Taxacom: summary of vicariance and dispersal

Mon Aug 4 10:18:59 CDT 2025

These are all certainly pertinent general questions and issues. All depends
on how one gets to whatever conclusions are reached. When it is said
"land-locked snails will be sooner affected by an arising water barrier
(e.g. the submerging  of Doggersland creating the North Sea, or of the
continental shelf between New Guinea and Australia, creating the Arafura
Sea, Torres Strait and Coral Sea) than some birds or wind-dispersed seed
plants." Assuming you are referring to the 'affect' being on
differentiation, that might be true in some cases, but not necessarily in
others. Birds and wind-dispersed seed plants can have allopatric taxa with
distribution patterns corresponding to those of snails or supposedly other
not so dispersible taxa (and keep in mind that various geographers,
including darwin' have theorized trans-oceanic dispersal  for land
snails).

In panbiogeographic approaches the concept of 'barrier' is not central.
Instead it is the fact that there are distributions of different taxa that
have distributional boundaries that correspond with tectonics or tectonic
related geology (i.e. not just stratigraphy) such as suture zones, normal
and transform faults, shear zones, propagating fractures, magmatic belts,
rifts (inclung failed rifts) etc. Chance dispersal studies pretty much have
ignored these correlations. usually justified as being 'too old". As you
say, when time can be included, the reconstruction becomes more realistic
and richer" - sure. A great example was recently shown for the
Coenonymphina butterflies involving 14 tectonic correlations with age
sequence matching the phylogeny.

But usually time is derived from fossils or fossil calibrated molecular age
that are misrepresented as actual or maximal ages and incorrectly used to
falsify earlier tectonic events. As Heads noted, the general fact is that
patterns are shared by large numbers of taxa in a community, and that these
taxa will show a wide range in their ecology and means of dispersal. Nice
case example is the correlation of distributions with the Vitiaz
Lineament (former subduction zone) where such theoretically non-dispersible
ghost moths show the same Vitiaz correlation as some damselflies. Everyone
'knows' damselflies and dragonflies can 'get around'. Perhaps they don't
quite do that in the way they imagined. The Vitiaz correlation is shared by
a broad range of plant and animal taxa (Heads 2014).

That paper on the biogeography of oranges gives extensive examples and
discussion of both differentiation and ecological aspects of distribution,
including the impact of sea transgression or regression.

Cheers, John

On Mon, Aug 4, 2025 at 10:51?AM turner at casema.nl <turner at casema.nl> wrote:

> Vicariance occurs when a barrier to (chance) dispersal arises that splits
> a distribution pattern into (last least) two parts. If the barrier
> affects several co-occurring taxa a general pattern is established that
> might be reconstructed and tentatively ascribed to the process that caused
> the barrier. General patterns are relative, of course: not all taxa will be
> affected equally by a barrier: land-locked snails will be sooner affected
> by an arising water barrier (e.g. the submerging  of Doggersland creating
> the Noth Sea, or of the continental shelf between New Guinea and Australia,
> creating the Arafura Sea, Torres Strait and Coral Sea) than some birds or
> wind-dispersed seed plants. On the other hand, existing barriers may
> disappear, allowing (long-distance) dispersal of many different taxa
> across the same previous border. This will also create a general pattern of
> sorts which may be very similar to a vicariance pattern.
> Time is a factor in these processes too: not all taxa respond to the
> origin or disappearance of a barrier at the same moment, but the general
> pattern can still be identical. The same process that created the Atlantic
> Ocean is creating a rift between the western and eastern parts of
> (tropical) Africa, but the barrier between S America and Africa is much
> older and affects more taxa than the Great Rift Valley.
> In my view, the pattern of multiple sister taxa on both sides of a barrier
> is interesting in itself and worth analysing, but if the factor of time can
> be included the reconstruction becomes morte realistic and richer.
>
> My two-cents worth?.
>
> *From: *Taxacom <taxacom-bounces at lists.ku.edu> on behalf of John Grehan
> via Taxacom <taxacom at lists.ku.edu>
> *Date: *Monday, 4 August 2025 at 16:03
> *To: *John Grehan <calabar.john at gmail.com>
> *Cc: *taxacom <taxacom at mailman.nhm.ku.edu>
> *Subject: *Re: Taxacom: summary of vicariance and dispersal
>
> I had an offlist response as below. Since the perspective may be of broader
> interest, I have response to the list in general, along with my comments.
>
> "I'd put it another way. Vicariance involves a search for biogeographic
> patterns and associated earth history mechanisms (explanations) for
> observed patterns. Dispersal appeals to ad hoc independent mechanisms for
> each taxon, and is therefore both unscientific and uninteresting!"
>
>
> I figure anyone is free to put the issues in any way that suits them. I?m
> not sure that it would be quite correct to say that ?Vicariance? searches
> for biogeographic patterns and associated earth history since vicariance
> represents a historical mechanism. Panbiogeography, as a research program,
> has the purpose of searching for biogeographic patterns ? with the caveat
> that this includes BOTH geographic and phylogenetic structure.
>
> Regarding dispersal ? note that there is a distinction between ?chance
> dispersal? where movement explains divergence, and ecological dispersal
> where movement explains distribution range (expansion, contraction, overlap
> etc.).
>
> Whether any particular approach or model is considered either unscientific
> or uninteresting is very much a personal perspective, and one that does not
> always correlate with scientific veracity (take that classic US example of
> jumping gene theory).
>
> I think a more direct consideration is that, as noted in the excerpt from
> Heads, that chance dispersal theory does not make any particular
> predictions while vicariance predicts that similar distributions will be
> shared by large numbers of taxa in a community, and that the taxa will show
> a wide range in their ecology and means of dispersal. All of this has been
> empirically verified over the decades and never refuted. But of course it
> is extremely intensively opposed in varous quarters, and even by at least
> one entire research organization.
>
> Cheers, John
>
> On Sat, Aug 2, 2025 at 1:42?PM John Grehan via Taxacom <
> taxacom at lists.ku.edu>
> wrote:
>
> > I see from reading the literature, and from comments some on Taxacom have
> > made, that many are still confused about dispersal and vicariance. I have
> > excerpted a summary (my color highlights) from Heads (2015) that puts it
> > all together with some great clarity I think. Going forward there is no
> > justified basis for further confusion about these concepts and
> > panbiogeography - in my opinion.
> >
> > Cheers, John
> >
> > *Summary on vicariance and dispersal*
> >
> > Traditional (dispersal) theory and most current (program-based)
> > biogeography explain allopatry by chance dispersal, and overlap by normal
> > dispersal.  Instead, the vicariance approach followed here explains
> > allopatry by vicariance, overlap by normal dispersal.
> >
> >    The centre of origin/dispersal model relies on idiosyncratic, chance
> > explanations for each individual clade. Long-distance dispersal involves
> > one-off events, that all occur at different times and are not related to
> > means of dispersal, or to any other biological or environmental factor.
> It
> > does not make any particular predictions. In contrast, vicariance
> predicts
> > that similar distributions will be shared by large numbers of taxa in a
> > community, and that the taxa will show a wide range in their ecology and
> > means of dispersal.
> >
> > --
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