[Taxacom] Dispersal clarifications

[Michael Heads]

Hi Jason,
 
About the terms you mention: 'dispersalist theory' and its advocates accept chance dispersal as a mode of speciation. An organism 'disperses across a barrier', often by mysterious means. For example, an organism that moves a few centimeters in its lifetime is theorized to have jumped - just once - 10 000 km across the Pacific. Vicariance theory accepts that allopatric speciation is caused by vicariance and that range expansion (physical movement) only explains overlap.   
   
Chance: this term can be used in the sense of sweeping something under the carpet - 'oh, event x must have just been an accident'. Alternatively, you can go out and collect data and actually work out the 'probabilities'. These are also referred to as 'chance' - as in 'the chance of event x happening', but 'chance' here is a completely different concept. For example, a road accident may have happened on a corner where it turns out, following a statistical analysis, 'accidents' regularly happen. It wasn't simply an accident, there was a general, underlying explanation. This change in approach was the psychological revolution of Fermat and Pascal that gave birth to probability theory. 
 
Dispersal theory as seen in biogeography and systematics journals deals with phylogeny and relies on 'chance' in the earlier sense. In contrast, dispersal studies in ecological journals deal with observations (not inferences) of simple movement and are full of probability analyses - they use chance in the modern sense.
 
   'Ecological dispersal' is just the simple movement of organisms observed every day. A weed disperses into a garden, but it doesn't differentiate into a new taxon. An organism can be where it is because it moved there or because it evolved there.
 
  Neatly allopatric (dovetailing) geographic structure repeated in different groups can't be explained by (mere) 'chance'. It could be the result of a general process such as vicariance.
   
Vicariance might be falsified by finding that the distributions of taxa in a region don't share similar phylogenetic/biogeographic breaks. But this could be due to original allopatry followed by range expansion. Unlike the text-book examples which everyone learns, most broad theories don't fall over instantaneously because someone finds a single fact (see the detailed critiques of 'naive falsificationism' by Lakatos, Feyerabend etc.). Usually there is a slow accumulation of 'anomalous' data and eventually there is a broad shift of opinion. If all the molecular phylogenies showed no geographic structure and instead taxa tended to sort on colour or size or hairiness, rather than locality, no-one would be looking at  vicariance and it would just fade away. But the clades nearly all show impressive allopatry - this is why so many authors are  putting distribution maps of their clades in the graphical abstracts at Mol. Phylogen. Evol. and lots
 of people are discussing vicariance. If you Google scholar 'vicariance', here are the numbers of articles mentioning the word for 5-year periods, starting with 1960-1965 when Croizat wrote his big books on the topic: 27, 40, 66, 218, 770, 1040, 1380, 2280, 4140, 6600 (2005-2010). 
  
Michael
 

Wellington, New Zealand.

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

--- On Fri, 3/6/11, Jason Mate <jfmate at hotmail.com> wrote:


From: Jason Mate <jfmate at hotmail.com>
Subject: [Taxacom] Dispersal clarifications
To: "Taxacom" <taxacom at mailman.nhm.ku.edu>
Received: Friday, 3 June, 2011, 8:16 AM



Michael,
sorry for not replying sooner. After reading your email I am even more confused than before. Do you think you could answer the following questions?:
What is a dispersalist? Name tags may be useful to create a sense of us-and-them but it seems like a red herring.
Chance. I´ve seen your reply to Richard but it is not very clear. Maybe you are talking about probability? If so, are you saying that "dispersalists" (whatever they may be) are invoquing some sort of magical forces? 
What is ecological dispersal? I am interested in your operating definition.
Geographic structure: are you claiming this is evidence for vicariance?
How would you falsify vicariance? Just curious.

Best
Jason                           
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