[Taxacom] Seed plants of Fiji

[Karl Magnacca]

On Thu, November 16, 2006 8:30 am, John Grehan wrote:
> Thank you for the clarification of your perspective. I can see that it
> is the traditional view that geological theory (i.e. interpretation of
> the evidence) determines biogeographic theory.

To a certain extent, it does.  If you have a barrier that cannot be easily
crossed by the organism in question, 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.

> You say "there is no
> evidence at all that the Hawaiian Islands have been anything other than
> always isolated". One would need to ask what "evidence" you would
> require. If you are looking only for the presence, for example, of
> continental rock remains, you might never find evidence (although such
> evidence has been found for at least one mid Atlantic ridge island).

*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.

> If one limits geological 'evidence' to the rocks themselves and the
> accepted interpretations of geologists biogeography will never be a
> science since it cannot independently predict history. As for Hawaii
> there is indeed biogeographic evidence for its biota having been
> inherited in part from an in situ Pacific biota.

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.

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.  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.

> As to what is the final answer to the pacific, I don't know. Perhaps it
> is an expanding earth, mobile micro-continents, upraised basaltic
> plateaus, or mobile island arcs. It is that uncertainty which makes the
> Pacific of particular interest - at least to me. Perhaps rather than
> subordinating biogeography to the Pacific Ocean model biogeographers
> could be in a position to really investigate alternative possibilities.

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

Karl
=====================
Karl Magnacca, UC-Berkeley
ESPM Dept., 137 Mulford Hall #3114
510-642-4148