[Taxacom] Biogeography of Australasia

[Stephen Thorpe]

The distinction between vicariance vs. dispersal scenarios really only makes sense for terrestrial allopatric species separated on oceanic islands. Then we can ask if one of the species is derived from ancestors which did not need to swim or fly from another island. This would be vicariance. Amphibians are good examples, as they can't fly (actively or passively) and they can't tolerate seawater.
 
Stephen
 

________________________________
From: Michael Heads <m.j.heads at gmail.com>
To: JF Mate <aphodiinaemate at gmail.com> 
Cc: Taxacom <taxacom at mailman.nhm.ku.edu> 
Sent: Tuesday, 25 March 2014 4:27 PM
Subject: Re: [Taxacom] Biogeography of Australasia


Hi Jason,

You said:

'This is a play on words. There is no valid/clear-cut distinction
between "chance dispersal", "range extension" or your "dispersal" vs
the meaning of the word as used by most biologists. I understand that
panbiogeography requires this (non-existant) difference to distinguish
itself, but in the end you only need a few observed cases of organisms
crossing barriers to show that, given the right conditions, dispersal
is a valid mechanism.'

The difference between normal, observed dispersal discussed by ecologists
(e.g. weeds dispersing into a garden), and chance, 'jump' or 'long
distance' dispersal as invoked by evolutionists, is that the former does
not involve differentiation, whereas the latter is proposed as a mode of
speciation.

Dispersal theory explains range overlap by dispersal, but also explains
allopatry by dispersal. Vicariance theory explains range overlap by
dispersal, but explains allopatry by vicariance. Note that the dispersal
invoked in vicariance theory is caused by geological change, whereas
dispersal as invoked by dispersal theory to explain allopatry, is caused by
chance.

Michael


On Sun, Mar 23, 2014 at 8:03 AM, JF Mate <aphodiinaemate at gmail.com> wrote:

> Mostly a reply to John but a sprinkling to Michael as well
>
> The use of quotes such as "It was Darwin who invoked the concept of
> miracles for anyone denouncing his theory of centers of origin and
> dispersal.  You are welcome to believe in extraordinary events ..."
> suggests that, either by accident or design, you (John) are implying
> dispersal is a mechanism akin to religion. That and the daily readings
> suggest baiting.
>
> As to why congruence of phylogeny and known geological events is
> important (your words):  "...sequence of geological events may
> indicate that the phylogeny predates the geology, is related to a
> different geology, or that the geological reconstruction is wrong."
> John, this makes Panbiogeography unfalsifiable. Your fallback line is
> "geology/genes/phylogeny" could be wrong if they don´t match a purely
> vicariant model. Yes, I am sure that as more evidence acumulates the
> biogeographical scenarios of certain groups will have to change. But
> where panbiogeography fails is in the closed, one size-fits-all
> mechanism department. Science is never "the last word" but the best
> fit to facts. By using this to shield Panbiogeography you are
> purposefully using scientific uncertainty to protect your ideas.
>
> As to "The significance of observed cases of dispersal of highly
> vagile species as evidence of chance dispersal being a significant
> force in biogeography is questionable and does not predict the
> tectonic correlations between good and poor dispersers (in the sense
> of means of dispersal)." There are plenty of examples of species
> (mostly good flyers) which have crossed significant barriers (even
> oceans) and colonized new areas in recent history. How are these
> examples not appropriate to the discussion? As for successful
> colonization, just look at gardeners in Europe or NA. Thousands of
> introduced, carefully nurtured plants, often cultured for generations
> and only a small fraction ever becomes naturalized. I acknowledge the
> fact that successful dispersal over significant barriers (sea, major
> ranges) can be an unlikely event on a daily event but over millions of
> years a small probability can really make a impact. The mechanism is
> certainly common enough to suggest it does not require divine
> intervention to happen.
>
> "Similarly, repopulation does not substanciate chance dispersal as a
> significant force in the sense of chance dispersal being a major
> mechanism in biogeography." and Michael "No-one is arguing that
> dispersal is a significant force. All organisms have dispersed to
> their current locations. Dispersal can be observed every day.
> Vicariance biogeography has never denied dispersal - you can't just
> have vicariance otherwise there would only be a single taxon in any
> area."
>
> This is a play on words. There is no valid/clear-cut distinction
> between "chance dispersal", "range extension" or your "dispersal" vs
> the meaning of the word as used by most biologists. I understand that
> panbiogeography requires this (non-existant) difference to distinguish
> itself, but in the end you only need a few observed cases of organisms
> crossing barriers to show that, given the right conditions, dispersal
> is a valid mechanism. Maybe not 99% of the time, but chance plays a
> bigger part in evolution than 0, and that is what matters.
>
> Best
>
> Jason
>
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> Celebrating 27 years of Taxacom in 2014.
>



-- 
Dunedin, New Zealand.

My recent books:

*Molecular panbiogeography of the tropics.* 2012. University of California
Press, Berkeley. www.ucpress.edu/book.php?isbn=9780520271968

*Biogeography of Australasia:  A molecular analysis*. 2014. Cambridge
University Press, Cambridge. www.cambridge.org/9781107041028 

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Celebrating 27 years of Taxacom in 2014.