Taxacom: summary of vicariance and dispersal

[turner at casema.nl]

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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