[Taxacom] Taxacom Digest, Vol 185, Issue 14

[John Grehan]

Rich - I'm not going to jump in on this conversation since it is directed
to Heads, but I would point out that I have seen one instance where the
proponents of a 'freak' event did indeed use that term. Cheers, John

On Wed, Sep 22, 2021 at 10:19 PM Richard Pyle via Taxacom <
taxacom at mailman.nhm.ku.edu> wrote:

> Thanks!  This is great feedback, and exactly the sort I was hoping for.
>
>
>
> > 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'.'.
>
>
>
> Perfect!  I chose my words carefully in articulating Scenario 3 with the
> hope of focusing on exactly the issue you raised.  So let’s compare these
> two things:
>
> 1.      The conditions that facilitated that geographic expansion are
> short-lived, resulting in a limitation or elimination of gene flow.
> 2.      The circumstances that facilitated that geographic expansion are
> exceedingly rare (e.g., only occurred once in 10 million years), resulting
> in a limitation or elimination of gene flow.
>
>
>
> I would argue that #2 above represents the same as what you report that
> most authors mean when they refer to “chance dispersal”.  I know that I
> didn’t use words like “random”, “chance”, and “freak” in my expression of
> the concept, but it’s debatable whether “random” even exists in the
> universe (“haphazard” might be a better term), “chance” suffers some of the
> same pitfalls as “random”, and “freak” is a bit hyperbolic.  But in both
> your wording and mine, the situation is essentially the same:
>
> *       A patch of Earth was never occupied by any individuals of a given
> population prior to time X
> *       After time X, at least one individual* of said population occupied
> that patch of Earth
>
> *If we’re talking about sexually reproducing organisms, and we’re talking
> about subsequent allopatric differentiation through evolutionary processes,
> then presumably at least two such organisms shared the same patch of novel
> Earth for a period that overlapped both of their lifespans.
>
>
>
> But the key thing is that we have different patches of Earth, and
> eventually we have organisms derived from the same source population
> occurring on those different patches, and genetic connectivity between them
> is not maintained (for whatever reason).
>
>
>
> I can’t speak for “most authors”, but my area of interest & expertise
> concerns the majority of Earth’s surface (and likely the majority of
> organisms that have ever lived; and hence the majority of biogeographic
> history on Earth) – that is, the Oceans.  Specifically, my area of
> interest/expertise involves coral-reef organisms, most of which have
> planktonic larvae -- which very likely facilitate the majority of
> historical genetic transfer over large distances via movements of said
> larvae.  In that paradigm, what you describe as “random”, “chance”, “freak”
> events that lead to individuals traversing some distance to places not
> previously occupied by members of the same source population might not just
> be possible; but perhaps even represents the norm.
>
>
>
> > 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.
>
>
>
> Well… one could argue that all living things on Earth share exactly the
> same “age” (at least if we assume a single origin for all life on Earth)…
> but I get what you mean here (i.e., the “birth” of one group happened when
> members of its population first trod on novel ground/sea, never to share
> genes with their elder siblings in the source population).
>
>
>
> > 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.
>
>
>
> Yeah, but I see this more as one end of a spectrum of evolutionary
> (hi)stories, the opposite end of which would be a one-in-ten-million-year
> set of conditions that allowed a group of planktonic larvae from, say, the
> western Pacific to settle off the shores of, say, the Hawaiian Islands –
> ultimately yielding two genetically isolated populations (after which
> sufficient allopatric differentiation yielded what we hairless-ape
> taxonomists deem to represent as distinct species).
>
>
>
> And this is my point – emphasizing the extreme ends of the spectrum might
> present a false dichotomy in modes and “circumstances” that allow an
> expanded subset of a source population to fail to share genes with its
> brethren for long periods of subsequent time.
>
>
>
> > 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
>
>
>
> I can’t imagine any examples of an “event” that was “unrelated to any
> particular factor (conditions)”.  I think what you might mean here is that
> the set of factors/conditions affected only a small set of organisms, as
> opposed to many different kinds of organisms at the same time? I mean… laws
> of physics are at play with pretty-much any organism’s movements across
> space.  But maybe I’m interpreting factors/conditions too generally here?
>
>
>
> > 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
>
>
>
> OK, I think I’m getting a better sense of what you’re talking about here.
> But it still seems like a bit of semantics.  I’ll propose two more
> hypothetical scenarios to focus on the distinction:
>
> 1.      A one-in-10-million-year anomalous set of circumstances (precise
> combination of wind, current, temperature, storm pattern, whatever)
> facilitates the transfer of planktonic larvae of multiple different species
> from one part of the Pacific to another, leading to allopatric
> differentiation between source populations and founder populations.
> 2.      A one-in-10-million-year anomalous set of circumstances (precise
> combination of wind, current, temperature, storm pattern, spawning
> behavior, genetic mutation, whatever) facilitates the transfer of
> planktonic larvae of a single species from one part of the Pacific to
> another, leading to allopatric differentiation between source populations
> and founder populations.
>
> [differences highlighted in bold].
>
>
>
> Would you consider #1 vicariance and #2 chance dispersal, or both
> vicariance, or both chance dispersal, or have I failed to provide enough
> information to distinguish these modes?
>
>
>
> Sorry to belabor this, but I’m genuinely not trying to be combative
> (‘though maybe just a bit snarky in some places – not to be insulting, but
> to keep the tone of the conversation light).  I’ve watched these
> discussions for many years on Taxacom, and while I generally follow them
> with at least some level of understanding, I’ve always been a little bit
> fuzzy on where to draw the line between vicariant vs. dispersal modes of
> population expansion and subsequent allopatry.
>
>
>
> 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
>
>  <http://hbs.bishopmuseum.org/staff/pylerichard.html> 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.
>
>
>
> From: Michael Heads <m.j.heads at gmail.com>
> Sent: Wednesday, September 22, 2021 3:15 PM
> To: Richard Pyle <deepreef at bishopmuseum.org>
> Cc: Brendon E. Boudinot <boudinotb at gmail.com>; Taxacom <
> taxacom at mailman.nhm.ku.edu>
> Subject: Re: [Taxacom] Taxacom Digest, Vol 185, Issue 14
>
>
>
> 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
> <mailto: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 <mailto: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 <mailto:
> 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 <mailto:boudinotb at gmail.com>
> >
> > Cc: Taxacom <taxacom at mailman.nhm.ku.edu <mailto:
> 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 <mailto: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 <
> http://www.cambridge.org/9781107041028>
> >
> >
> > *Molecular panbiogeography of the tropics. *University of California
> Press,
> > Berkeley. 2012. www.ucpress.edu/book.php?isbn=9780520271968 <
> http://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 <
> http://books.google.co.nz/books?id=Bm0_QQ3Z6GUC&dq=panbiogeography&>
> &dq=panbiogeography&
> > source=gbs_navlinks_s>
> > _______________________________________________
> > Taxacom Mailing List
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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 <
> http://www.cambridge.org/9781107041028>
>
>
>
> Molecular panbiogeography of the tropics. University of California Press,
> Berkeley. 2012. www.ucpress.edu/book.php?isbn=9780520271968 <
> http://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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> The Taxacom email archive back to 1992 can be searched at:
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> Nurturing nuance while assailing ambiguity for about 34 years, 1987-2021.
>