[Taxacom] Chile-New Zealand distributions (rafting as a rare alternative)
michael.heads at yahoo.com
Mon Jul 26 03:44:10 CDT 2010
Geologists often cite the 'principle of multiple working hypotheses' (Chamberlin, T.C., 1890. reprinted 1965. Science 148, 754-759). This holds that it is dangerous in science to have just a single explanation for a phenomenon. The idea of chance dispersal is popular and can explain any distribution, for example Nothofagus could have dispersed from Chile to New Zealand, or vice versa, or dispersed to its current range from Kalamazoo and gone extinct there, etc. Different clades in any biogeographic pattern, such as the New Zealand - Chile disjunction, show differing degrees of evolution. Under the evolutionary clock model that has become very popular they all evolved at different times. The clock model suggests that all speciation is by dispersal, none by vicariance.
As an alternative, the vicariance model looks outside the group rather than within it, and assumes that different groups will evolve to different extents at a given biogeographic break because of different prior genome architcture. It stresses that Nothofagus (Nothofagaceae) is basal in the world-wide group Fagales and that it is the only member endemic to Australasia - Patagonia (fossils in Antarctica). Instead of a local center of origin this model proposes a world-wide ancestral Fagales. The first split is between Nothofagus, which develops more or less in its current range, and the rest of the families, which differentiate everywhere else. In this model Nothofagus does not have a centre of origin and does not disperse.
There is a complication because the northern Fagaceae overlap with Nothofagus in New Guinea (N. is at higher altitude) and the mainly central Australian Casuarinaceae overlap with N. in eastern Australia and New Caledonia, with a couple of species also in New Guinea. This can be accounted for by local range expansion of one or more of the families by the normal, observed means of dispersal, and is in contrast to the dispersal model which proposes extraordinary, long distance transoceanic dispersal in Nothofagus by means of dispersal that have never actually been observed, despite years of intensive ecological research.
A vicariance model accounts simply for the absence of Nothofagus in the mountains of Borneo, central Asia, the northern Andes etc. and the absence of Fagaceae etc. in New Zealand and Patagonia. A dispersal model has to bring in extra ad hoc hypotheses to explain these.
Nothofagus is (a) a large tree (b) dominant over large areas and (c) is wind-pollinated, producing vast amounts of pollen. Not surprisingly it has a good fossil pollen record and all four extant subgenera are known from the Cretaceous. Countless other groups with a very similar distributions can be explained by vicariance occurring at similar phylogenetic/biogeographic breaks (nodes), e.g. the Hebe complex (Orobanchaceae) with individual species (!) such as H. elliptica and H. salicifolia restricted to New Zealand and Patagonia. Most of these are much smaller, insect-pollinated plants that are not nearly as abundant as Nothofagus, and it is not surprising that they have virtually no fossil record.
Wellington, New Zealand.
My papers on biogeography are at: http://tiny.cc/RiUE0
--- On Mon, 26/7/10, Stephen Thorpe <stephen_thorpe at yahoo.co.nz> wrote:
From: Stephen Thorpe <stephen_thorpe at yahoo.co.nz>
Subject: Re: [Taxacom] Chile-New Zealand distributions (rafting as a rare alternative)
To: "Kenneth Kinman" <kennethkinman at webtv.net>
Cc: taxacom at mailman.nhm.ku.edu
Received: Monday, 26 July, 2010, 3:53 PM
well I'm not suggesting that it is to be "explained" by magic or "divine
intervention" - it is "unusual" only in the sense that I don't think there are
any other species of beetle (or even terrestrial lifeforms of any kind) whose
native distribution is N.Z. + Chile only ... plenty of higher taxa, but not
From: Kenneth Kinman <kennethkinman at webtv.net>
To: stephen_thorpe at yahoo.co.nz
Cc: taxacom at mailman.nhm.ku.edu
Sent: Mon, 26 July, 2010 3:38:08 PM
Subject: Re: [Taxacom] Chile-New Zealand distributions (rafting as a rare
Whether these weevils took the long route (around Antarctica) or
rafted directly on a Nothofagus tree, or even took the longer route
around Antarctica in the opposite direction, the MAIN point is that
there are several explanations that explain such a seemingly "curious"
distribution. Which one is the best supported remains to be seen, but I
don't find any of them particularly curious or unusual. That R.
tenuirostris is pretty much identical in Chile and New Zealand would
make me lean more toward a rafting hypothesis.
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