[Taxacom] Incongruent phylogeny and geology

[David Campbell]

For whatever taxon you happen to be looking at it's necessary to check each
of these.  What fossil record do we have?  Where did they occur in the
past?  What changes occur over time?  How long ago were they present?  How
do the fossils fit into the phylogeny?  For example, the first appearance
of a representative of a stem group is not the date of the split between
the two members of the crown group that happen to be in your molecular
analysis (names omitted to protect the guilty).  How good is the fossil
record?  Can you say anything at all about it?

Dispersal abilities: Some organisms can readily survive a long float or
swim, some can't.  Some can run, swim, fly, or hitchhike, some can't.  Real
organisms are neither limitless dispersers nor completely passively
vicariant, but fall somewhere in between.

If you establish what a given organism likely could or couldn't cross, then
you can carefully research current geological understanding to see what
barriers and bridges may have been present at different times.

But too often geology in biological papers is a handwaving "my dubiously
calibrated molecular clock says 357.497 million years ago, and I think
that's about when some continent collided, so I have proved that mountain
building led to the origin of the clade."


On Thu, Apr 10, 2014 at 6:41 AM, John Grehan <calabar.john at gmail.com> wrote:

> 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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>>
>
>


-- 
Dr. David Campbell
Assistant Professor, Geology
Department of Natural Sciences
Box 7270
Gardner-Webb University
Boiling Springs NC 28017