[Taxacom] Taxacom Digest, Vol 185, Issue 14

Wed Sep 22 20:15:25 CDT 2021

Rich,
It's always good to have your input. I agree with all your points except
one.

Your 'Scenario 3 was:' A population rapidly expands the size of its
geographic footprint over time.  The *conditions that facilitated that
geographic expansion are short-lived*, resulting in a limitation or
elimination of gene flow...'. [bold added] . You commented: 'I *think*
Scenario 3 is an example of chance = jump = long distance dispersal', but
this is not 'chance dispersal' as used by most authors. In chance
dispersal, dispersal doesn't stop because the *conditions* change - it's
random, and not related to any other factor, physical or biological. A
single, one-in-ten million years freak event that no-one could have
predicted. It is thought, by its proponents, to be mediated not by normal
means of dispersal but by 'non-standard means'.'.

At a single biogeographic break zone, the breaks in all pairs of sister
groups, with one on one side and one on the other, are thought to have
different ages. The ages vs taxa graph for a single break zone always shows
a smooth curve, and the break in each pair is attributed to a different,
idiosyncratici event (chance dispersal) unrelated to conditions. If you
accepted the dates, this would be excellent evidence that allopatry is
*never* the result of vicariance, a community-wide process caused by
changing conditions.

One example of your scenario 3 would be the great expansion of many coastal
taxa with the epicontinental marine transgressions of the Cretaceous, seen
in all the continents.  When the *conditions changed* and the seas
regressed, huge numbers of widespread coastal groups were stranded inland
and differentiated. This is not chance dispersal, but a series of events
that has affected the whole community globally.

We are not saying that unique, one-in-ten million year events that are
unrelated to any particular factor (conditions) don't happen in individual
groups, We are saying that these types of unique events in single
individuals of a single species in a community do not explain the main
biogeographic/evolutionary patterns that are observed. These are all
repeated in large numbers of clades, e.g. the 'enigmatic' Hawaii - SE
Polynesia connection seen in so many marine and land groups.

On Thu, Sep 23, 2021 at 10:46 AM Richard Pyle <deepreef at bishopmuseum.org>
wrote:

> I already know I'm going to regret sending this message.  But wisdom is
> hard-won, and I've evidently not won it yet, so here goes:
>
> Michael heads wrote:
> > [...] nearly always
> > by in situ allopatric differentiation (vicariance) (not chance = jump =
> long
> > distance dispersal)
>
> With the *sincere* hope of not stirring up (yet another) dispersal vs.
> vicariance debate (ha... fat chance...), I want to parse the statement
> above and ask for a bit of clarification, and a sincere question.
>
> First, we have the statement, "nearly always by in situ allopatric
> differentiation".  I'm on board with that much, for sure (I certainly
> believe "sympatric differentiation" *can* happen, and it might even happen
> in a situation that isn't really just something I might label as
> "micro-allopatry").  So that's not my request for clarification or my
> question.
>
> My request for clarification is this:
>
> When you structure the sentence, "in situ allopatric differentiation
> (vicariance) (not chance = jump = long distance dispersal)", are you saying:
> 1) "Allopatric differentiation is associated with vicariance; and not
> associated with chance=jump=long distance dispersal"; or
> 2) "Allopatric differentiation is associated with both vicariance and
> chance=jump=long distance dispersal, but when I say "nearly always" I am
> referring to vicariance as the basis of allopatric differentiation"
> 3) Something else altogether?
>
> Regardless of the answer, I want to get my head around the seemingly
> dichotomous and mutually exclusive notions of "vicariance" vs.
> "chance=jump=long distance dispersal".
>
> My premises are:
> 1) "Taxa" exist as sets of many individuals that exist across space and
> time somewhere/when on planet Earth.
> 2) For simplicity, I will refer to such sets of individuals as
> "populations".
> 3) Populations of many/most organisms do not encompass the entire planet,
> and are generally bounded in some way (e.g., terrestrial vs. aquatic; one
> continent or another; only on one island; etc.).
> 4) Population boundaries may fluctuate over time; sometimes occupying a
> smaller footprint on Earth, and at other times occupying a larger footprint.
> 5) Allopatric differentiation occurs among subsets of populations over
> time, presumably as the result of various evolutionary processes, and
> probably involving uneven or discontinuous rates of gene flow among
> individuals between the different subsets.
>
> If I haven't slipped up on the phrasing of the above premises, I'd like to
> frame my question.
>
> Scenario 1: A population slowly expands the size of its geographic
> footprint over time.  Something happens that limits or eliminates gene flow
> between one part of the population and another part of the population.
> Over time, the separated parts of the population accumulate differences
> allopatrically.
>
> Scenario 2: A population rapidly expands the size of its geographic
> footprint over time.  Something happens that limits or eliminates gene flow
> between one part of the population and another part of the population.
> Over time, the separated parts of the population accumulate differences
> allopatrically.
>
> Scenario 3: A population rapidly expands the size of its geographic
> footprint over time.  The conditions that facilitated that geographic
> expansion are short-lived, resulting in a limitation or elimination of gene
> flow between one part of the population and another part of the
> population.  Over time, the separated parts of the population accumulate
> differences allopatrically.
>
> I *think* Scenario 1 is an example of vicariance.  And I *think* Scenario
> 3 is an example of chance = jump = long distance dispersal.  So my question
> is:  Are these really dichotomous and mutually exclusive alternatives?  Or
> are they more like end-points on a spectrum, with many "flavors" of
> intermediate scenarios (e.g., Scenario 2) in-between?
>
> In other words, how much of the debate between alternate modes of
> establishing circumstances for allopatric differentiation (i.e., vicariance
> vs. dispersal) ultimately boils down to semantics, rather than evolutionary
> biology?
>
> ...climbing back under a rock for shelter...
>
> Aloha,
> Rich
>
> Richard L. Pyle, PhD
> Senior Curator of Ichthyology | Director of XCoRE
> Bernice Pauahi Bishop Museum
> 1525 Bernice Street, Honolulu, HI 96817-2704
> Office: (808) 848-4115;  Fax: (808) 847-8252
> eMail: deepreef at bishopmuseum.org
> BishopMuseum.org
> Our Mission: Bishop Museum inspires our community and visitors through the
> exploration and celebration of the extraordinary history, culture, and
> environment of Hawaiʻi and the Pacific.
>
> > -----Original Message-----
> > From: Taxacom <taxacom-bounces at mailman.nhm.ku.edu> On Behalf Of
> > Michael Heads via Taxacom
> > Sent: Wednesday, September 22, 2021 11:47 AM
> > To: Brendon E. Boudinot <boudinotb at gmail.com>
> > Cc: Taxacom <taxacom at mailman.nhm.ku.edu>
> > Subject: Re: [Taxacom] Taxacom Digest, Vol 185, Issue 14
> >
> > Good questions.
> >
> > 1. The original distribution of a clade is established by evolution -
> nearly always
> > by in situ allopatric differentiation (vicariance) (not chance = jump =
> long
> > distance dispersal). The spatial pattern of differentiation is repeated
> in a large
> > number of taxa in the region and so has a general (tectonic or climatic)
> cause.
> > This original area of a clade may be very large, e.g. if a worldwide form
> > differentiates into northern and southern hemisphere forms. The original
> > distribution may be modified by subsequent range expansion as part of a
> > community-wide 'geodispersal' (not by chance dispersal, a mode of
> speciation)
> > caused by geological/climatic change, or by range contraction.
> >
> > 2. As far as I know, areas of endemism for ants are always repeated in
> other
> > groups, consistent with the processes in 1. One example is  the group of
> > Leptomyrmex species in Australia with a phylogenetic/biogeographic node
> at
> > the McPherson-Macleay Overlap (see my Australasia book, Fig. 4.16).
> >
> >
> >
> > On Wed, Sep 22, 2021 at 5:54 PM Brendon E. Boudinot
> > <boudinotb at gmail.com>
> > wrote:
> >
> > > Dear John and Michael,
> > >
> > > Would you explain in ≤ 300 words for each point (vis à vis an
> abstract):
> > > 1. How extant (and extinct) species came to be distributed as they are.
> > > 2. How ant biogeography can be explained by point 1 above.
> > >
> > > This would be the best way of proceeding in our conversation.
> > >
> > > All the best,
> > > Brendon
> > >
> >
> >
> > --
> > Dunedin, New Zealand.
> >
> > My books:
> >
> > *Biogeography and evolution in New Zealand. *Taylor and Francis/CRC, Boca
> > Raton FL. 2017.
> > https://www.routledge.com/Biogeography-and-Evolution-in-New-
> > Zealand/Heads/p/book/9781498751872
> >
> >
> > *Biogeography of Australasia:  A molecular analysis*. Cambridge
> University
> > Press, Cambridge. 2014. www.cambridge.org/9781107041028
> >
> >
> > *Molecular panbiogeography of the tropics. *University of California
> Press,
> > Berkeley. 2012. www.ucpress.edu/book.php?isbn=9780520271968
> >
> >
> > *Panbiogeography: Tracking the history of life*. Oxford University
> Press, New
> > York. 1999. (With R. Craw and J. Grehan).
> > http://books.google.co.nz/books?id=Bm0_QQ3Z6GUC
> > <http://books.google.co.nz/books?id=Bm0_QQ3Z6GUC&dq=panbiogeography&
> > source=gbs_navlinks_s>
> > _______________________________________________
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> >
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> > http://mailman.nhm.ku.edu/cgi-bin/mailman/listinfo/taxacom
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> > searched at: http://taxacom.markmail.org
> >
> > Nurturing nuance while assailing ambiguity for about 34 years, 1987-2021.
>
>

-- 
Dunedin, New Zealand.

My books:

*Biogeography and evolution in New Zealand. *Taylor and Francis/CRC, Boca
Raton FL. 2017.
https://www.routledge.com/Biogeography-and-Evolution-in-New-Zealand/Heads/p/book/9781498751872

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

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

*Panbiogeography: Tracking the history of life*. Oxford University Press,
New York. 1999. (With R. Craw and J. Grehan).
http://books.google.co.nz/books?id=Bm0_QQ3Z6GUC
<http://books.google.co.nz/books?id=Bm0_QQ3Z6GUC&dq=panbiogeography&source=gbs_navlinks_s>