More on Biogeographic memory
Hovenkamp at NHN.LEIDENUNIV.NL
Fri Oct 12 10:24:34 CDT 2001
At 03:16 PM 10/11/01 +0200, Pierre Deleporte wrote:
>>>(...) when Craw et al. draw the
>>>shortest straight line between the distribution of two sister groups,
>>>Hovenkamp draws a separation between adjacent distribution areas of two
>>>sister groups. The line or track has to cross the separation or barrier. On
>>>this limited point, the two approaches look very similar, and this could
>>>give a handle to contrast them (Peter : why do you draw your line between
>>>the spatially closest borders of the extension ranges? Or are there
>>>exceptions to this rule?).
>>Good question! Come to think of it - the earth being round, it is of course
>>possible to view any area as being completely surrounded by any other one.
>>So why the spatially closest border? It somehow seems logical to do it this
>>way. Perhaps all depends on the scale of the analysis - if sister taxa
>>occur in close proximity it makes little sense to take the possibility
>>seriously that they may have vicariated round the world completely - if
>>they are farther apart, it seems to make little sense to try to measure
>>whether the one way round is 20000 miles and the other way 19000... In the
>>latter case, one might want to consider both options.
>This "simple" answer just illustrates that the slightest technical aspect
>in historical biogeography IS theory loaded, and that the often implicit
>underlying explanatory laws are to be made explicit for evaluating methods:
>disclose their implications, in every detail, and contrast these to their
>goals, themselves being explicited.
>That the shortest spatial distance is in itself the right thing to take
>into account for any "biogeographic" purpose seems highly questionable.
But that was not the original question, I believe. This was - which line to
draw (either connecting, or dividing) between the distributions of two
So the first criterion is taxonomic: sister groups. That excludes many of
the possible "spatially nearest" neighbours. That solves many of your
questions, I believe:
>"Come to think of it" (!)... Examples of speciation "all around the world"
>do exist (sea gulls...). If we consider freshwater fishes in adjacent upper
>river basins separated by a narrow mountain ridge: is the "spatial nearest
>neighbour" downstream, or across the ridge? Do both approaches make equal
>biological sense ?
Look for the sister-group relationship!
>We could say "I don't care", but the fishes could care
>and we could suspect this.
By looking at sister-group relationships.
>And thus we could argue an a priori
>justification for considering a given taxon as relevant for biogeography
>dealing with a given kind of geological feature (e.g. connected freswater
>networks for fishes...), and not "anything you hit around as the crow flies".
Indeed not - we are specifically looking at sister-group relationships only...
>Am I suggesting an apparent entorse to a possible "general requirement of
>independence" of biogeography? Maybe, but first of all we need biological
>sense. The choice of relevant taxa and geological features to be considered
>may be guided by specific hypotheses to be tested and by particular
>underlying asumptions... if explicited. Extracting and comparing patterns
>under different logics (or for different taxa with different ecological
>skills) may also be useful (fishes locations considered as the crow flies
>on the map versus following freshwater networks, or fishes versus
>terrestrial snails, to take trivial examples...).
Hoping for congruence to emerge...
>Just my two pence in the pot... or my two steps in the woods, as you like it.
And another two pence ... how many eurocents is that?
Nationaal Herbarium Nederland - Leiden
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hovenkamp at nhn.leidenuniv.nl
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