More on Biogeographic memory

Pierre Deleporte Pierre.Deleporte at UNIV-RENNES1.FR
Thu Oct 11 15:16:51 CDT 2001

At 12:58 11/10/2001 +0200, Peter Hovenkamp wrote:

>I know this is in fact off-topic, but I can't help pointing out that
>monophyly of terminal taxa is a red herring - at least if one applies this
>term to species. More inclusive taxa should be monophyletic groups of
>species - but one cannot apply the term to the species themselves - for the
>perfectly simple reason that one cannot be a monophyletic group of oneself.
>But let's continue with biogeography... Although this illustrates how easy
>it is to be led astray by easy but inapplicable analogies.

I agree with the general warning against hasty analogies. Let's say that in
phylogeny reconstruction a species should really be a species, under a
definition of "species" relevant for phylogenetic analysis. Nevertheless
there is a problem of definition and delineation of operational terminal
units, and this was my point: nothing is "self evident" in this respect, be
it character, character state, group, species, area or spatial network. By
the way, I am not trying to introduce the "species concept" debate one more
time: please don't jump in, there is already a lot in TAXACOM archives on
this question, but reading them I concluded to "different definitions
fitting different uses" rather than "red herring".

>>(...) 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.
"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 ? We could say "I don't care", but the fishes could care
and we could suspect this. 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".

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...).

Just my two pence in the pot... or my two steps in the woods, as you like it.


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