NOT all individuals belong to a species

Wilbert Hetterscheid W.HETTER at PBN.AGRO.NL
Wed Mar 29 14:55:59 CST 1995

Dear Taxacomers,

During the recent discussions in this eminent circle on
species, hierarchy, Linnean matters etc. I noticed parti-
cularly the question "do all individuals belong to a
species?" of someone trying to identify garden roses to
species. This nicely illustrates the bias of such discus-
sions towards the "taxon" concept in general. Has it
occurred to anyone of you that there are such things as
cultivars? "Things" that do NOT per se belong to a spe-
cies. The urge to demand that every individual belongs to
a species, or, worse, to a "thing" that we can give a
Latin binomial (I refer to Latin hybrid binomials invented
to "collect" hybrids of cultivated origin under) has
heavily intruded in the taxonomy of cultivated plants.
Biodiversity vs. cultodiversity and can they be classified
with one and the same system? For those who are not put
off with this small intro, please read on. It is a manu-
script submitted to Acta Horticulturae for a volume with
the proceedings of the 2nd International Conference on the
Taxonomy of Cultivated Plants, organised August last year
in Seattle. I think it has quite some "interfaces" with
several discussions I've seen recently in Taxacom. I'm
curious to see how you all percieve this.


Wilbert Hetterscheid
w.hetter at


W.L.A.Hetterscheid            W.A.Brandenburg
V.K.C.                        DLO Centre for Plant Bree-
Linnaeuslaan 2a               ding and Reproduction Re-
Aalsmeer                      search
Netherlands                   P.O.Box 16


   The primacy of botanical nomenclature as ruled by the
ICBN (International Code of Botanical Nomenclature) has
always overshadowed the nomenclature of cultivated plants
and it is often stated that the latter is a subset of the
former. Since nomenclature is a corollary of proposed
classifications, which themselves are based on a taxono-
mists' systematic thinking, it would seem that the syste-
matics of cultivated plants and wild plants are the same.
They are not. Classifying biodiversity resulting from
evolution and classifying artificial diversity resulting
from man manipulating and changing organisms at his will,
ought to be subject of totally different systematic prin-
ciples. The use of the taxon concept for systematic groups
of wild plants and cultivated plants is the cause of the
confusion. In order to clarify the position of systematic
thought in the taxonomy of cultivated plants a new concept
has been proposed, viz. the culton (Hetterscheid, 1994;
Hetterscheid & Brandenburg, 1995). The culton is a general
concept for "systematic groups of cultivated plants" and
its consistent use will avoid further confusion with taxa,
in use for systematic groups of wild plants. It will also
guide to a more consistent and stable nomenclature of
cultivated plants.

1. Introduction

   The taxon concept was introduced by botanist Prof. Lam
in 1948 to indicate "taxonomic groups" as used in the
ICBN. This is still its definition in recent editions.
Also part of the definition is that taxa have a rank. This
being a rank in the taxonomic ("Linnean") hierarchy. It is
sometimes said that "taxon" encompasses any group of
organisms but this is too liberal a definition because
there is no room in systematic science for random (non-
systematic) groups. In conclusion taxa are systematic
groups of organisms given a rank in the taxonomic hierar-
chy. This rank is established by assigning the taxon to a
Linnean category.

   In the ICNCP (International Code of Nomenclature for
Cultivated Plants) four systematic groups of cultivated
plants are treated, viz. cultivar, group, graft chimaera
and "hybrids", the latter referring to hybrids produced in
cultivation. However no mention is made of a general
concept encompassing these systematic groups. On the other
hand there is a statement in ICNCP (Brickell et al., 1980,
art. 10) saying that the cultivar is the "lowest category"
for nomenclatural purposes but it is not stated in which
system this category fits. Another statement about ranking
is found in ICNCP, art. 26 concerning groups (assemblages
of similar cultivars):

"This category is intermediate between species and culti-

   This suggests that cultivar, (cultivar-)group and
species are part of the same categorical system. This
system can only be the taxonomic hierarchy, since that is
where the species category is fitted in, suggesting that
cultivar and (cultivar-)group are Linnean categories. But
do they fit the mechanisms that are inherent to the taxo-
nomic hierachy and its nomenclature (mutual inclusion and
exclusion of nested sets of taxa, extensive hierachy,
autonymy, typification etc.)? We will show that systematic
groups of cultivated plants are by their nature incompati-
ble with these mechanisms and as such cannot be fitted in
the taxonomic hierarchy. This fact and the current majori-
ty use of the taxon concept as dealing with systematic
groups of organisms evolved in nature led us to introduce
a new general concept for systematic ("taxonomic") groups
of cultivated plants, viz. "culton".

2. The cultivar

   The most important systematic group of cultivated
plants is the cultivar. For breeders it is the final
product of their efforts and for the trade, growers,
researchers, law etc. it is the main object of (part of)
their interest (and income). In circles of amateur bree-
ding and plant lovers, the cultivar has gained much less
attention and often wild plant species or self-produced
non-commercial hybrids are the main attraction. However by
far the largest number of cultivars is being produced in
commercial enterprise and human society has a strong
demand for them. As a result millions of people deal
directly or indirectly with cultivars of agricultural,
horticultural or forestry merit. Therefore taxonomists
dealing with cultivated plants should first of all work
out a system of taxonomy that deals properly with culti-
vars. Since this system must be servant to the many diffe-
rent uses of cultivars in society, it must yield a stable
nomenclature, less sensitive to destabilising mechanisms
as are current in the nomenclature of wild plants.

   Cultivars are being produced and maintained in various
ways. When a cultivar is ready for marketing purposes it
must receive a name (Latin name + cultivar epithet) and
must be described (ICNCP) or its name must be entered in a
statutory register. Society demands that a cultivar, once
introduced, retains its distinguishing characters during
its entire life span. This demand is e.g. institutionali-
sed by law in countries where plant breeder's rights are
granted. For that purpose distinction, uniformity and
stability are tested (D.U.S. testing). It appears that
society puts demands on cultivars as it does on other
undustrial products. Changing demands lead to changing
assortments by the improvement of characters leading to
new cultivars. All in all it is quite clear that the
cultivar as an entity is 100% embedded in human society
and derives its existence purely from that context.
Without man's action, no cultivar could exist.

   Cultivars are produced intentionally, they do not
evolve. A cultivar may cease to exist for a while but may
equally well be reconstituted. ICNCP covers this behaviour
by stating that the way in which a cultivar originates is
of no consequence to its recognition. One and the same
cultivar may thus be produced in more than one way and at
various moments in time. From this follows that apparently
a cultivar is recognised solely by the original combinati-
on of name and description. In order to fulfill the de-
mands of uniformity and stability this original descripti-
on cannot be "emended" as is often the case for e.g.
species descriptions. This and the reconstitution possibi-
lity of a cultivar make it a typical class-like entity.
The class consisting of all individuals that exist or may
come to exist in the future and that conform to the
class's (cultivar's) circumscription.

3. Classifying cultivars

   Taxonomy of cultivated plants starts when users (bree-
ders, researchers, lawyers, customers, traders, growers,
etc.) call for establishing and naming groups of cultiva-
ted plants. As stated the most important systematic group
of cultivated plants is the cultivar. When the number of
cultivars in a particular crop or assortment increases
there may develop a demand for classifying the cultivars
into distinguishable cultivar groups. For this purpose an
inventory must be drawn up of the criteria upon which the
cultivar groups are to be established. These criteria must
be provided by the users. A taxonomist may select from
this inventory the characters that are best suited to the
purposes of a particular group of users. It may also be
that different groups of users provide different charac-
ters relevant to their particular interest in the crop.
Therefore more than one classification of a crop or as-
sortment into cultivar groups may have to be prepared. The
circumscription of a cultivar group is often drawn up from
only the few relevant user-provided characters (e.g. a
certain pest resistance profile, a certain habit, colour
etc.). All cultivars of the assortment conforming to the
criteria are classified in that cultivar group, irrespec-
tive of their origin, unless origin is the grouping crite-
rion. When an assortment is being classified into cultivar
groups using the chosen criteria, it may result in several
cultivars remaining that do not conform to the chosen
criteria (an example of such a classification is to be
found in Bos et al, 1992). They cannot be accommodated in
one cultivar group since that would be a group defined by
a lack of characters, which is in contradiction to the
purpose of classifying cultivars (positive intentional).
Here an important difference with botanical classificati-
ons of taxa is encountered. Whereas a classification of
taxa into higher taxa of one particular level must be
exhaustive (all taxa must be classified into higher taxa,
a so-called "closed" classification), a classification of
cultivars, as described above, by contrast is an "open"
classification (Brandenburg et al., 1982; Brandenburg,

   Since a cultivar classification is based on user-crite-
ria and several user groups may have an interest in a
particular assortment of cultivars, more than one classi-
fication of a particular assortment of cultivars may be
coexistent without them being evaluated as contradictory
at the same time. One and the same cultivar may be part of
more than one cultivar group at the same time and this is
not a token of conflicting classifications but of equally
valid classifications. This is another fundamental diffe-
rence with biological classifications of taxa. Two diffe-
rent classifications of one and the same set of taxa are
always conflicting because the central tenet of life being
monophyletic, leads to the inescapable conclusion that
life's phylogeny has only one true topology and thus only
one "true" phylogenetic classification. Therefore of two
different classifications of the same set of taxa, at
least one is wrong. The search is for the "final" classi-
fication (Holy Grail) and therefore classifications of
taxa have very different test implications, based on a
systematic philosophy totally different than that for
cultivated plants. Classifications of cultivated plants
are tested on the grounds of their appeal to the users and
whether they fulfill their demand and so they cannot be
tested in terms of "wrong" or "right" but in terms of
"useful" or not.

   It has been suggested that assigning cultivars to a
(notho-)genus or (notho-)species is also an act of classi-
fication. Be that as it may (we feel the act is overra-
ted), it is for totally different purposes compared to
what has been said above. In breeding programmes, the
generic assigment of the plant material used is already
known and so the resulting cultivars are automatically
labelled with the relevant generic name unless intergene-
ric hybridisations are part of the programme, in which
case nothogeneric names may be proposed when necessary.
This can hardly be considered a classification effort but
is a convenient escape from the problem of not being able
to assign a cultivar to an existing genus. The intention
is to be able to produce a binomial guaranteeing that the
cultivar can be given a unique name. The description of a
genus is in the hands of taxonomists dealing with the
natural species that belong to it and therefore the as-
signment of a cultivar to a genus is not an intentional
act of classifying the cultivar, since no user criteria
are taken into consideration. Treating a genus as a major
grouping-device for cultivars is therefore in conflict
with the general intention of classifying cultivated
plants. The same line of argument holds for the species
category. ICNCP makes clear that the species-epithet is
not an obligatory part of a cultivar's name.

   Despite all this, history learns that taxonomists are
often of the opinion that the full name of a cultivar MUST
be a Latin binomial + cultivar epithet. This opinion is
taken farthest by taxonomists who create Latin hybrid
binomials for results of artificial interspecific crosses,
whether they lead to cultivars or not and thereby leading
the nomenclature of cultivated plants into the realms of
the ICBN and introducing an extra danger of instability of
nomenclature. The Botanical Code does indeed allow notho-
specific epithets to be proposed for artificial inter-
specific hybrids but the establishment of such epithets
follows the rules as for ordinary species epithets. There-
fore the nothospecies category suffers from the same flaws
as classification entity for cultivars as do the genus and
species (see above). On top of that it introduces mecha-
nisms of nomenclatural priority, synonymy, typification,
Latin diagnoses etc., that are irrelevant to the typically
"special purpose" taxonomy of cultivated plants.

    History has also seen various efforts to force the
classification (and nomenclature) of cultivars into the
taxonomic hierarchy. For these efforts entirely cultigenic
taxa have been proposed, often using all possible subspe-
cific categories to fulfill the demand of using the entire
axiomatic taxonomic hierarchy and all categories mentioned
in ICBN. Some taxonomists were apparently not satisfied
with the few subspecific categories provided by the ICBN
and introduced several additional categories (e.g. speci-
oid, convar, subconvar etc.; Jirasek, 1966; Jeffrey,
1968), all subject to ICBN mechanisms. For example, this
line of working led to the following "name" of the "taxon"
White Headed Cabbages, displaying no less than five subge-
neric hierarchic categories:

Brassica oleracea subsp. oleracea convar. capitata var.
capitata f. alba

In contrast to this cumbersome name, Siemonsma and Kasem
Piluek (1993) present a perfectly simple and adequate
alternative in proposing a classification of Brassica
oleracea cultivars featuring the White Headed Cabbage
Group, replacing all ICBN-based categories in the "taxon"-
name above.

   As a result of trying to rank cultivars fully in the
taxonomic hierarchy, taxonomic phantom problems have been
created that are often adressed in literature as "Problems
with the subspecific taxonomy of .....", dealing entirely
with nomenclatural problems that are self-inflicted.
Needless to say a taxonomy of cultivated plants servant to
needs in human society should not concern itself with nor
produce such academic excesses. We also feel that no
nomenclatural code should support this line of working and
must eliminate rules from their texts that enhance it.

3. Introducing the culton

   Nomenclatural and classificatory problems and complexi-
ties as outlined above emanate from intentionally or unin-
tentionally treating systematic groups of cultivated
plants as taxa, whereas by their nature such groups do not
at all conform to taxa. First of all they do not fit a
pre-determined hierarchy nor is it requested that they be
classified in such a hierarchy, quite the contrary is
true. Taxa nowadays are the central players in evolutiona-
ry studies and ever since Darwin have always had that
connotation (which is why phenetics failed the test of
being a relevant part of evolutionary biology), whereas
systematic groups of cultivated plants are decidedly non-
evolutionary. Taxa are used to describe naturally occur-
ring biodiversity whereas classifying cultivated plants
serves to describe artificial biodiversity. As early as
1923 L.H. Bailey was fully aware of this distinction when
he proposed to distinguish between the "indigen" (groups
of plants of the wild flora) and the "cultigen" (groups of
plants in cultivation). Evolutionary relevant taxa are
proposed using criteria to reconstruct them as monophyle-
tic groups. In this light they are to be seen as concepts
behaving as individuals (they come into existence in a
singular event and go extinct, never to reappear again),
whereas cultivars and groups derived of them are class-
concepts (see above).

   Why then, in the face of this, should we consider that
systematic groups of cultivated plants are taxa? From a
historical point of view they cannot be taxa because they
are not an integral part of the taxonomic hierarchy and
from a practical point of view, they appear to behave
radically different from taxa as conceptualised today by a
majority of biologists. The bottom line is that their
systematic background is totally different. Whereas syste-
matic philosophy concerned with developing systems of
classifying biodiversity is entirely based on the assump-
tion that evolution has occurred, systematic philosophy
concerned with classifying cultivated plants should incor-
porate the fact that man's influence is the all important
factor that produced the diversity it deals with. Therefo-
re entities figuring in the systematic philosophy of
cultivated plants must not be the same as those figuring
in the evolutionary context of most biological systematic
philosophies. Such a general concept of systematic groups
of cultivated plants is lacking and we propose to introdu-
ce the term "culton" (plur. culta) for this concept. The
definition reads:

A culton is a systematic group of cultivated plants based
on one or more user-criteria. A culton must have a name
according to the rules of the International Code of Nomen-
clature for Cultivated Plants.

With this concept difficulties in the taxonomy and nomen-
clature of cultivated plants can be solved. Extensive
hierarchies are not required thus simplifying taxonomy and
being better in conformity with the demands of everyday
users. Unprofitable mechanisms in use for the naming of
taxa can be avoided. If drawn to its logical conclusion,
ICBN must restrict its rules pertinent to cultivated
plants to the establishment of nothogeneric names. All
other references (notably the establishment of nothospe-
cies epithets for artificial hybrids) can be eliminated.
The ICNCP must be adapted so as to eliminate rules that
refer to mechanisms that are based on treating culta as
taxa and so must regain its prime status of code to be
used for the nomenclature of cultivated plants. The ICNCP
must make clear that classifying culta leads to very
different types of classifications as in use for taxa. No
complementarity is required, no extensive hierarchy of
nested sets of entities is required, no forced establish-
ment of categories is required. A positive spin-off for
the nomenclature of taxa is that it is no longer invaded
by Latin (hybrid) epithets actually referring to culta and
that e.g. compete for nomenclatural priority.

4. Conclusions

   Being confronted with the discrepancy of a vast litera-
ture dealing with apparent complexities in the taxonomy of
cultivated plants on the one hand and on the other hand
noticing the quite simple demands that human society
places on classifications of cultivated plants, we have
considered that the taxonomy of cultivated plants is
unnecessarily complicated by a wrong systematic approach.
The failure to recognise the essential difference between
taxa and what we now call culta, has led many taxonomists
to make use of principles and mechanims of classification
and nomenclature that are in use for taxa. This is the
core of all problems and has destabilised and complicated
the nomenclature of cultivated plants for many decades.
Recognising the culton concept and its logical consequen-
ces will restore the taxonomy of cultivated plants to its
full potential as a biological science servant to the
immediate needs of human society instead of being a batt-
lefield of discussions that is out of touch with that same

5. References

Bailey, L.H., 1923. Various cultigens, and transfers in
   nomenclature. Gentes Herb. 1(3): 113-115.
Bos, J.J., W.L.A. Hetterscheid and J.J. van de Wege, 1992.
   Wild and cultivated Dracaena fragrans. Edinb. Journ.
   Bot. 49(3): 311-331.
Brandenburg, W.A., Oost, E.H. and van de Vooren, J.G.,
   1982. Taxonomic aspects of the germplasm conservation
   of cross-pollinated plants. In: E. Porceddu and G.
   Jenkins (eds.). Seed Regeneration of cross-pollinated
   plants. Rotterdam: 33-41.
Brandenburg, W.A., 1986. The objectives in classification
   of cultivated plants. In: Styles, B.T. (ed.), Infraspe
   cific classification of wild and cultivated plants.
   Clarendon Press, Oxford: 87-98.
Brickell, C.D. et al., 1980. International Code of Nomen
   clature for Cultivated Plants. Regn. Veg. 104.
Hetterscheid, W.L.A., 1994. The culton concept: recent
   developments in the systematics of cultivated plants
   (abstract). Acta Bot. Neerl. 43(1): 78.
Hetterscheid, W.L.A., and Brandenburg, W.A., 1995 (in
   press). Culton versus taxon: conceptual issues in
   cultivated plant systematics. Taxon.
Jeffrey, C., 1968. Systematic categories of cultivated
   plants. Taxon 17: 109-114.
Jirasek, V., 1966. The systematics of cultivated plants
   and their taxonomic categories. Preslia 38: 267-284.
Siemonsma, J.S. and Kasem Piluek (eds.), 1993, Prosea 8.
   Vegetables. Pudoc, Wageningen. 412 p.

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