[Taxacom] RE Dispersal clarifications -wallaces line

[John Grehan]

In reference to Jason's reference to Wallace's line, this turns up a lot in dispersalist literature as some kind of barrier, but as Heads notes, the distribution of taxa either side has no necessary meaning for dispersal. Barriers are funny inventions. If something is only on one side of the 'barrier' that just proves it's a barrier. If something is on both sides that just proves the barrier was crossed (i.e. the barrier withstands falsification.

An example of where Wallace's line may as well be non existent is found with the ghost moth genus Endoclita which ranges all the way between Sri Lanka and Southern India to northern Japan and Far East Russia, Taiwan, and South East Asia all the way to the Island of Halmahera (with an endemic species) about 120 km from the New Guinea mainland. The typical dispersal story would probably argue that this group dispersed from an imagined center of origin in Asia and where the southern limit just happens to be where dispersed to at this time, and yet the group is not recorded from the lesser Sunda where the geographic gap between Java and Bali is just a hop, skip and a jump by comparison (20 km). The absence of otherwise widespread taxa on the Lesser Sunda is found in many eastern Asian taxa that are also present in Borneo-New Guinea (and quite a few that are disjunct to Madagascar or nearby islands). So this absence may be real for Endoclita as well, but that will have to await future corroboration.

Further, Endoclita vicariates with respect to Aenetus, a potentially sister genus widespread through Australasia with its north-western limit in New Guinea, Ambon and Aru (the latter at the eastern end of the lesser Sunda). This vicariant pattern is sensible in the context of a formerly widespread ancestor ranging over both eastern Asia and Australasia, with the point of differentiation perhaps being focused, not on Wallace's line, but convergence of the Asian, Australia, and Philippine plates.

John Grehan



-----Original Message-----
From: taxacom-bounces at mailman.nhm.ku.edu [mailto:taxacom-bounces at mailman.nhm.ku.edu] On Behalf Of Michael Heads
Sent: Friday, June 10, 2011 10:52 PM
To: taxacom at mailman.nhm.ku.edu
Subject: Re: [Taxacom] Dispersal clarifications

 
 I cannot speak for the flora of New Zealand but I can vouch for the dozens of butterflies and scarabs species  that span over the Wallace line. The point is we can find examples to support both mechanisms. 
 
MH: A clade's presence on both sides of Wallace's line does not support dispersal, it just supports lack of differentiation.  It's like saying a group at A and B implies dispersal from one to the other, but in fact it might have got to A and B by evolvng there. If you have a very precise break like Wallace's line in many (not all) scarabs, lepidoptera, plants,  fish,  birds, mammals,etc. it's probably an interesting phenomenon, not just an accident of 'chance dispersal'. 



MH: Yes, but no-one does that, not even dispersalists. Biogeographers look at many examples of allopatry and develop a concept of chance dispersal or vicariance. Then they test those concepts on other examples. Obviously, 'chance' can explain anything at all and I'm sure you'll be able to explain all your beetle distributions easily. But as more and more new examples of very precise, complex allopatry pile up in the molecular work, even in marine groups with pelagic larvae and now protozoans, people are starting to re-examine the old concepts. 

Best
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