[Taxacom] Taxacom Digest, Vol 185, Issue 14

Wed Sep 22 21:19:34 CDT 2021

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>
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> 
> 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> 