Characters, identification (x Delta-l)

[Mike Dallwitz]

                                                                 31 March 1995

> From: Renaud Fortuner <fortuner at MATH.U-BORDEAUX.FR>
> To: Taxacom
>
> Should we attempt to use "all" the characters or only the characters that
> have been "critically evaluated and reconciled"?
>
> First, we should clearly make the distinction between "recording" and
> "using" the characters (e.g., for identification). It is obvious that
> critical evaluation cannot even begin without a list of the things that need
> to be evaluated.

It can, and must, BEGIN without a full list. You try to devise characters from
a sampling of the literature or of specimens. You then start recording against
these characters, and HOPE that you don't have to change them as you see more
material (because that might require re-examination of all previous material).

If you don't do it critically, the end result may just be a collection of
rubbish - little advance on rubbish scattered all over the place (Watson, L.
(1971). Basic taxonomic data: the need for organization over presentation and
accumulation. Taxon 20, 131-136).

Incidentally, one of the advantages of recording against an explicit character
list is that it is less likely for the taxonomist's own concept of the
character to drift during the work. Even so, I strongly advise DELTA users to
write `character notes' at the time they are thinking about the definition of
a character, to further guard against this drift. There is a tendency to think
that these are only for the benefit of the end user, and to postpone writing
them.

> Second, we should make a distinction among qualitative characters between
> ordinal and nominal characters. Ordinal chr can be ordered - e.g., color
> from red to violet ...

Bad example. Colors can't, in general, be ordered because they are
three-dimensional (specified by red/green/blue, cyan/magenta/yellow,
hue/saturation/lightness, etc.). In a limited context, such as the colors
occurring within a specified group, it may be possible to order them. Even
then, you have to be careful with the terminology used, e.g. `azure, light
blue, dark blue' can't be ordered, because `azure' overlaps or is a subset of
`light blue'.

I agree that large numbers of states don't cause any problems with ordered
characters; on the contrary, the more the better. An infinite number (i.e.
numeric character) is best of all.

> The other problems raised ... can be solved if the traditional concept of
> character is re-evaluated in the light of modern computer science.

OK, but in view of the already confusing use of terminology in this area
(Colless, D. H. (1985). On "character" and related terms. Syst. Zool. 34,
229-233) it might be best to coin a new term, or at least use a qualifier, to
denote your new kind of `character'. I look forward to seeing your paper.

> "Number of differences" is one thing, "coefficient of overall similarity" is
> something else. Do I prefer one or the other? No, I want both. I have a
> similarity algorithm for individual specimens and another for populations.

So have I. So what have we been arguing about?

> I do consider discriminating power of the characters for the group of taxa
> that remain to be discriminated.

Then your identification program uses dichotomy, after all! It divides the
taxa into those that remain to be discriminated, and those that don't.

Mike Dallwitz                                  Internet md at ento.csiro.au
CSIRO Division of Entomology                   Fax +61 6 246 4000
GPO Box 1700, Canberra ACT 2601, Australia     Phone +61 6 246 4075