[Taxacom] Dispersal example

[John Grehan]

Fred,



My earlier response was more levity than explanation, but dispersalist have
for a long time asserted that chance dispersal can account for any
distribution. But as Mike points out, any such assertion for any individual
case only sets up a litany of contradictions between theory and actual
observation. Mike provides numerous examples in his book where appeal has
been made for chance dispersal that appears to be fully satisfactory for an
individual instance only so long as one avoids looking at any wider
context. For example, Murienne (2009) asserted that long-distance dispersal
was the ?only valid interpretation? for the origin of New Caledonian
groups. But then this insight led Murienne to conclude that the presence of
numerous relict groups ?remains puzzling?. Similarly Gradcolas et al (2008)
concluded that the phylogenetic relicts ?remain puzzlingly enigmatic?. This
is only one of the examples that Mike cites in his book that shows how
dispersalist theory seems more capable of generating mysteries than real
solutions. If somehow the idea of dispersalist theory generating puzzles
and enigmas is considered more convincing to the majority of biogeographers
then ok, but it does not seem a very productive avenue for me.



Mike also cites the case of polycentropoid Tricoptera where the New
Caledonian species belong to three clades, NC-Australia, NC-NZ/Aust-Chile,
NC-Chile/northern neotropics. The three clades are allopatric except for
New Caledonia. Johanson et al 2012 said this was because NC was colonized
three times, once from each of Aust, NZ and Chile. But why only once? It?s
not a simple matter of established populations since the dispersal theory
allows for three individual dispersals. And this is not to even consider
why the taxa are not anywhere else if they were so capable of long-distance
dispersal.



As one list member pointed out, just considering only one option falls
short. The authors above failed to consider vicariance and what Mike has
done is to show that for all the vast number of dispersalist accounts there
is a vicariance alternative to be considered.



I noticed that in the dispersalist reviews of Mike?s first book there was
no real effort to refute the vicariance evidence and I suspect that this
will prove to be true of such reviews for his current book. I don?t know if
any on this list have yet read the book, but I certainly hope that the
skeptics will provide the proof of their pudding when they do.



John Grehan


On Tue, Apr 15, 2014 at 1:55 AM, Michael Heads <m.j.heads at gmail.com> wrote:

> Hi Fred,
>
> Your founder-takes-all model would be a very good explanation for the
> common arrangement of simple allopatry, and it has recently been promoted
> by Waters et al. (2013 in TREE). But it doesn't account for another very
> common pattern that can be illustrated by New Zealand groups such as the
> plants Hebe, Olearia and Astelia. These groups comprise several clades,
> each in New Zealand and one other area, e.g. NZ + SE Australia, NZ + New
> Guinea, NZ + New Caledonia, NZ + Fiji, NZ + Tahiti, NZ + Chile.
>
> In this 'star pattern' there are two main aspects to explain: (1) the high
> level of allopatry of the clades, and (2) their overlap in NZ. The
> founder-takes-all (competition) model  explains the allopatry, but it is
> inconsistent with the overlap in NZ. I think the pattern is the result of a
> widespread ancestor, followed by vicariance around a break in NZ, followed
> by local overlap in NZ. If the stages in the process are mediated by
> tectonics, for example, rather than chance dispersal, this would explain
> why the pattern is repeated in different groups.
>
> Michael
>
>
> On Tue, Apr 15, 2014 at 7:15 AM, Fred Schueler <bckcdb at istar.ca> wrote:
>
> > On 4/14/2014 1:17 PM, John Grehan wrote:
> >
> > > I think it is less a case of considering long-distance dispersal as
> > > unlikely and ask why it did not occur twice, but rather why did it
> occur
> > > just once, time and time again for group after group...
> >
> > * as the old Irish bards used to say: "Not difficult." Once the group is
> > established, a second colonization is made unlikely by the presence of
> > the established populations of the taxon.
> >
> > fred.
> > ------------------------------------------------------------
> >           Frederick W. Schueler & Aleta Karstad
> > Mudpuppy Night in Oxford Mills - http://pinicola.ca/mudpup1.htm
> > Daily Paintings - http://karstaddailypaintings.blogspot.com/
> >           RR#2 Bishops Mills, Ontario, Canada K0G 1T0
> >    on the Smiths Falls Limestone Plain 44* 52'N 75* 42'W
> >     (613)258-3107 <bckcdb at istar.ca> http://pinicola.ca/
> > The 2014 Bishops Mills Calendar Festival:
> > Fred:
> >
> >
> http://karstaddailypaintings.blogspot.ca/2013/12/the-fred-calendar-for-2014.html
> > - data input
> > Aleta: http://www.lulu.com/content/14204755 - paintings
> > Jennifer:
> > http://www.zazzle.com/free_range_wall_calendar-158319768903951661  -
> > free ranging livestock
> > ------------------------------------------------------------
> >
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>
>
> --
> Dunedin, New Zealand.
>
> My recent books:
>
> *Molecular panbiogeography of the tropics.* 2012. University of California
> Press, Berkeley. www.ucpress.edu/book.php?isbn—80520271968
>
> *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.
>