[Taxacom] Incongruent phylogeny and geology

[John Grehan]

David,



Variability in timing from a particular geological event is definitely a
worthwhile consideration although specifics may be problematic where
assumptions are made when a taxon located either side of a previous
'barrier' is assumed to have crossed that barrier. The veracity of such
arguments would have to be assessed case by case. There are taxa north and
south of Panama that may have already occupied land to the north and south,
there are taxa that may have originated from the Panama terranes out of the
Pacific, and there are taxa to the north and south that never occupied or
'crossed' the Panama isthmus.



With Wallace's line (as example of a boundary that is often represented as
a dispersal barrier) assumptions are often made that a taxon on one or
other side is a demonstration of that taxon having been restricted by
Wallace's line acting as a barrier, and yet on the other side there may be
a sister taxon which would show that the 'line' did not function in this
way at all.



I would be interested to know in more detail as to what you mean by careful
examination of the fossil record, dispersal abilities, and detailed
geological analysis for correlating a particular point on a tree with the
event in question. Perhaps you could give an example.



John Grehan


On Wed, Apr 9, 2014 at 4:35 PM, David Campbell <pleuronaia at gmail.com> wrote:

> Geological evidence is often relatively coarse-scale, and neontologists'
> ideas of geology often much vaguer still.  In the real world, connections
> and gaps between continents or ocean basins do not abruptly jump from
> impassible to easily traveled and vice versa; different species have
> different dispersal ability and are stopped or not by different barriers.
> Thus, an event such as the closing of Panama does not have a magic fixed
> date at which every land and freshwater species crossed and every marine
> species stopped crossing.  Land mammals (probably decent swimmers) began to
> cross by about 10 million years ago, but the trickle became a flood about
> 2.5 million.  The southernmost part of the isthmus geologically is
> geographically northwestern Colombia, and probably was in place about 10
> million years ago, but shallow marine connections persisted as recently as
> 2.5 million.
>
> These issues are quite commonly a problem in molecular clock calculations.
> Calibrations based on tectonic events rarely involve careful examination of
> the fossil record, dispersal abilities, and detailed geological analysis
> that would be necessary to actually correlate a particular point on the
> tree with the event in question.  Likewise, when dates are calculated, many
> different things are happening geologically at any given time, and honest
> error bars for molecular clocks are quite wide, so the range of possibly
> correlating events is in fact very wide.
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