Rankless again [was Re: Fungi and Four Kingdoms]

Frederick W. Schueler bckcdb at ISTAR.CA
Sun Oct 3 11:34:37 CDT 1999

Kipling Will wrote:

> Elizabeth Frieders wrote:
> > Recently someone posted that Linnaean ranks have served their
> > purpose for 250+ years.
> >  The new evidence is here. Let's do something about it rather than
> > clinging to ancient, creationist-based views of the classification of
> > life on earth.

>  just because something is new does not make it better. The reason
> the current system has weathered and grown with our knowledge for more
> than 250 years has little to do with "clinging" or "creationist-based
> views". The recognition of a hierarchy in nature
>  is the very reason an explanation was/is needed.

* but the observation of a hierachy, and its compelling explanation by
descent with modification, doesn't provide any support for continuing to
use Linnean categories (ranks). Shrubs are hierarchically branched too,
but if I want to know the rank of any branch, I can section it and count
the annual rings, in a way that I can't compare the differences among
diverse organisms, though some sort of philosophically opaque
comparability continues to be implied by the Linnean categories.

I think that time of divergence is the only possible criterion for
ranking taxa. Even phenetic divergence between totally sequenced genomes
won't do, because different taxa have different genes. And you can't
tally a phyletic analog to stream order, because of missing taxa. I also
think that the orginal cladists made their big mistake in trying to
wrench the Linnean categories into service for this purpose. I propose
that rather than continuing to struggle with Linnean categories, that,
whenever there's any idea of the age when sister groups diverged, the
time of divergence be used to form a rank, so that rather than (or
in addition to) "Family Hylidae" we'd say something like "Upper
Jurrasic-iformes Hylidae" (to distinguish this useage from "Upper
Jurrasic Hylidae," meaning Hylids known from that era). The advantage of
a totally new system of ranks is that it could grow up alongside the
existing one, and then perhaps eventually replace it, as DNA is
hypothesized to have taken over from RNA as the genetic material.

The advantages of ranking by time are that the names (or numbers) of the
geological periods are already well-known and stable, and that the rank
would put the best estimate of the age of a group right out in public.
The huge disadvantage, of course, is that the vast majority of these
would be guesses or interpolations. But many modern techniques, from
vicariance biogeography to genetic clocks, give some information about
the age of groups, and when cosmologists can't decide on the age of the
universe surely there's no shame in uncertainty about the age of a
lineage of Bugs.

I'll bet, however, that such ages would rapidly converge on
relatively stable values, and then, when herpetologists saw the familiar
"Upper Jurrasic-iformes Hylidae" changed to "Lower Jurrasic-iformes
Hylidae" they'd know that somebody had had a new idea or data about the
ancestry or history of the group, in a way that a change from 'Family' to
'Superfamily' doesn't imply ("Looks like a splitter is loose among the
Treefrogs, eh?").

just reasoning from first principles...

fred schueler.
         Eastern    Ontario    Biodiversity    Museum
                Grenville Co, Ontario, Canada
(RR#2 Oxford Station, K0G 1T0) (613)258-3107   bckcdb at istar.ca

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