Bad taxonomic keys

[Robin Panza]

>On Fri, 29 Sep 1995, Warren Lamboy wrote:
>>
>>  A user of a taxonomic key must
>> invest some time in learning the key features of the taxa of interest.  Even
>> if one does this, however, there are still problems with keys of the above
>> sort.  First, one may not have all of the requisite characters available in a
>> particular specimen (this is hardly the fault of the maker of the key,
>> however!).
>
>Joseph Laferriere responded (in part)
>
>Sure, it is!    [snip]    sometimes a person doing an
>ecological or a floristic survey will be forced to collect specimens that
>are less than perfect. Writers of keys should realize this and enable the
>users to figure out the specimen's identity anyway. They should, if at
>all humanly possible,  avoid using obscure characters which they know are
>difficult or impossible to observe, so long as alternatives are
>available. I once saw a key separating two genera using the following
>couplet:
>
>Embryo curved
>Embryo straight
>
>The seeds of both taxa were about a millimeter in diameter. Another book
>separated the exact same genera thus:
>
>Inflorescence a spike
>Infloresence a panicle
>
>Which would you rather use? Embryo shape might be a more useful character
>in determining the relationships of these genera to other groups, but
>inflorescence shape is infinitely easier to determine.

This argument is self-contradicting.  The very fact that a particular specimen
is damaged is exactly why the key's creator cannot hope to make a key for all
uses.  Granted, inflorescence shape is more likely to be of use on future
identifications.  Granted, if seed and inflorescence are both available, the
latter would still be easier to use.  However, if I had to do an environmental
survey right now (fall, around here), I couldn't possibly identify this plant
based on inflorescence shape.  I would be better off with the embryo character.

Insects often have antennae or legs broken off; birds may be shot from too
close and have severely-damaged or blood-stained areas that are critical to ID;
Plants may lack characters at certain seasons; in many bird species, the
immature stage (or several stages) look quite different from a full-adult (and
internal anatomy in birds is rarely of use at the species level); sometimes we
need to identify a fragment (such as in stomach contents or pieces that may
have been collected illegally); no key can possibly cover all contingencies.

A good key will concentrate on characters that are usually present and easy to
see.  However, if my particular specimen lacks parts, that's not the writer's
fault.

Robin Panza
Section of Birds                     panzar at clp2.clpgh.org
Carnegie Museum of Natural History
Pittsburgh  PA  15213