[Taxacom] Dispersal clarifications

[Michael Heads]

Hi Jason, 
 
I've added my new comments below. 
 
Michael


Wellington, New Zealand.

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

--- On Sat, 4/6/11, Jason Mate <jfmate at hotmail.com> wrote:


From: Jason Mate <jfmate at hotmail.com>
Subject: Re: [Taxacom] Dispersal clarifications
To: "Taxacom" <taxacom at mailman.nhm.ku.edu>
Received: Saturday, 4 June, 2011, 10:21 AM



Hi Michael, my thoughts on your comments below:

MH:About the terms you mention: 'dispersalist theory' and its advocates accept chance dispersal as a mode of speciation. 
JFM:So is dispersal of a species from a mainland source to an island acceptable dispersal?

MH: Many mainland populations expand their range onto an island, e.g. weeds after a volcanic eruption. This can be observed and studied. On the other hand, simply assuming that a distinctive island endemic 'dispersed there' from the mainland by 'biogeographic', 'jump' or 'waif dispersal' is not acceptable. It could be there for many other reasons (e.g. it could be a relic inherited from a previous island in the vicinity).    
 
MH: 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.
JFM:Are all purported cases of dispersal similarly extreme? 
 
MH: No, of course not. I think you're aware of examples. But the principle is the same - dispersal here is just a word that is invoked, a sound you make with your mouth. You don't have to actually study anything. You just say: 'oh, it must have been dispersal, although strangely enough we don't know of any possible means'. Editors seem quite happy with this.  
 
JM: Do you consider the dispersal of flying organisms equally mysterious? 
 
MH: I don't think dispersal is mysterious. It is the dispersalists who refer to means of dispersal as mysterious ('we can only speculate how it got here...').
 
JM: Don´t you think that the probability of dispersal correlates with the organisms ability to disperse?
 
MH: It depends which concept of dispersal you're referring to. I don't think 'chance dispersal', with speciation, sensu Mayr etc., exists. On the other hand, all plants and animals move, and their pattern of movement is related to their means of dispersal. This movement may have little or nothing to do with the geographic distribution of their clade. A migratory bird moves in a very different way to an earthworm, and yet major clades in both groups can have very similar distribution patterns.  
 

MH: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. 
JFM:This is a qualitative remark. A 1 in a million event, although rare, although ´chance´, still has a probability, and the sum of all the probabilities describes and event. Thus, although accidents can cluster and these are what people may find interesting, a well lit corner in a speed-controlled zone with good visibility can also have the odd accident. The underlying cause of this accident may be something completely random, with a low repeatability (maybe old drivers with astigmatism at dusk fail to see pedestrians). But this isolated case is still part of the totality of accidents for the area under study. So I´d say that you are confusing probability with repeatability. 
 
MH: So you don't think statistical analysis is useful in science (or road design)?
 
 
JM: By studying the biota of an area you work out the frequency (probability) of each mechanism. Maybe dispersal is not of interest to you since you are only interested in vicariance, but to taxonomists or ecologists dispersal can be equally interesting.
 
MH: Why do you think I'm only interested in vicariance and not dispersal? I'm very interested in ecological movement in groups, range expansion, daily and annual migrations, and, above all, dispersal in the most general sense: 'any and all changes of position' (Clements, c.1930). Allopatry is caused by vicariance, overlap is caused by normal range expansion (not chance dispersal).

MH: 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.
JFM:Chance in the earlier sense? You mean the magical sense? Each throw of the dice is independent from the previous ones. So even if I could work out the probability of dispersal under your system (I can´t because your philosophical frame does not allow it) it would be based on studying the origin of a certain number of clades and 
 
MH: I'm not sure what happened to the rest of your sentence here. You can work out the probability of dispersal easily by making observations. A Rhipidura fantail appears outside my window once a day. Zosterops is there maybe three or four times a day.  

MH:'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.
JFM:The simple movement observed every day? So if you don´t observe it every day it does not occur, i.e. migration? 
 
MH: Dispersal can be observed every day: birds, insects, etc.  
 
JFM: What about vagrant specimens? 
 
MH: All plants and animals are vagrants, at least at one stage in their life. 
 
JFM: Dispersal is not an everyday, week or even year event. It is rare, precisely because it is a high failure, overwhelmingly lethal, no second prize strategy.
 
MH: I see dispersal happening all ther time. Every organism (including everyone reading this) has dispersed to their current position.
 

MH: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.
JFM:You are the only one here saying that vicariance is not an accepted mechanism that accounts for organisms distributions. 
 

MH: Vicariance is still mainly accepted in theory. In practice, most papers on intercontinental groups of higher plants or animals treat fossil-calibrated dates (illogically) as maximum ages, automatically giving Tertiary dates for clades that they then use to rule out intercontinental vicariance. They then use a program that assumes a basal grade occupies the center of origin and - voila - your new molecular center of origin/dispersal paper. (The phylogenies are fantastic, though).  
   The idea that a fossil gives a minimum date only is another one still accepted mainly in theory but it is making progress. 
 
JFM: At the same time, you would expect that sources of colonists would be limited to certain areas that have some sort of advantage (i.e. currents, prevailing winds, shortest distance).
 
MH: Of course. The weeds in my garden tend to come from the neighbors, not from the other side of town.
 
MH: 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. 
JFM:Once again, vicariance is an accepted mechanism.
 
MH: Yes, and by a growing number. But chance dispersal is still used to explain most patterns.  

MH: 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). 
JFM:So vicariance is the only mechanism allowable unless everything there shows otherwise? Why would you assume that the entire biota of an area got there by one mechanism only? What is the logical basis for this?  
 
MH: No-one has ever suggested that vicariance is the only process. That would lead to a situation where every tiny area on Earth had only one organism present but it was endemic there.   
 
Michael
                         
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