[Taxacom] Dispersal clarifications

[Michael Heads]

Hi Sergio,
 
I'm looking at your beautiful map of Pacifigorgia (J Biogeogr. 2008, I used it in a course recently): Baja California to Panama, Pacific coast, including Revillagigedo and Galapagos. Then crossing over at Panama to the Atlantic side: Colombia to Brazil. As you stressed in the paper, the absence from the Antilles and the Atlantic coast of Mexico-central America is significant and probably an ancient absence. The Revillagigedo/Galapagos area is also significant - and probably an ancient presence. The groups here are mostly old Pacific groups that were surviving in the region long before the present islands. Galapagos-Caribbean affinities in both Darwin's finches and the mockingbirds are now corroborated in molecular work and several groups on the Galapagos have been dated as older than the islands. The mechanisms are well-known (prior islands, subsidence, burial under new eruptions etc.) and you could make a stronger case for this on
 Revillagigedo. I don't think Pacifigorgia dispersed to the Galapagos. I think pre-Pacifigorgia (not P. itself) was in the Galapagos region before the Galapagos existed, and both entities developed there. Just as a mental exercise, you could imagine a scenario where Pacifigorgia starts off widespread over its present range and then differentiates, into several Panama-Galapagos lines and all the others. There is no a priori need to have dispersal between Panama and Galapagos, in either direction. In practice, the first question is: are the Panama - Galapagos clades distributed exactly alike?  For example, there could be a E Panama - E Galapagos clade and a W Panama - W Galapagos one. If not, they may have been something like this originally and then overlapped within the Galapagos and within Panama (again, no dispersal is needed between the two). But in most cases of supposed sympatry there is often a trace of the original allopatry, so any
 difference at all in the distributions is often significant. 
 
Here's that distribution in mockingbirds: the four Galapagos species of Mimus (‘Nesomimus’) form a clade that is sister to M. gundlachii: Jamaica, keys off northern Cuba, the nearby Turks & Caicos Islands, and the Bahamas (Arbogast et al., 2006). The sister of this Galapagos - Antilles group is a clade of three species in which M. graysoni of Socorro I. (Revillagigedo Is.) is strongly supported as basal to (not nested in!!) the widespread M. polyglottos: US to Mexico, + M. gilvus: Mexico to SE Brazil. 
 
Michael
    

Wellington, New Zealand.

My papers on biogeography are at: http://tiny.cc/RiUE0

--- On Sun, 12/6/11, Sergio Vargas <s.vargas at lrz.uni-muenchen.de> wrote:


From: Sergio Vargas <s.vargas at lrz.uni-muenchen.de>
Subject: Re: Dispersal clarifications
To: jfmate at hotmail.com, taxacom at mailman.nhm.ku.edu, michael.heads at yahoo.com
Received: Sunday, 12 June, 2011, 7:26 AM


Hi guys,

I've been reading your emails with great interest. After following the discussion, it seems to me that you are actually talking about the same thing, when you talk about dispersal. Don't know, but Jason dispersal's seem faraway from what Michael refers to chance dispersal. I like to talk about naive dispersalism to refer to Mayr-like chance dispersal or long-distance dispersal because the reference to either both rarely involve any mechanism whatsoever, while what I would like to call modern dispersalism does in fact looks for a mechanism explaining the inferred dispersal (range expansion) events. Sadly, I cannot say there is modern dispersalism because long-distance dispersal sensu Mayr continues to be use as an "explanation" when something doesn't fit a vicariance model.

Anyways, you have been talking about oceanic islands, and I think I have a good example of the difference between naive and modern dispersalism in a marine context. In the Galapagos archipelago there are 4 described species (probably there will be 5 at some point) of the sea-fan genus Pacifigorgia. The genus has its diversity hot-spot in the panamic province with ca. 20 of the 35 described species occurring there. The 4 species of Pacifigorgia occurring in Galapagos are not closely related in a morphological phylogeny of the genus, but form part of different clades and almost invariably have a Panamic sister species. If you try to explain this pattern: Galapagos-Panamic sister groups, which by the way, is fairly common, you would more likely need to postulate recurrent independent "long-distance dispersal events" of panamic ancestors towards Galapagos. I hope we all agree with this. Now, here's the difference between a Mayr-like explanation and a
 Croizat-like explanation for the recurrent pattern, at least in my opinion. If would use Mayr, you would simply say that the four peripheral endemics are the result of repeated (chance) long-distance dispersal events from mainland Central America towards Galapagos. How the ancestor reached the Galapagos, and why it stop reaching the archipelago is a matter of speculation, and the direction of dispersal is obviously clear: mainland->oceanic island. The problem is that dispersal here is regarded as an "event" that occurs one time and never happens again, and its improbability makes it obscure and quasi-magic. A Croizat-like explanation, which rejects chance dispersal, needs to provide a mechanism accounting for both the establishment of a range-expansion process towards Galapagos, and the interruption of gene-flow between the mainland and the archipelago. In this case (Pacifigorgia), the mechanism needs to be recurrent in geological time. Curiously, if
 you search for such a mechanism in the literature you find a good candidate: several (recurrent) interruptions in the wind jets responsible for the seasonal formation of the Panama Bight, a current that reaches Galapagos during part of the year. These interruptions which disrupt the current, only occurred after the closure of the panama isthmus. This is in accordance with the fact that Pacifigorgia, most likely, reached its current diversity after this event (the closure). This explanation account for both periods of range-expansion (increased likelihood of reaching Galapagos) and of vicariance (reduced likelihood of reaching Galapagos) resulting in speciation, are testable (because the jet interruptions can be dated with some precision and should correlate with the age of speciation events), and require no mistery or speculation on how the ancestors reached the galapagos or why they stop reaching the archipelago. If you like to talk about
 probabilities, I guess, you need to account for a process making the probability of dispersal to a place higher during some period and then lower during some other period. In Mayr-like (naive) dispersalism, dispersal events cannot be explained mechanistically: are the result of chance (given enough time...). Modern dispersalism should be able explain mechanistically the process resulting in dispersal (range-expansion).

cheers

sergio