Biogeography with or without Pan: Science or not?

Quite some time ago I wrote about two papers by panbiogeographer Michael Heads in the journal Australian Systematic Botany. In the latest issue of the same journal, Matt McGlone has now published a rebuttal.

He focuses on arguing the following:

First, it does not appear to him as if there was institutional or hiring bias against panbiogeographers in New Zealand in the 1980s and 1990s, as claimed by Michael Heads. Instead he sees a general tightness of the job market as the likely reason that panbiogeographic PhD students went overseas, as did many others. (I am of course entirely unable to assess this either way.)

Second, there is a lot of indirect evidence for long distance dispersal: time-calibrated phylogenies, observed dispersal events happening around us today, groups with easily dispersed propagules are more likely to show trans-oceanic disjunctions than groups with heavy and/or short-lived propagules. Panbiogeographers tend to doubt that the first are reliable, argue the second away as 'normal dispersal' that does not demonstrate that speciation will follow, and explain the latter with widespread ancestors.

Third, McGlone argues that the model of speciation underlying the panbiogeographic assumption of the widespread ancestor is at best ill-defined and at worst incompatible with the current best understanding of speciation processes in evolutionary biology.

What I find most interesting about the paper is, however, its analysis of the motivation behind panbiogeographers' rejection of long distance dispersal as an explanation in biogeography. It cites Michael Heads himself as follows: long distance dispersal "is unfalsifiable and explains all distributions and none at the same time". In other words, mainstream biogeography is not a science because it is at heart ad-hoccery. No matter where something occurs, it can just be assumed to have got there through dispersal, end of story.

Conversely, and as seen above, mainstream biogeographers do not consider panbiogeography a science because it always assumes the ancestor to be widespread, building the conclusion of vicariant speciation into its premises. Another way of saying the same thing is that the panbiogeographic method has no way of falsifying the hypothesis of vicariance; instead it is designed to merely find a rationalisation of that assumption.

So which is it? Or is all of biogeography just-so stories? Unsurprisingly, given what I wrote before on this issue, I feel that the panbiogeographic analysis method at least as presented in the two Australian Systematic Botany papers by Heads does indeed come across as circular reasoning, and thus I find it hard to accept it as scientific.

On the other hand, I believe that a rejection of dispersal as ad-hoc misses an important point. It sees the issue as binary: either dispersal or no dispersal, for any given group of organisms. Instead, it becomes a matter of statistics when we shift our perspective to larger numbers of cases, and here it should become clear that we can again formulate testable hypotheses and even models or 'laws'. For example about how certain traits of a species influence its range size or its likelihood of establishing in a distant area, or how the distance and size of two landmasses and the wind and current patterns between them influence the likelihood of organismal exchange in both directions.

In summary, the panbiogeographic critique of mainstream biogeography as unscientific does not convince me. One could just as well argue that science cannot study radioactivity if it permits a random element to when exactly an atom decays.