[Taxacom] Taxacom Digest, Vol 185, Issue 14
Michael Heads
m.j.heads at gmail.com
Thu Sep 23 21:05:47 CDT 2021
Your last point: '...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'.
I agree completely. A bird or insect or whatever might be restricted to
its region for millions of years, surviving there by its normal means of
dispersal, physiology etc., but a change in conditions can lead it and its
community to enter a phase of mobilism, in which massive range expansions
take place, facilitated, again, by the groups' ordinary means of
dispersal.
On Fri, Sep 24, 2021 at 1:48 PM Richard Pyle <deepreef at bishopmuseum.org>
wrote:
> 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
>
> BishopMuseum.org <http://hbs.bishopmuseum.org/staff/pylerichard.html>
>
> *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.
>
>
>
> On Fri, Sep 24, 2021 at 6:57 AM Richard Pyle <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
>
> BishopMuseum.org <http://hbs.bishopmuseum.org/staff/pylerichard.html>
>
> *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 12:59 AM
> *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
>
>
>
> 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>
> 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
>
> BishopMuseum.org <http://hbs.bishopmuseum.org/staff/pylerichard.html>
>
> *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>
> 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>
> > _______________________________________________
> > Taxacom Mailing List
> >
> > Send Taxacom mailing list submissions to: 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 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
>
>
>
> *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>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
> --
>
> 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>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
> --
>
> 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>
>
>
>
>
>
>
>
>
>
>
>
>
>
--
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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