Which assumption should we choose?
Of the three event–based options described above for treating widespread taxa, there is none that is ideally suited to all kinds of problems. Each option has its strengths and weaknesses, and the choice should therefore depend on the nature of the data. The free option is more general in that it allows more processes in explaining widespread terminal distributions. On the negative side, it is computationally more demanding than the other options and because it allows more solutions, it may also be associated with loss of information concerning the optimal GAC. To some extent, however, the potential information loss may be counteracted by the differences in the cost associated with the allowed solutions. The ancient option makes the boldest assumptions about the origin of the widespread distributions. If the assumptions are warranted, the search for the optimal GAC should gain in power; if they are not, the result of the analysis may be flawed. For instance, the ancient option might be useful in analyzing the distribution history of old groups that are very unlikely to have dispersed, or in which the widespread taxon has lost the ability to disperse (e.g., a wingless species in a fully winged group).
In many cases, it is quite clear that the widespread terminals are younger than any of the ancestral areas in the GAC, in which case the recent option would be the only defensible choice. The recent option may also be advantageous in the identification of phylogenetically constrained biogeographic patterns because it does not allow vicariance events within widespread terminals, in contrast to the free and ancient options. Assume that we test for constrained distributions by comparing the cost of the observed TAC with that of random TACs obtained either by randomly drawing new TAC topologies or randomly shuffling the TAC terminals. Because the widespread terminals are the same in both the observed and random TACs, the terminal events will not contribute to distinguishing the observed TAC from the random TACs. However, it is quite likely that several of the “terminal” events could be pushed onto the ancestral nodes in the observed TAC but not in the random TACs. This potential support for the GAC is ignored by the free and ancient options. The recent option forces vicariance events onto ancestral nodes in the TAC and is therefore more powerful in separating phylogenetically constrained distribution patterns from random data in this kind of test. For an empirical example, see SANMARTÍN et al. (2001).