[Taxacom] Seed plants of Fiji

[John Grehan]

> bounces at mailman.nhm.ku.edu] On Behalf Of Karl Magnacca
> 
> To a certain extent, it does.  

To a certain extent?

If you have a barrier that cannot be easily
> crossed by the organism in question,

How do you know the 'barrier' is pertinent to the distribution in the
first place, or that it is a 'barrier'?

 and there is no evidence that it did
> not exist at some time in the past (especially if there *is* evidence
that
> it has always existed), then until you come up with contradictory
evidence
> the conclusion is that the organisms dispersed across the barrier.

Don't follow the logic here.

> *Any* evidence would be enough to have a discussion.  So far you keep
> saying that there is evidence that the Hawaiian Islands are vicariant
but
> have given nothing in support.

The biogeographic evidence is that the Hawaiian islands support biota
that comprise vicariously distributed groups in the Pacific and standard
tracks across the Pacific. There are innumerable examples in the
literature, but take a look at the Meterosideros example as a starter,
and Springer's book on Pacific shorefishes, Heads articles on Coprosma,
etc.

> Sure, but if you're talking about a biota that is called "Pacific"
because
> it is composed of species that disperse among the islands, it has
nothing
> to do with vicariance.

No - I am taling about a biota that is Pacific because their
distributions are centered on and sometimes located within the Pacific
basin.
> 
> quoting from your reply to Neal:
> > For example, the connection between Hawaii and Tahiti may be
correlated
> > with the geological theory suggesting some of the Hawaiian chain
> > originated in the south. But then some would argue that the island
> > integration was never close enough to allow direct terrestrial or
> > shallow marine transfer. Perhaps so, but the meta-population theory
does
> > not require that such organisms cannot have the opportunity to move
over
> > water through their means of dispersal. The vicariism just means
that
> > the process was local rather than regional.
> 
> I think there are two problems with this discussion.  First is that
you
> seem to be using a different definition of "vicariance" than everyone
> else.  

Everyone? I am referring to vicariant form-making 

My interpretation of it is that you have one continuous,
> interbreeding population (or metapopulation, if you like) that
diverges
> into separate species when a barrier arises; it does not just mean
> short-distance dispersal.  Island-hopping, even across an island arc,
is
> still dispersal because you have a rare event of crossing a
pre-existing
> barrier to establish a population where there was none before.  In
some
> cases speciation might be considered vicariant if the barrier begins
small
> and increases to the point that dispersal across it is too rare to
> maintain interbreeding, but that depends on the kind of organism and
> barrier you're talking about.

All of this is nice theory, but since most biogeographers follow the
Darwinian model of biogeography the concept of vicariance is limited to
its opposition to dispersal whereas vicariism is an empirical
biogeographic pattern that calls for vicariant form-making. Mobilism
within vicariant ranges maintains the existence of the taxon without
obliterating the underlying vicariism.

> The second is that your idea of not allowing the geologic evidence to
> dictate the biogeographic evidence seems to have been taken to the
point
> of ignoring the geologic evidence.  

No

In the case of Hawaii in particular,
> you're proposing that major geologic events took place and left
virtually
> no trace. 

No - just did not leave behind a stratigraphic trace as the current site
of the islands, just as there is no trace left at the Galapagos even
though many geologists accept the possibility of a former island arc
connection.

If you look at a map of the Pacific (e.g.
>
http://upload.wikimedia.org/wikipedia/commons/b/be/Pacific_elevation.jpg
),
> the only possible structures of the kind you're talking about are a
ridge
> to the west that snakes off towards the Marshall Islands, and a
plateau to
> the south (Palmyra).  Since the biota of Hawaii is not closely related
to
> that of Micronesia, and the latter would still require two dispersals
of
> ~1000 miles to get from Tahiti, I don't think either of those help
your
> cause.

They neither help nor hinder if one accepts that biogeography
constitutes an independent research program with its own methods and
principles. That is true of panbiogeography, but it may not be true of
all other methods. There are geologists who have suggested such
structures as I have talked about, but you wont see them by just looking
at a map of the Pacific.

John Grehan