FUNDAMENTAL DIFFERENCES IN THE CLASSIFICATION AND NOMENCLATURE OF CULTIVATED AND WILD PLANTS
In the interest of nomenclatural stability it may at first glance seem wise to standardize the nomenclature of biological objects once and for all. This would not allow, however, for changes to accommodate new theories of evolution and classification. For example, Linnaeus’s very useful development of binomial nomenclature predated modern ideas of evolution espoused by Darwin (1859). If traditional nomenclature was “frozen” since the time of Linnaeus we would be burdened with an archaic system with little biological relevance. Hence, classification rules change along with new data and needs (every six years for the
ICBN, irregularly for the ICNCP).
A. Ambiguity of the Term Variety
The term variety has caused much confusion. One meaning, as used by the ICBN (the “botanical variety”), is a particular rank in the taxonomic hierarchy below the rank of species and subspecies and above the rank of form (form /variety /subspecies / species). Another meaning, as used in the ICNCP (the “cultivated variety” or “cultivar”), refers to cultivated variants originating through human influence. Regarding the use of variety in the ICBN, species variation has been subdivided through infraspecific classifications. The relationships of the infraspecific categories allowed in the ICBN are strictly hierarchical, and as such they are differentiated by their degree of uniqueness: subspecies within a species should differ less among themselves than separate species, varieties should differ less among themselves than subspecies, and forms less than varieties. In practice, however, different taxonomists treat variation patterns differently. For example, sometimes a species is subdivided in subspecies and these in varieties, but in other cases a species is subdivided directly into varieties and the subspecies rank is not used at all (Hamilton and Reichard 1992). Some taxonomists feel that recognizing subspecies indicates a geographical component, with subspecies being mostly allopatric, while varieties may be sympatric. This is not a formal or universally held distinction between these ranks, however. The term becomes especially confusing with the wish to assign cultivated plants to the species from which they originate, resulting in the application of the term to cultivated plants in a form that appears as a botanical variety. The use of the rank variety for cultivated plants goes back to Linnaeus (1753). In many cases Linnaeus started his treatment of a species with the wild plant, mentioning cultivated varieties at the end (Wijnands 1986). Linnaeus clearly considered varieties as minor variants due to the influence of climate or soil, or in the case of cultivated varieties, of human influence. He later stated that the grouping of cultivated plants should be the task of beginners in botany, while qualified botanists should study species and higher taxonomic levels (Linnaeus 1764). Many later workers on the taxonomy of cultivated plants continued the practice of applying variety names for cultivated plants, burdening nomenclature with formal names with all the inherent problems of typification and priority that these entail. In these systems (e.g. Helm, 1957, 1963) the varieties are often grouped in artificial higher categories like convariety (or convar). Convarieties can be roughly comparable to cultivar groups, but convarieties, unlike cultivar groups, do not necessarily contain named varieties, and convarieties are members of traditional “Linnaean” ranks. The ICNCP replaced this term with the term cultivar-group, and convarieties should not be used in modern cultivated plant taxonomy (Trehane et al., 1995). Some modern influential works, however (e.g., Hanelt 2001), ignore rules of the ICNCP and continue to use the term convar (convariety). The term cultivated variety (cultivar) in the ICNCP, in contrast, is used in a very different way. The botanical variety has its fixed position in the taxonomic hierarchy. The cultivated variety stands outside this hierarchy because it could have resulted from many different processes as selection or a complex series of interspecific hybridizations, making it impossible to assign it a position in the hierarchy. Because of this, the nomenclature of the cultivated variety follows the ICNCP, dispensing with Latin epithets used in hierarchical ranks in the ICBN. Presently, however, names originally published as botanical varieties still refer to cultivated material. These entities can be reclassified as cultivars, or if a botanical variety was described to encompass many cultivated morphotypes (as is the case in the classification of Brassica oleracea) they can be reclassified as cultivar-groups (van den Berg 1999). Thus, the botanical variety Brassica oleracea var. gemmifera can be reclassified as Brassica oleracea Gemmifera Group, encompassing the many cultivars of brussels sprouts. However, the term variety for cultivar is still in wide use in legal documents all over the world. ICNCP deals with this in stating that the term variety as used in such texts is fully equivalent to cultivar. Legally, variety can have additional definitions. For example, the U.S. Plant Variety Protection Act (PVPA) uses the term “cultivar” in a manner similar to the botanical “variety,” but with the additional stipulation that “development” must take place from wild stock, as through breeding or genetic engineering. That is, discovery of unique variants alone does not make a cultivar eligible for protection under the PVPA. In addition, PVPA protection of varieties is granted with the additional requirements that it is “new,” “distinct” from other cultivars, “uniform,” and “stable.”
The botanical rank form has also been used extensively to describe minor variants of cultivated plants. Its use in the classification of wild plants is generally discouraged because the entities that could be described as forms are usually such minor morphological variations that it is arguable whether their distinction is useful. For cultivated plants these forms may easily be reclassified as cultivars. The same goes for the many informal and often ill-defined terms like strain, sport, type, and so on. If any such entity is worthy of recognition and description, it will be best to employ the general term cultivar for all of these.
B. Culton Versus Taxon
A fundamental difference between the ICNCP and ICBN is their respective
approach toward classification. Groups of plants used in the ICBN to classify and name are collectively designated as taxa (singular: taxon). The ICNCP uses the terms cultivar and cultivar-group for cultivated plants. Although it claims that they are taxa, these terms do not fit the definition of taxa for several reasons. This may become clear by the definition in the ICNCP of the term cultivated plant: A cultivated plant is one whose origin or selection is primarily due to the intentional activities of mankind. Such a plant may arise by deliberate or, in cultivation, accidental hybridization, or by selection from existing cultivated stock, or may be a selection from minor variants within a wild population and maintained as a recognizable entity solely by deliberate and continuous propagation. A key point is the influence of humans on the origin of cultivated plants, disrupting natural evolutionary and environmental factors and constraints. Plants in the wild are subject to natural selection, whereas cultivated plants are subject to conscious or unconscious human selection. Hetterscheid, van den Berg, and Brandenburg (1996) have argued that classifications of cultivated plants and wild plants have different goals. Whereas wild plants are classified in a system that seeks to clarify evolutionary relationships, cultivated plants are (or should be) classified according to special purpose user-defined criteria, with stability of names as primary, requiring a totally different classification philosophy. Practitioners of the taxonomy of cultivated plants have not yet completely accepted this (Hetterscheid and van den Berg 1996). Since the term taxon is used as a basis for evolutionary classifications, it seems illogical to use the same term for very different kinds of classifications. The most important consequence of this is the substitution of the concept of “culton” for “taxon” for systematic groups of cultivated plants (Hetterscheid and Brandenburg 1995a, 1995b), but this term has not yet been included in ICNCP rules to the full extent. 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 Nomenclature for Cultivated Plants. This definition emphasizes the essential role of human activity, in using the term “user-criteria” as the sole basis for the creation of systematic groups of cultivated plants (culta). This does not preclude studies of the origin of cultivated plants from existing natural populations. The point here is to divorce the nomenclature of cultivated plants from closed classifications that imply relationships, because artificial selection and hybrid origins often render this system nonsensical and nomenclaturally unstable.
C. Open Versus Closed Classifications
Classifying plants involves putting sets of individual plants in boxes, where the boxes are the ranks in the taxonomic hierarchy (e.g., species, genus, family, order). On the basis of classification criteria, a number of individuals are put in a box. This system of boxing has one important principle: every box belongs in a higher, more inclusive (larger) box and, vice versa, every box contains one or more boxes itself, with the largest box being “life.” In classification terms, this equates to: one or more species add up to form a genus, one or more genera add up to form a family, all the way through the taxonomic hierarchy. When we supplant the term box with taxon we have described the classification system of the ICBN and which is called a closed classification system. The ICBN says that there are an infinite number of levels (ranks) that can be constructed and named. Some ranks are specifically mentioned (e.g., the ranks called subspecies, species, genus, family, order) but their number may be increased infinitely. This is the nature of the hierarchy of levels typical in traditional nomenclature. Another mechanism typical for closed classifications is that when the individuals in a certain box (taxon) are going to be put in smaller boxes, all those individuals must be in smaller boxes and not one may be left on its own in the larger box. For that one leftover, a separate box has to be created and even named. The content of the boxes (taxa) is determined for a particular group of individuals by a taxonomist studying that group. Currently, evolutionary relationship is the primary criterion for grouping plants in taxa. Such classifications are called natural classifications because they describe relationships that taxonomists think are “real” in nature (see “Culton Versus Taxon”). Typical for such classifications is the use of as many characters as possible to gain a solid evolutionary basis. This frequently is contrasted to artificial classifications that are based on nonevolutionary special-purpose criteria. Whereas taxonomists of wild plants follow this closed classification system, taxonomists of cultivated plants have good reason not to use it (Hetterscheid, van den Berg, and Brandenburg 1996). Cultivated plants are created according to the whim of individuals. They arise by considerable hybridization and rapid selection for extreme forms. Classifying cultivated plants is directed by needs of individual groups of users, completely unlike the reason for classifying wild plants. Whereas one group may want to classify certain cultivated plants based on resistance against pests, another group may want to classify those same plants based on ornamental value. As such, the classification criteria for a certain group of cultivated plants may vary considerably, leading to the need of several coexisting special purpose classifications. Typically such classifications use few criteria and are sometimes called “artificial” classifications. In the philosophy of open classification (Brandenburg, Oost, and van de Vooren 1982; Brandenburg, 1986) special purpose classifications are allowed. In this system, the only boxes created are those needed for utility to users. That is, cultivated plants not possessing the characters of interest are left out of that particular classification. This is quite logical, because if we would include this last group, it would have to be based on not having a number of characters, which is contrary to the goal of the classification in the first place. For instance, in a classification of a crop the attribute of leaf shape may have led to the recognition of a box called the Laced-leaf Group and a box called the Dentate-leaf Group. The mere existence of these two boxes does not mean that all plants of that crop not having laced or dentate leaves automatically define a third box because this would have to be defined as “plants not having dentate or laced leaves,” which is quite the opposite of the original classification intent. That particular “left-over” group would contain a very heterogeneous assemblage of cultivars, which is diametrically opposite the whole idea of user-criteria driven classification (for examples see Hetterscheid and van den Berg 1996; Hoffman 1996; Hetterscheid et al. 1999). Another simplifying attribute of open classifications compared to closed ones is the avoidance of complex hierarchies and hierarchy names. Currently, classifying cultivated plants in an open classification system only requires two categories, the cultivar and the cultivar-group (van den Berg 1999). Fewer names means greater stability of names (Hetterscheid 1999) and this is further aided by the fact that the ICNCP provides substantially fewer nomenclatural mechanisms compared the ICBN.
In the interest of nomenclatural stability it may at first glance seem wise to standardize the nomenclature of biological objects once and for all. This would not allow, however, for changes to accommodate new theories of evolution and classification. For example, Linnaeus’s very useful development of binomial nomenclature predated modern ideas of evolution espoused by Darwin (1859). If traditional nomenclature was “frozen” since the time of Linnaeus we would be burdened with an archaic system with little biological relevance. Hence, classification rules change along with new data and needs (every six years for the
ICBN, irregularly for the ICNCP).
A. Ambiguity of the Term Variety
The term variety has caused much confusion. One meaning, as used by the ICBN (the “botanical variety”), is a particular rank in the taxonomic hierarchy below the rank of species and subspecies and above the rank of form (form /variety /subspecies / species). Another meaning, as used in the ICNCP (the “cultivated variety” or “cultivar”), refers to cultivated variants originating through human influence. Regarding the use of variety in the ICBN, species variation has been subdivided through infraspecific classifications. The relationships of the infraspecific categories allowed in the ICBN are strictly hierarchical, and as such they are differentiated by their degree of uniqueness: subspecies within a species should differ less among themselves than separate species, varieties should differ less among themselves than subspecies, and forms less than varieties. In practice, however, different taxonomists treat variation patterns differently. For example, sometimes a species is subdivided in subspecies and these in varieties, but in other cases a species is subdivided directly into varieties and the subspecies rank is not used at all (Hamilton and Reichard 1992). Some taxonomists feel that recognizing subspecies indicates a geographical component, with subspecies being mostly allopatric, while varieties may be sympatric. This is not a formal or universally held distinction between these ranks, however. The term becomes especially confusing with the wish to assign cultivated plants to the species from which they originate, resulting in the application of the term to cultivated plants in a form that appears as a botanical variety. The use of the rank variety for cultivated plants goes back to Linnaeus (1753). In many cases Linnaeus started his treatment of a species with the wild plant, mentioning cultivated varieties at the end (Wijnands 1986). Linnaeus clearly considered varieties as minor variants due to the influence of climate or soil, or in the case of cultivated varieties, of human influence. He later stated that the grouping of cultivated plants should be the task of beginners in botany, while qualified botanists should study species and higher taxonomic levels (Linnaeus 1764). Many later workers on the taxonomy of cultivated plants continued the practice of applying variety names for cultivated plants, burdening nomenclature with formal names with all the inherent problems of typification and priority that these entail. In these systems (e.g. Helm, 1957, 1963) the varieties are often grouped in artificial higher categories like convariety (or convar). Convarieties can be roughly comparable to cultivar groups, but convarieties, unlike cultivar groups, do not necessarily contain named varieties, and convarieties are members of traditional “Linnaean” ranks. The ICNCP replaced this term with the term cultivar-group, and convarieties should not be used in modern cultivated plant taxonomy (Trehane et al., 1995). Some modern influential works, however (e.g., Hanelt 2001), ignore rules of the ICNCP and continue to use the term convar (convariety). The term cultivated variety (cultivar) in the ICNCP, in contrast, is used in a very different way. The botanical variety has its fixed position in the taxonomic hierarchy. The cultivated variety stands outside this hierarchy because it could have resulted from many different processes as selection or a complex series of interspecific hybridizations, making it impossible to assign it a position in the hierarchy. Because of this, the nomenclature of the cultivated variety follows the ICNCP, dispensing with Latin epithets used in hierarchical ranks in the ICBN. Presently, however, names originally published as botanical varieties still refer to cultivated material. These entities can be reclassified as cultivars, or if a botanical variety was described to encompass many cultivated morphotypes (as is the case in the classification of Brassica oleracea) they can be reclassified as cultivar-groups (van den Berg 1999). Thus, the botanical variety Brassica oleracea var. gemmifera can be reclassified as Brassica oleracea Gemmifera Group, encompassing the many cultivars of brussels sprouts. However, the term variety for cultivar is still in wide use in legal documents all over the world. ICNCP deals with this in stating that the term variety as used in such texts is fully equivalent to cultivar. Legally, variety can have additional definitions. For example, the U.S. Plant Variety Protection Act (PVPA) uses the term “cultivar” in a manner similar to the botanical “variety,” but with the additional stipulation that “development” must take place from wild stock, as through breeding or genetic engineering. That is, discovery of unique variants alone does not make a cultivar eligible for protection under the PVPA. In addition, PVPA protection of varieties is granted with the additional requirements that it is “new,” “distinct” from other cultivars, “uniform,” and “stable.”
The botanical rank form has also been used extensively to describe minor variants of cultivated plants. Its use in the classification of wild plants is generally discouraged because the entities that could be described as forms are usually such minor morphological variations that it is arguable whether their distinction is useful. For cultivated plants these forms may easily be reclassified as cultivars. The same goes for the many informal and often ill-defined terms like strain, sport, type, and so on. If any such entity is worthy of recognition and description, it will be best to employ the general term cultivar for all of these.
B. Culton Versus Taxon
A fundamental difference between the ICNCP and ICBN is their respective
approach toward classification. Groups of plants used in the ICBN to classify and name are collectively designated as taxa (singular: taxon). The ICNCP uses the terms cultivar and cultivar-group for cultivated plants. Although it claims that they are taxa, these terms do not fit the definition of taxa for several reasons. This may become clear by the definition in the ICNCP of the term cultivated plant: A cultivated plant is one whose origin or selection is primarily due to the intentional activities of mankind. Such a plant may arise by deliberate or, in cultivation, accidental hybridization, or by selection from existing cultivated stock, or may be a selection from minor variants within a wild population and maintained as a recognizable entity solely by deliberate and continuous propagation. A key point is the influence of humans on the origin of cultivated plants, disrupting natural evolutionary and environmental factors and constraints. Plants in the wild are subject to natural selection, whereas cultivated plants are subject to conscious or unconscious human selection. Hetterscheid, van den Berg, and Brandenburg (1996) have argued that classifications of cultivated plants and wild plants have different goals. Whereas wild plants are classified in a system that seeks to clarify evolutionary relationships, cultivated plants are (or should be) classified according to special purpose user-defined criteria, with stability of names as primary, requiring a totally different classification philosophy. Practitioners of the taxonomy of cultivated plants have not yet completely accepted this (Hetterscheid and van den Berg 1996). Since the term taxon is used as a basis for evolutionary classifications, it seems illogical to use the same term for very different kinds of classifications. The most important consequence of this is the substitution of the concept of “culton” for “taxon” for systematic groups of cultivated plants (Hetterscheid and Brandenburg 1995a, 1995b), but this term has not yet been included in ICNCP rules to the full extent. 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 Nomenclature for Cultivated Plants. This definition emphasizes the essential role of human activity, in using the term “user-criteria” as the sole basis for the creation of systematic groups of cultivated plants (culta). This does not preclude studies of the origin of cultivated plants from existing natural populations. The point here is to divorce the nomenclature of cultivated plants from closed classifications that imply relationships, because artificial selection and hybrid origins often render this system nonsensical and nomenclaturally unstable.
C. Open Versus Closed Classifications
Classifying plants involves putting sets of individual plants in boxes, where the boxes are the ranks in the taxonomic hierarchy (e.g., species, genus, family, order). On the basis of classification criteria, a number of individuals are put in a box. This system of boxing has one important principle: every box belongs in a higher, more inclusive (larger) box and, vice versa, every box contains one or more boxes itself, with the largest box being “life.” In classification terms, this equates to: one or more species add up to form a genus, one or more genera add up to form a family, all the way through the taxonomic hierarchy. When we supplant the term box with taxon we have described the classification system of the ICBN and which is called a closed classification system. The ICBN says that there are an infinite number of levels (ranks) that can be constructed and named. Some ranks are specifically mentioned (e.g., the ranks called subspecies, species, genus, family, order) but their number may be increased infinitely. This is the nature of the hierarchy of levels typical in traditional nomenclature. Another mechanism typical for closed classifications is that when the individuals in a certain box (taxon) are going to be put in smaller boxes, all those individuals must be in smaller boxes and not one may be left on its own in the larger box. For that one leftover, a separate box has to be created and even named. The content of the boxes (taxa) is determined for a particular group of individuals by a taxonomist studying that group. Currently, evolutionary relationship is the primary criterion for grouping plants in taxa. Such classifications are called natural classifications because they describe relationships that taxonomists think are “real” in nature (see “Culton Versus Taxon”). Typical for such classifications is the use of as many characters as possible to gain a solid evolutionary basis. This frequently is contrasted to artificial classifications that are based on nonevolutionary special-purpose criteria. Whereas taxonomists of wild plants follow this closed classification system, taxonomists of cultivated plants have good reason not to use it (Hetterscheid, van den Berg, and Brandenburg 1996). Cultivated plants are created according to the whim of individuals. They arise by considerable hybridization and rapid selection for extreme forms. Classifying cultivated plants is directed by needs of individual groups of users, completely unlike the reason for classifying wild plants. Whereas one group may want to classify certain cultivated plants based on resistance against pests, another group may want to classify those same plants based on ornamental value. As such, the classification criteria for a certain group of cultivated plants may vary considerably, leading to the need of several coexisting special purpose classifications. Typically such classifications use few criteria and are sometimes called “artificial” classifications. In the philosophy of open classification (Brandenburg, Oost, and van de Vooren 1982; Brandenburg, 1986) special purpose classifications are allowed. In this system, the only boxes created are those needed for utility to users. That is, cultivated plants not possessing the characters of interest are left out of that particular classification. This is quite logical, because if we would include this last group, it would have to be based on not having a number of characters, which is contrary to the goal of the classification in the first place. For instance, in a classification of a crop the attribute of leaf shape may have led to the recognition of a box called the Laced-leaf Group and a box called the Dentate-leaf Group. The mere existence of these two boxes does not mean that all plants of that crop not having laced or dentate leaves automatically define a third box because this would have to be defined as “plants not having dentate or laced leaves,” which is quite the opposite of the original classification intent. That particular “left-over” group would contain a very heterogeneous assemblage of cultivars, which is diametrically opposite the whole idea of user-criteria driven classification (for examples see Hetterscheid and van den Berg 1996; Hoffman 1996; Hetterscheid et al. 1999). Another simplifying attribute of open classifications compared to closed ones is the avoidance of complex hierarchies and hierarchy names. Currently, classifying cultivated plants in an open classification system only requires two categories, the cultivar and the cultivar-group (van den Berg 1999). Fewer names means greater stability of names (Hetterscheid 1999) and this is further aided by the fact that the ICNCP provides substantially fewer nomenclatural mechanisms compared the ICBN.