[Taxacom] Clade age

Richard Jensen rjensen at saintmarys.edu
Tue Nov 15 11:35:06 CST 2011

Just a few comments provided below.

Dick J

On 11/15/2011 11:41 AM, Curtis Clark wrote:
> To me, "triangle" is a bad example. It has a definition, "A polygon 
> with three vertices." I suspect that in the realm of non-Euclidean 
> geometries, there are cases where that definition is no longer useful; 
> whether it has impeded understanding by mathematicians, I will leave 
> to them. 
OK  - I was not precise enough: In the realm of Euclidean geometry, an 
obtuse triangle is a two-dimensional construct defined by the possession 
of a specific set of attributes.

>> "Homo sapiens" is a shorthand expression for
>>>   a natural entity defined by a given set of attributes.
> I would counter that a species (as a natural entity) is "defined" by its
> membership, not by its attributes; the attributes are known only
> post-hoc.

The attributes determine membership, not vice-versa.  We cannot 
recognize a set unless we specify the attributes that members of that 
set must have (i.e., have defined the set). Once we have defined the 
set, we can then ask if any particular entity belongs to that set.  But 
we can't assign membership until the set is defined.

> And adherence to definition can weigh an argument: Certain
> definitions of Homo sapiens would pretty much require that the
> stabilization and establishment of HeLa cells be regarded as a
> speciation event, since the entity no longer matches the definition.
That's exactly why definitions matter.  As you note, "certain 
definitions" may require recognition of HeLa cells as a separate 
species.  Other definitions would not require that view.  If we want to 
know if we are talking about the same thing, we have to agree on the 
definition a priori (or, a posteriori, we come to agree on one of the 
available definitions so we can make sure that our future conversations 
avoid the confusion that might arise).  Regardless of the situation, 
definitions matter.

> Taxonomists' views of the species they study are subject to change. To
> me, that seems to require that definitions based on attributes must also
> change. And I suppose that one could look at that sort of taxonomy as a
> dialectic, with definitions-as-hypotheses being falsified, rejected, and
> new definitions formed. But I'm not sure that taxonomy actually works
> that way, and I don't think it's useful.
Isn't that what taxonomists are doing when declaring one species to be a 
later homonym of another?  Or, when transferring a species to a new or 
different genus?  A hypothesis has been falsified and rejected.  In most 
cases, it's not that the attributes have changed, it's that our 
interpretation of the attributes has changed, or that we have new 
information about the group - the definition is more specific.

Richard J. Jensen, Professor
Department of Biology
Saint Mary's College
Notre Dame, IN 46556
Tel: 574-284-4674

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