[Taxacom] categorical systematics

[Fred Schueler]

Quoting John Grehan <calabar.john at gmail.com>:

>> Fred said "Surely we're at the state now where we can characterize at
>> least these higher taxa by their estimated age of divergence, and the
>> genomic difference between their surviving descendents."
>
> 'Surely' it depends. What constitutes an 'estimate' of age of divergence
> when all estimates are minimal?

* well, as in all things, an estimate is a best estimate on the basis  
of available evidence. But you can argue about an age of divergence in  
a rational way that's not available in arguing whether a taxon is a  
genus or a subfamily.

> And we might have 'genomic difference' but we have always had that (in its
> morphogenetic aspect).

* that's the third measure of divergence which ought to be tabulated,  
as a measure of ecological difference, but the pheneticists goal of  
measuring overall morphological similarity seems to have foundered.

Ken Kinman wrote:

> I think Linnaean categories are (and always have been) a good,  
> useful tool.  The human brain organizes things by using categories,  
> and Linnaean categories give classifications a much needed structure.

* but the Linnaean categories are "merely conventional signs" as the  
Bellman's crew affirmed - geological ages are also named entities,  
which are tied to something that can be measured, and which relate to  
evolution, and both genomic and morphological divergence are numbers.  
Evolved taxa are naturally categorical classifications, but Linnaean  
categories are a partitioning of a continuum into segments, and I  
suppose I'm just biased against this practice.

In our autobiographical whinge - http://pinicola.ca/transition.ca - we  
write "Fred also realized, during the composition of his thesis, that  
in every case where he'd classified a phenomenon into categories, the  
situation was better explained as an ordination or regression ? a  
response to continuous variables, rather than a chopping-into-parts.  
This doubtless had roots in taking plant ecology from Bob Whittakker  
(who had explained plant distribution as species independently arrayed  
along environmental gradients, rather than as co-adapted  
phyto-societies), and infra-specific systematics from Bill Brown (who  
had debunked the classical subspecies as spurious partitions of clinal  
variation). He was also immersed in the 1970's culture of multivariate  
analysis at the University of Toronto Department of Zoology, and  
further back, had read Peter Freuchen's 'Book of the Eskimos,' with  
its exposition of seasonal, as opposed to calendric, time. This leads  
us to suspect all classifications, boundaries, and partitions, and  
prefer explanations based on combinations of continuous causal  
factors. We're then struck with the often-surprising discovery that  
categorical partitions are acceptably congenial to many who employ  
them, and who don't feel that they need to replace them by relational,  
regression-like, explanations. Our loathing of categorical  
classifications is probably associated with our skittishness of  
national borders, parks, and all other arbitrary geographical  
partitions."

fred.
------------------------------------------------------------
          Frederick W. Schueler & Aleta Karstad
Bishops Mills Natural History Centre - http://pinicola.ca/bmnhc.htm
Mudpuppy Night in Oxford Mills - http://pinicola.ca/mudpup1.htm
Daily Paintings - http://karstaddailypaintings.blogspot.com/
          South Nation Basin Art & Science Book
          http://pinicola.ca/books/SNR_book.htm
     RR#2 Bishops Mills, Ontario, Canada K0G 1T0
   on the Smiths Falls Limestone Plain 44* 52'N 75* 42'W
    (613)258-3107 <bckcdb at istar.ca> http://pinicola.ca/
------------------------------------------------------------