[Taxacom] Taxacom Digest, Vol 185, Issue 14

[Richard Pyle]

OK, thanks.  I guess I’m focused more on the conceptual, than on what people in the broader biogeography biz are actually doing/saying/promoting/claiming/etc.

 

I’m still not sure I share the same definition for vicariance and dispersal as you are using for these terms, but at least I now better understand how you are using them.

 

And, of course, NO population is panmictic or homogeneous.  Again, we’re talking about a spectrum of degrees of homogeneity of gene exchange across space and time.  Some patterns are more homogeneous than others.

 

I suppose my final point is that “dispersal potential” (for lack of a better term) should not be thought of as fixed for any given species/subspecies/population/whatever.  It’s not constant – it varies in response to many different biotic and abiotic variables.  

 

But again, I’m not trying to stir up a debate about biogeography – my interest was in better understanding the language and terminology.  Again, I’m grateful for your help in sharing your thoughts.

 

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: Thursday, September 23, 2021 3:11 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

 

RP:  Because we’re talking about “chance” dispersal, and because “chance” can be represented mathematically as a function of probability...,'. 

MH: Chance as a calculated probability has been the normal, scientific understanding of chance ever since Pascal and Fermat. But the concept of chance in 'chance dispersal' is not the same. The probability of a chance dispersal event cannot be calculated. It can happen at any time, over any distance and is not related to any particular circumstances   or means of dispersal, behaviour,physiology etc. This is 'chance' in the old, pre-Pascal sense of 'anything can happen' or 'we don't know why or how' - it's all just 'luck' (the Roman goddess Fortuna). Perhaps this is why people often avoid the term 'chance dispersal'. In the very widely used biogeography package BioGeoBEARS it's called 'jump dispersal', but that's just as bad really - any aerial dispersal is a jump.

 

MH: Whether the pantropical oceanic species comprise many 'species' or many 'subspecies' isn't of special interest - the point is that they are not panmictic and homogeneous, despite having such excellent means of dispersal, larvae and adult. It looks as though they should be pantropical and the same everywhere - if dispersal is the factor - but they are not.  Likewise, the 'paradox' of local endemism and the surprising immobilism of clades in birds and butterflies, commented on by many experts. 

 

The general rule these days is to resolve vicariance/chance dispersal on clock dates. If the date matches a well-known geological event, the node is attributed to vicariance. If it does not, to chance dispersal ('we don't know'). One problem with clock dates is that meta-analyses of many groups' clock ages at single biogeographic boundaries show that the ages form a smooth curve, indicating that vicariance (a community-wide process affecting many groups) never occurs (see earlier mail on this thread).  Who would accept that? 

 

Geological developments are more interesting for practical biogeography than arguments about chance dispersal. For example, revolutionary new proposals of a huge island arc in the East Pacific that extended from off North America south to off Chile. It began to be accreted /subducted to the west coast of the mainland with the opening of the Atlantic.

Mohammadzaheri, A., Sigloch, K., Hosseini, K., & Mihalynuk, M. G. (2021). Subducted lithosphere under South America from multifrequency P wave tomography. Journal of
Geophysical Research: Solid Earth, 126, e2020JB020704.

We discussed their earlier papers on North America in our Galapagos paper.

  <https://ssl.gstatic.com/ui/v1/icons/mail/images/cleardot.gif> 

 

On Fri, Sep 24, 2021 at 6:57 AM Richard Pyle <deepreef at bishopmuseum.org <mailto:deepreef at bishopmuseum.org> > wrote:

Thanks, Michael.

 

> MH: I don't think this is what they mean, because chance dispersal, as invoked, isn't related to circumstances or conditions. 

> A general, community-wide change in conditions or circumstances will not be reflected in rare cases, 

> but across the fauna and flora as a whole.

 

I guess this depends on how broadly one defines “circumstances or conditions”.  If an organism (or minimally two organisms for sexually reproducing things that lead to allopatric differentiation) mange to get to a place where they weren’t born, and where no other members of their kin had ever previously managed to get to, then surely there were *some* circumstances or conditions that facilitated such movement (they walked, flew, or swam; were carried by wind or current; were transported by some other organism; etc.).  Just because we have no idea what circumstances or conditions facilitated such an unprecedented (once in 10 million years!) event, doesn’t mean there were no such circumstances or conditions.  But perhaps now I’m the one of being overly focused on semantics?

 

> MH: Yes, dispersal by planktonic larvae is the norm in many species, just as flight is in birds, 

> and crawling is in worms; it happens all the time (not just once in tens of millions of 

> years in one individual), so it's not chance dispersal – 

 

Yeah, that’s obvious (and not my point).  Gene flow facilitated through regular planktonic larvae dispersal within a defined geographic footprint is just that – gene flow (not dispersal).  My point was in the context that I introduced for this discussion, whereby a population *expands* its distribution through *irregular* planktonic larvae dispersal (once in ten million years), under such circumstances/conditions that are not maintained (and, therefore, gene flow is not maintained).  When I referred to the “norm”, what I meant was that speciation events among marine organisms (especially insular) facilitated by rare/unusual colonization events may be the norm – not that planktonic dispersal maintaining gene flow on shorter timescales is the norm (shorter in this case meaning that gene flow is regular enough that allopatry is never really achieved).

 

But I don’t want to drift into a “is dispersal or vicariance the norm in generating allopatry among marine organisms” debate (we’re not even close to having a true answer to that question).  I wanted to understand how we parse out events in historical biogeography as being either vicariance or chance dispersal.

 

> Now in practice, many entities that were thought to be pantropical oceanic species have turned out 

> (with sequencing) to be complexes of more or less cryptic species, often allopatric. 

 

Well…. I’m HIGLHY tempted to launch into a rant on one of my pet peeves about the artificial distinction between “pantropical oceanic species” and “complexes of more or less cryptic species” … but I’ll do my best to stay on the topic at hand.  But the good news is that I can focus on this distinction to illustrate something on the topic at hand, so here goes…

 

If we assume that “complexes of more or less cryptic species” are collectively evolutionary siblings, and we choose to recognize them as distinct (more or less cryptic) species (instead of succumbing to lumper temptations of thinking about them as representing a single “pantropical oceanic species”) – and especially where evidence from sequencing is what led us to that conclusion – then what we are really suggesting is that historical gene flow between the different cryptic species has been lower than the gene flow within each cryptic species.  Because we’re talking about “chance” dispersal, and because “chance” can be represented mathematically as a function of probability, then it might be helpful to characterize this as:

 

“The probability of gene exchange *between* populations that represent distinct more or less cryptic species is lower than the probability of gene exchange *within* each such species.”  For consistency and simplicity, let’s set the threshold at 10 million years (and ignore the complex mathematics of differing rates of gene flow with respect to population sizes and how they affect our characterization as hybridization vs. introgression vs. heterogenous gene flow vs. homogeneous gene flow, etc.).  What I’m trying to illustrate here is that there is a spectrum of cases ranging from those where genes are exchanged between two geographically separated populations robustly on an annual basis, to those where genes are exchanged only once in 10 million years (and every possible intermediate between these extremes).  In this context, gene exchange is short-hand for “organisms moving from one place to another and mating with other organisms when they get there”.

 

If we focus on the “rare” (once in 10 million years) end of the spectrum, then that implies low probability of movement of organisms from one place to the other, and hence “chance” (“freak”?) dispersal of individual organisms from one place to another.  Sometimes when these improbable geographic movements take place, the travelers end up where other members of their kin had previously made the move (10 million years prior).  But sometimes they are the very first of their kind to arrive at the new place, in which case it might be characterized as “chance dispersal”.

 

> 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?

> 

> MH: In 1, it's not so much the dispersal from a to be that causes the allopatric differentiation – 

> it is the cessation or slowing of dispersal, in this case presumably because the geological event is over.

> So this looks like a normal cycle of 1. a phase of biological mobilism followed by . a phase of immobilism, 

> with both caused, not by chance but by changing circumstances.

 

I don’t really follow your logic here.  I guess it comes back to our apparently different notions of what counts as “circumstances”.  I certainly agree that the dispersal from a to b does not “cause” the allopatric differentiation.  Rather, it establishes the conditions to allow it to happen in time (i.e., it establishes the allopatry).  It seems to me that the exact same thing is true for vicariant events – such events do not “cause” the allopatric differentiation; rather, like the “chance” part of chance dispersal, they simply cause the allopatry (time and evolutionary processes provide the differentiation).

 

> In 2, the question is, what particular change in circumstances would favor the expansion of just a single species? 

> What would be an example? 

 

I deliberately added (and highlighted) “spawning behavior” and “genetic mutation” as hints in that direction.  Both could lead to atypical situations in how organisms behave differently, or can tolerate different environmental circumstances in different ways.  The “chance” part is probably rarely (if ever) a single factor.  Rare “chance” events are much easier to imagine when you consider two (or more) rare things happening together.  For example, a group of reef fish spawn at an atypical time of the lunar cycle the exact same month that meteorological events (unusual hurricane?) altered the pattern of water movement in the area.  Or a mutation shared by several members of a brood of offspring cause them to head south when all their kin are heading north, coinciding with a climactic event that shifted thermal regimes in such a way that the south-bound travelers were able to survive when they otherwise wouldn’t.  The possibilities are almost literally endless. 

 

I know that this sounds like a bunch of “just so” stories, but here’s the thing:  circumstances that happen only once in 10 million years seem implausibly unlikely… until you remember that there are about ten-duotrigintillion* combinations of things that happen.  In that context, one-in-ten-billion-year events are not just commonplace, they’re inevitable.

 

*Googol it… ;-)

 

> I suppose it could happen in theory, but in practice the same, striking 

> distribution patterns are repeated in large numbers of taxa with completely different means of dispersal and ecology. 

 

Agreed!  And I think everyone would categorize these towards the “vicariance” end of the spectrum.  

 

> If a general explanation for a pattern is possible, that would be preferable to proposing separate explanations for each 

> of all the taxa in a pattern - often hundreds or more. 

 

Depending on how well-supported the pattern actually is, I agree.

 

Again, I’m not trying to say that “vicariance never happens”, nor am I trying to say that “chance dispersal happens more often than you give it credit for”.  What I’m trying to do is parse out what separates vicariance from chance dispersal.  Perhaps the former always involves many species, and the latter involves only one or a few species?  Or, maybe the former is about splitting existing sympatric populations into separate allopatric populations, whereas the latter is about organisms expanding their range as a consequence of one-in-ten-million-year circumstances?  Or perhaps vicariance is mostly involving abiotic factors, whereas chance dispersal is more about biotic factors?  I’ve seen all of these distinctions mentioned (or, at least, alluded to) in explanations and debates on this topic – but I’m still looking for a clear explanation for why they are really different classes of things, rather than extremes along the same continuum.

 

I apologize that this message has been a bit rambly, and I have not woven the tapestry of my point as cleanly as I would have liked, but here’s what I’m ultimately getting at:

I often see concepts of “vicariance” and “chance dispersal” represented as though they were cleanly disparate and mutually exclusive alternatives.  But all I can see out there in nature (and in my imagination) are multiple different continua and spectra.  That includes a spectrum of circumstances in biogeography we would characterize as “vicariant events” ranging to circumstances we would characterize as “chance dispersal”.  Perhaps it is still useful to debate about which end of this spectrum has played (and continues to play?) the larger role in shaping evolutionary history, but I don’t often see the arguments framed that way.  And this is a big part of the reason I find these seemingly endless debates so exacerbating.

 

And on the topic of “exacerbating”, I feel like this email has earned that characterization itself, so I’ll reign in my rantings (for now, at least).

 

Thank you very much for taking my questions seriously, and providing very helpful responses.  This exchange has enabled/forced me to re-think some of my own perspectives, so at least I (for one?) have found it helpful.

 

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> 

 <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 <mailto:m.j.heads at gmail.com> > 
Sent: Thursday, September 23, 2021 12:59 AM
To: Richard Pyle <deepreef at bishopmuseum.org <mailto:deepreef at bishopmuseum.org> >
Cc: Brendon E. Boudinot <boudinotb at gmail.com <mailto:boudinotb at gmail.com> >; Taxacom <taxacom at mailman.nhm.ku.edu <mailto:taxacom at mailman.nhm.ku.edu> >
Subject: Re: [Taxacom] Taxacom Digest, Vol 185, Issue 14

 

Hi Richard. Some responses to your points:

 

RP: 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”. 

MH: I don't think this is what they mean, because chance dispersal, as invoked, isn't related to circumstances or conditions. A general, community-wide change in conditions or circumstances will not be reflected in rare cases, but across the fauna and flora as a whole.  

 

RP: [Most oceanic species 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'.  

MH: Yes, dispersal by planktonic larvae is the norm in many species, just as flight is in birds, and crawling is in worms; it happens all the time (not just once in tens of millions of years in one individual), so it's not chance dispersal - the groups are not speciating. Now in practice, many entities that were thought to be pantropical oceanic species have turned out (with sequencing) to be complexes of more or less cryptic species, often allopatric. (I reviewed this fascinating new development in 'Towards a panbiogeography of the seas'. Biol J Linn Soc 84: 675. 2005). 

 

RP: 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?

 

MH: In 1, it's not so much the dispersal from a to be that causes the allopatric differentiation - it is the cessation or slowing of dispersal, in this case presumably because the geological event is over. So this looks like a normal cycle of 1. a phase of biological mobilism followed by . a phase of immobilism, with both caused, not by chance but by changing circumstances.

   In 2, the question is, what particular change in circumstances would favor the expansion of just a single species? What would be an example? I suppose it could happen in theory, but in practice the same, striking distribution patterns are repeated in large numbers of taxa with completely different means of dispersal and ecology. If a general explanation for a pattern is possible, that would be preferable to proposing separate explanations for each of all the taxa in a pattern - often hundreds or more. For example, the Colorado Plateau is a major centre of endemism, and has a history of uplift followed by  erosion (leading to the Grand Canyon etc). Isn't it likely that this history has had fundamental, underlying, effects on evolution in the region? Or do we follow the old model of: 1. the plateau is raised, then 2. it is invaded by dispersal from the lowlands or other highlands, with the endemic species differentiating after they get there?

   

 

 

On Thu, Sep 23, 2021 at 2:19 PM Richard Pyle <deepreef at bishopmuseum.org <mailto:deepreef at bishopmuseum.org> > 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 <mailto: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 <mailto:m.j.heads at gmail.com> > 
Sent: Wednesday, September 22, 2021 3:15 PM
To: Richard Pyle <deepreef at bishopmuseum.org <mailto:deepreef at bishopmuseum.org> >
Cc: Brendon E. Boudinot <boudinotb at gmail.com <mailto:boudinotb at gmail.com> >; Taxacom <taxacom at mailman.nhm.ku.edu <mailto: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
> 
> Send Taxacom mailing list submissions to: taxacom at mailman.nhm.ku.edu <mailto:taxacom at mailman.nhm.ku.edu>  For
> list information; to subscribe or unsubscribe, visit:
> http://mailman.nhm.ku.edu/cgi-bin/mailman/listinfo/taxacom
> You can reach the person managing the list at: taxacom-
> owner at mailman.nhm.ku.edu <mailto:owner at mailman.nhm.ku.edu>  The Taxacom email archive back to 1992 can be
> 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> 

              

 

 

 

 

 




 

-- 

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> 

              

 

 

 

 

 




 

-- 

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>