the oldest (basal) lineages (i.e. they branch-off first). This set of relationships is inconsistent with a continental drift/vicariance scenario for their evolution and distribution (which would predict the Madagascar taxon as the oldest and a closer relationship between SA and African taxa as this reflect the sequence of spiltting of the landmasses, see Noonan 2000).

Vicariance-dispersal scenarios can be evaluated using predictions based on extent of genetic divergence and timing of lineage splitting when using molecular markers and invoking a "molecular clock" (the idea that sequence changes accrue in a regular, clocklike manner such that sequence divergence can be equated with time).

The example below (from Avise 1994) shows the predicted distances (shaded areas) in albumin divergences based on a vicariance model of divergence in various Caribbean vertebrate taxa. The major vicariant events are the separation of the "proto-Antilles" from the North American mainland and the separation of Cuba, Hispanolia, and Puerto Rico. The dots represent pairwise divergences among taxa found in the various areas. Note: (i) the wide variability in divergences, and (ii) that most of the divergences post-date the timing of the vicariant events. Both these observations are inconsistent with these particular vicariant events being important in the zoogeographic distribution of these taxa (which would predict more narrowly spaced divergence times and their general timing being coincident with the timing of the geological vicariant events).

(2) Phylogeography: congruent intraspecfic evolutionary lineages are strongly patterned geographically in the southeastern USA

A second example involves more recent vicariant events associated with Pleistocene glaciations. The figure immediately below shows the distribution of two major phylogroups (inferred from mtDNA) in the southeastern United States. One group ("Atlantc") tends to be restricted to east and north of the Floridian peninsula, while another ("Gulf Coast") predominates in the Gulf of Mexico area. Note that the same general pattern is shown is birds, fish, oysters, horseshoe crabs, and turtles (although they very in details of exact distribution and levels of sequence divergence). These concordant distributions, again, argues that evolutionary divergences in all the taxa (despite very different ecological characteristics and dispersal abilities) have been influenced by a common historical process that has generated a very non-random geographic distribution of phylogroups. The study of intraspecific lineages and their association with geography is known as "phylogeography", a subfield of zoogeography born out of the work of conducted by J.C. Avise and colleagues at the University of Georgia in the late 80’s and early 90’s.

Avise and colleagues coined the term "phylogeography" to describe the study intraspecific evolutionary divergences and their association with geography. This and other phylogeographic studies have provided evidence for the importance of vicariance in biogeographic pattern (but remember the green turtle example!).

Below is shown the effects of Pleisticene glacial advances on the position of the shoreline across the same area (A). Note that during glacial maxima, the sea level was much lower which resulted in a greater restriction in the area between the Floridian peninsula, Cuba, and the Gulf of Mexico. Such restriction may have limited gene flow between the Atlantic basin and the Gulf of Mexico and promoted divergence between populations on either side of this "barrier" (formed by restricton to deep water of fast currents and the reduction of nearshore salt march habitats that ring this area now).

The two areas are isolated somewhat presently by the strong northeastern orientation of the Gulf Stream current as well. Also shown in (A) is the shoreline during the Pliocene which flooded much of the Gulf and Atlantic coastal plain. Such flooding likely acted to isolate freshwater fish to the headwater areas of streams flowing into these two areas, again perhaps promoting divergence within species of fish across this area.

Freshwater fish show some of the strongest geographic partitioning (as above) between (a) and (b) areas indicated on map (B) (see Avise and Bermingham (1986) for more details).

In sum, phylogenetic systematic approaches led to a greater application of hypothesis testing under a strict set of assumptions and methods that has helped to quantitatively assess alternative hypotheses (e.g. importance of historical and current dispersal versus vicariance) in biogeography. Considerable evidence has been presented from these methods that has supported a role for vicariance, and of historical factors in general, in organizing biogeographic pattern. This does not mean, however, that dispersal is unimportant. In fact, an appreciation for the role of historical, long distance dispersal is increasing (see Berra et al. 1996; de Queiroz 2005, below). The important point is not that vicariance is the one and only explanation, but that vicariance biogeography ignited a move to empirically test biogeographic scenarios under a inductive reasoning and hypothesis testing framework. We shall gain more appreciation for the role of current dispersal in our next section!

Examples of trans-oceanic dispersal in plants and animals (de Queiroz 2005).

References:

Avise, J.C. 1994. Molecular markers, natural history and evolution. Chapman and Hall, New York.

Avise, J.C. 1992. Molecular population structure and the biogeographic history of a regional fauna: a case history with lessons for conservation biology. Oikos 63: 62-76. (strong evidence for vicariance).

Berra. T.M. et al. 1996. Galaxias maculatus: an explanation of its biogeography. Marine and Freshw. Res. 46: 845-849. (evidence for dispersal influencing distribution in widely distinct areas).

Bermingham, E. and C. Moritz. 1998. Special Issue - Phylogeography. Molecular Ecology 7: 367-545.

Bermingham, E. and J.C. Avise. 1986. Molecular zoogeography of freshwater fishes in the southeastern United States.Genetics 113: 939-965.

Brown, J.H. and M.V. Lomolino. 1998. Biogeography. 2nd ed. Sinauer Assoc. see Chapters 9, 11, and 12.

Brundin, L.Z. 1988. Phylogenetic biogeography. In "Analytical biogeography". Ed. by A.A. Myers and P.S. Giller. Chapman and Hall, London.

de Queiroz, A. 2005. The resurrection of oceanic dispersal in historical biogeography. Trends in Ecol. Evol. 20: 68-73.

Joseph, L., Moritz, C. and Hugall, A. 1995. Molecular support for vicariance as a source of diversity in rainforest. Proc. Royl. Soc. Lond. B 260: 177-182.

Noonan, B.P. 2000. Does the phylogeny of pelomedusoid turtles reflect vicariance due to continental drift? Journal of Biogeography 27: 1245

Steppan, S.J. et al. 1999. Molecular phylogeny of the marmots (Rodentia: Sciuridae): tests of evolutionary and biogeographic hypotheses. Syst. Biol. 48: 715-734.

Trewick, S.A. 2000. Molecular evidence for dispersal rather than vicariance as the origin
of flightless insect species on the Chatham Islands, New Zealand. J. Biogeog. 27: 1189.

Wiley, E.O. 1988. Vicariance biogeography. Ann. Rev. Ecol. Syst. 19: 513-542.