nominal characters

Lawrence Kirkendall Lawrence.Kirkendall at ZOO.UIB.NO
Fri Apr 7 12:37:44 CDT 1995

>Lawrence R.
>Kirkendall sent several very promising examples. Is there a plant taxonomist
>who can confirm that this pins and thrums stuff is nominal? On the other hand,
>color and shapes have been disqualified as being disguised ordinal characters.

I must have missed this.  How can different mimicry morphs in the same
butterfly population (e.g. Papilio memnon females--see discussion and
illustrations in Chap. 8 of Ridley's Evolution textbook) be a "disguised
ordinal character"? The different color/shape morphs here (and in "pins and
thrums", or Cepaea snails, but not Heliconia polymorphisms) are the results
of different combinations of alleles for tightly linked sets of genes
("supergenes"), and in at least some instances are maintained by frequency
dependent selection.  And I emphasize again, this is just one well-known
example of what I am sure would be an endless list of morphological
polymorphisms (yes, within species), many of which cannot readily be
ordered if they can be ordered at all.  The point here is your claim might
stump theoretical systematists, but not ecologists.

>So we now have several behavior/ecological characters which are both
>nominal and
>intraspecifically variable, but still no confirmed ISV for a morphological
>nominal character (if there is such a thing).[snip]
>"A truly nominal MORPHOLOGICAL character cannot be present in
>a valid species in more than one state"

Given, for many types of organisms, practicing taxonomists must deal almost
exclusively with external morphology; nonethelss, I see no good
justificaiton for distinguishing between behavioral and morphological
characters.  Both are the expressions of interactions between genes and
environment; both can result from the action of single genes, a few genes,
or many genes.  Both morphological and phenotypic characters can show no,
some, or much environmentally determined variability.  So, if a proposition
is not true for behavioral characters, it is not likely to be true for
morphological ones either.

Well, not being a taxonomist, there is probably something I have
misunderstood here--the proposition seems so obviously wrong.  Could
someone carefully define an ordinal character, and how one determines the
ordering?  Does there have to be biological "logic" in the ordering?
Doesn't the ordering mean that evolution can go only for A to B, or B to C,
but not A to C?I really don't see how the states of a polygenic or two- to
several-locus character can be logically ordered; even if  only one change
(in allele make-up) was necessary to go from form A to form B,  there is no
reason to think that form B is a pre-requisite for evolution of form C
(which in involves, for example, one change at a different locus).  In
fact, single changes are likely to produce totally unsuccessful genotypes,
and as a consequence these intermediates are very rarely found and would
not be encountered by a txonomist enumerating the character states for the
species; the gaps between morphs are apparently often several "steps".

Lawrence R. Kirkendall                        FAX:   +47 55 31 44 64
Univ. Bergen Zoological Inst.                 VOICE: +47 55 21 23 42
Allegaten 41, N-5007 BERGEN Norway
EMAIL:  lawrence.kirkendall at

ÿÿ    Re: nominal characters                                                  DU

More information about the Taxacom mailing list