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Taxonomy (part II)

In view of a more traditional approach to the classification of the Cactaceae, both genera and species

Time, reality, individuality

In the conclusions of the previous text, dedicated to the taxonomic aspect of our studies (Anceschi & Magli 2010, 19), we stressed that the results obtained using phylogenetic criteria to achieve a genealogical classification according to Darwin, or a natural classification according to Hennig, expressed through the Linnaean hierarchical system, make up one of the possible interpretations (the one used by current science) to classify living things. The reason why a phylogenetic classification is preferable to others (morphological, typological, etc..), is that living things are transformed over time, they have a beginning and an end, and in this sense are manifested as real processes, provided with individuality. A classification which does not consider this aspect, does not set as object of study real phenomena, but rather artificial projections. The idea is expressed by N. Hartmann: “The true characteristics of reality are not dependent on the categories of space and matter, but of those of time and individuality. And temporality is inseparably connected with individuality. It consists in nothing else but the onceness and the singleness” (Hartmann 1942, quoted from Hennig 1966, 81). For reality Hartmann means “the mode of existence of everything that has a place or a duration in time, its origins and its cessation” (ibid.). So the entities (individuals, populations, species) measured by phylogenetic criteria, are concrete and real entities, with a beginning, contrasting with those that are abstract and timeless, whose distinction is based on other parameters. In the last pages of Philogenetic Systematics, Hennig (1966, 238-239) stresses the importance of an exact chronology of the real historical events in phylogeny, to distinguish the monophyletic groups from those that are paraphyletic, and that terms like reality, individuality, origin, differentiation and extinction have a different significance for the different groups. The correct interpretation of the direction of time’s arrow is therefore an essential element in establishing the reality and proper assessment of the links between all members of an evolutionary line, coming from the same ancestor or monophyletic group.

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Higher taxa: methods and definition techniques

Many authors consider that only the individuals and the species are real entities, whereas the other taxonomic categories (from the genus up) are mere abstractions. For Plate, for example, the species occupies “a position distinct from the genus, family, etc. in that it exists in nature as an actual ‘complex of individuals’ independent of human analysis, and therefore as an objective entity. The members of a species recognize each other and reproduce together, whereas the higher groups of individuals (genus, family, etc.), are not formed through themselves, but by the comparing and reflecting mind of man. In this sense the species is real, whereas the genus, family, and other higher groups are abstractions”. (Plate 1914, in Uhlmann 1923, quoted from Hennig 1966, 78). As already highlighted in Taxonomy (Anceschi & Magli 2010, 9), the only higher taxon which we deal with, in our classification system of the Cactaceae (in addition to the family), is the genus, making the infrageneric ranks unnecessary (sub-genera and groups) (ibid., 13, 18) as well as the suprageneric (subtribes, tribes and subfamilies). For the definition of higher taxa, and the relationships between them, the current biological systematics uses:

a) Hennig‘s phylogenetic systematics theories (1966) and of his successors Wiley (1981); Wiley & Liebermann (2011).

b) The large amount of comparative information from molecular investigations (ie the DNA sequence data from different genomes).

c) Software able to handle large amounts of information in complex numerical analyses.

d) Likelihood models of DNA sequence evolution and statistical tests to explore and evaluate the probabilities of competing phylogenetic hypotheses.

Currently the most important work on the classification of the higher taxa in the Cactaceae is by Nyffeler and Eggli (2010, 6: 109-149).

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The interpretation of the arrow of time in the methods for the definition of the higher taxa. The recognition of monophyletic groups through the system based on symplesiomorphy / synapomorphy

As already mentioned, an exact chronology of the real historical events is what distinguishes the monophyletic groups from other non-natural groups. In the theory of the cladistical phylogenetic systematics (Hennig 1950, 1966; Wiley 1981; Wiley & Liebermann 2011), in the time which gave rise to the transformation process, which led to the current species (and genera), the first ancestor of the analysed group was born with the stem species. The time dimension during the process is marked by the moments of splitting, which during the series of transformations leads from the stem species to the current taxa. The ancestors of the current species are extinct, not only the first, but also those representing the successive points of splitting in the transformation process. The only possible connection based on a real time (even in the past), would be the reconstruction of the steps, through the instruments of paleontology, but for the family Cactaceae A. L. de Jussieu, there are no relevant fossils. The instruments provided by the cladistics school to recognize monophyletic groups, ie those that “... are subordinated to one another according to the temporal distance between their origins and the present; the sequence of subordination corresponds to the ‘recency of common ancesty’ of the species making up each of the monophyletic groups “(Hennig 1966, 83), are the characters that Hennig identified as synapomorphy. He considers the characters carried by the first ancestor (stem species) plesiomorphous, those derived during the process of transformation, and fixed on subsequent ancestors (later becoming extinct themselves) apomorphous. In the current species, the characters directly inherited from the first ancestor (plesiomorphous) are defined symplesiomorphous, while the characters derived from more recent ancestors (apomorphous), although not necessarily the most recent, are defined synapomorphous (ibid., 89). So let’s call synapomorphies a particular category of characters, i.e. those that distinguish a monophyletic group, which are inherited by all members of the group, or clade, from a recent common ancestor. In the identification of these characters, it is essential to have the distinction of those who are genuinely sinapomorphous from:

a) Those that are simplesiomorphous, which like the first, represent types of homologous characters, i.e. inherited from a common stem species.

b) Those resulting from evolutionary convergence or parallelism (analogous characters), either morphologically similar in different species, but not derived from a common ancestor (due to convergence), or similar characters, absent in the stem species of a monophyletic group, occurred independently in the subsequent species (parallelism) (ibid., 117).

Phylogenetic systematics start rather from the conviction that all correspondences and differences between species and groups of species, in the course of phylogeny, arose out of an alteration of characters of the common stem species (ibid., 128).

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The interpretation of synapomorphy: a problem of intuitive nature

In Hennig’s own opinion: “... there is no simple and absolutely dependable criterion for deciding whether corresponding characters in different species are based on synapomorphy. Rather it is a very complex process of conclusions by which in each individual case, ‘synapomorphy’ is shown to be the most probable assumption” (ibid., 128). Furthermore, “... the attempt to reconstruct the phylogeny, and thereby the phylogenetic relationships of species, from the present conditions of individual characters and the presumed preconditions of these characters has the nature of an integration problem. In mathematics, the most exact science, according to Michaelis (1927), ‘integration... is an art... since one is often faced with the problem of combining, from the numerous possible manipulations, those that make possible the solution of the problem.” (ibid., 128-129). The “manipulation” made available by the author for the distinction of the monophyletic groups between the higher taxa, is precisely the one based on the system of symplesiomorphy and synapomorphy. Hennig adds that the solution to a particular problem depends on capabilities that do not lie in the realm of the learnable (what we would call intuition), quoting the words of the mathematician Gauss: “I have the result, but I don’t know yet how I got it” (ibid., 129). Similar positions are reiterated by Wiley & Liebermann (2011, 122-123) which, referring to Hennig, underline how the basic principle of phylogenetic empiricism is made up of the fact that discovering homologies is an observational hypothesis, not a fact, because we have no perfect method of observing real homologies as they exist in nature. And they add: “... the assertion that two or more organisms share a homology or the assertion that a particular synapomorphy is a character property of a particular monophyletic group are both probabilistic conjectures (Patterson, 1982; Haszprunar, 1998; Sober, 2000) whose veracities are always open to further testing as opposed to deductive conclusions (e.g., Rieppel 1980).” (Wiley & Liebermann 2011, 123). On the basis of the selected characters to identify groups, Hennig defines those where similarity is based on synapomorphy as monophyletic; those in which similarity is based on symplesiomorphy as paraphyletic; if similarity is due to convergence then they are polyphyletic (1966, 146). The system based on symplesiomorphy / synapomorphy made available by the author to distinguish monophyletic groups in the phylogenetic study of the higher taxa, is in many cases indispensable, since it allows a possible interpretation of the evolutionary history of groups of taxa, even in the absence of fossils (as in the case of the family Cactaceae), but also of difficult and varied interpretation. Often, cladistic analysis based on morphological characters (Taylor & Zappi 1989), or molecular ones, have in our opinion been imposed with unreliable results. In fact, the application of a system designed for higher taxa (in Hennig 1966, mainly families, suborders, orders, subclasses, classes) it is not always possible for the analysis of infrageneric groups, genera or tribes. Therefore, groups consisting of a few species, which by themselves (sometimes by the author’s own admission, ibid., 14, 29, 39) cannot show quantitative, and qualitative characters, in order to be interpreted as ancestral or derivatives, and then to draw reliable phylogenetic conclusions on the analyzed taxa. A principle that Hennig summarizes: “For phylogenetic systematics this means that the reliability of its results increases with the number of individual characters that can be fitted into transformation series” (ibid, 132). We discussed the characters used in the classification of the genera and species of the Cactaceae in the first text dedicated to taxonomy (Anceschi & Magli 2010, 14-18), asking ourselves if the current preference given to molecular results is correct. In this regard, we stressed that the molecular data cannot be considered as absolute data, but must be evaluated in a ratio of relations with all other data of the characters making up the holomorphology of a taxon. Similarly, Wiley & Liebermann (2011, 121) point out that the behavioral synapomorphy does not have less value in systematics than morphological homologies, and (citing McLennann et al. 1988) that homoplasy (convergence) phenomena do not necessarily occur at higher frequency in behavioral characters than in morphological characters. In underlining this they state: “Similarly, in this view, morphological homologies are no less suited for phylogenetic analysis than DNA sequence homologies. The idea that one kind of data is inherently better than other kinds of data is not viable under this concept, and hypotheses of homology from whatever source can and should be allowed to compete on an even playing field as potential evolutionary innovations (e. g., discussion in Hillis, 1987).” (Wiley & Liebermann 2011, 121).

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The interpretation of time’s arrow in molecular analysis results

Although the methods based on chemical or molecular analysis in phylogenetic systematics that Hennig (1966, 104-107) founded were not particularly useful, it is clear that molecular characters exist, which are of great importance, occurring in what Hennig called “holomorphological characters” (ibid., 32), and which can be used in the system based on symplesiomorphy / synapomorphy. How is the exact chronology of the real historical events in the molecular data reconstructed? The nucleotide sequences that can be analyzed (after PCR, electrophoresis, sequence alignment) are photos of the current DNA, just as the species we can observe are just the current ones. In both cases the ancestral element no longer exists; some molecular characters are identified as ancestral, and others as derived. For example, in the previously cited study by Nyffeler & Eggli (2010), in the part concerning the tribe Notocacteae, the deletion of 23 nucleotides, highlighted in the representatives of Parodia s.l., is considered a derived character (sinapomorphy), and the presence of these in the other two groups in question, a primitive character (simplesiomorphy). We are not entirely convinced of the fact that molecular systematics (DNA sequences) almost completely avoid the similarity resulting from parallel evolution in order to infer relationships, because the molecular characters are not subject to the same external forces as the phenotype (Wallace 1995, 13: 2). We believe, in fact, that genes are part of the hardware of a living being, like any other biochemical substance, and that they receive instructions from the rest of the cell, no less than they give. The idea of an immutable DNA is not realistic, and the latest frontiers opened by epigenetic science seem to show a panorama that is a little more complex. In this regard Nyffeler & Eggli observes: “It is generally assumed that DNA sequences, in particular of ‘non-coding‘ regions of the genome, are not affected by evolutionary processes interfering with the phenotype of the individual organisms. However, there are also molecular evolutionary phenomena currently not yet well understood that may cast dust onto the preserved historical signal” (2010). We have seen that through the system based on symplesiomorphy / synapomorphy, using an interpretative approach of the holomorphological data, we can attempt to reconstruct the steps in the series of transformations linking the actual species to their extinct ancestors. The fossil records (not available in the family Cactaceae), when present and in a good condition, can help in the reconstruction of temporal events, but often this does not occur. As for the morphological characters, even the molecular ones, one of the most important methods for attributing homologies is the similarity in position; in the case of molecular characters it is used in the alignment of the sequences of the nucleotides (Wiley & Liebermann 2011, 124-129). In a group of taxa the similarity of the topographic position of a character relative to other characters, and of the body as a whole, is interpreted as evolutionary proximity; in the same way similar alignments are read in the sequences of the nucleotides. For molecular data, the temporal dimension of the splitting moments, by which new species are born, is given by a scan based on probability. As already said, the current molecular biology is able to handle a large amount of comparative information, from the molecular investigations through to appropriate software. Likelihood models of the analyzed DNA sequence evolution and statistical tests are used to explore and evaluate probabilities of competing phylogenetic hypotheses. Among the more used models we are using are: parsimony analysis, maximum likelihood analysis and Bayesian analysis. The common aim of these methods, albeit through different routes, is to reconstruct the best chances of parental relationships between the examined taxa, then showing the results through cladograms (diagrams of phylogenetic trees). We will briefly analyze the three approaches. Philosophically, in accordance with Wiley & Liebermann (2011, 152), the principle of parsimony is a methodological principle, which implies that simpler explanations of the data are to be preferred to more complex ones. In the construction of a phylogenetic tree, the principle is thus synthesized: “Parsimony differs from other approaches because trees are evaluated based on minimum length - the minimum number of changes in characters that are hypothesized to have occurred for any particular tree hypothesis. Trees of minimum length fulfill the principle. Parsimony is then built around the proposition that the ‘best tree’ is the tree that describes the evolution of any particular set of characters using these smallest number of evolutionary changes of the characters analyzed” (ibid., 153). Maximum likelihood and Bayesian analysis are both parametric phylogenetic models, i.e. based on a specific evolution model chosen by the investigator. In the maximum likelihood methods, according to Wiley & Liebermann, the basic criterion is: “... the preferred tree is the tree that has the highest probability of producing the data we observe [the observed DNA sequences], given a specific [stochastic] model of evolution adopted by the investigator, the tree topology and the branch lengths between nodes” (ibid., 203). Maximum likelihood uses an explicit evolutionary model. We assume that the data we observe are identically distributed from this model. Even Bayesian analysis uses likelihood calculations, but the criterion employed is that of maximizing the posterior probability of the tree, given the data and model of influence. Returning to the arrow of time, likelihood calculates the probability that an event that has happened in the past would yield a specific outcome, while Bayesian analysis explores the posterior probability to find the model / tree topology, the largest posterior probability is conditioned by what the investigator is willing to accept as true before the analysis.

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The latest taxonomic changes in the higher taxa of the Cactaceae. The genus level. The importance of molecular evidence

Since Wallace’s study (1995, 13: 1-12), during the last decades, changes at the genus level and the higher taxa in the family Cactaceae, have almost always been followed by new evidence emerging from molecular analysis. As already said, probably the most comprehensive molecular biology study, applied to the higher taxa (genera, subtribes, tribes and subfamilies) on Cactaceae, is that of Nyffeler & Eggli, which appeared in Schumannia (2010, 6: 109-149). The two authors recognize, at the genus level, 128 taxa (ibid.) versus the 124 recognized in Hunt et al. (2006, text: 5), and the 125 recognized in Anderson (2001). 128 genera are still accepted by Eggli, as author of the latest German edition of Anderson’s book, Das Grosse Kakteen Lexikon (2011), but removing 4 genera: Borzicactus Riccobono, Rimacactus Mottram, Strophocactus Britton & Rose, Vatricania Backeberg, and adding 4 others: Acharagma (N. P. Taylor) Glass, Cintia Knize & Riha, Pygmaeocereus H. Johnson & Backeberg, Sulcorebutia Backeberg (in relation to the list of the genera accepted together with Nyffeler). The 2010 study by Nyffeler & Eggli substantially confirms the positions of the previous literature (Anderson 2001, 2005; Hunt et al. 2006), about the formation of certain macro-genera, including many ex-genera especially loved by the enthusiasts (who might demand their reintroduction). Among these genera, in the tribe Notocacteae Buxbaum, the authors confirm that Parodia Spegazzini s.l. (Nyffeler 1999, 7: 6-8) is a well-supported monophyletic clade, which includes the previous segregated genera Brasilicactus Backeberg, Brasiliparodia F. Ritter, Eriocactus Backeberg, Notocactus (K. Schumann) Frič, and Wigginsia D. M. Porter (ibid.). In the same tribe, the data of the analysis do not support Eriosyce Philippi s.l. (Kattermann 1994), nor the current expanded concept of the genus, which includes the previously segregated genera Horridocactus Backeberg, Islaya Backeberg, Neoporteria Britton & Rose, Pyrrhocactus (Berger) A. Berger and Thelocephala Y. Itô, nor a more restricted concept, since the relationships remain unresolved, requiring further analyses with additional data (ibid.). The most interesting news has arrived from the subtribe Trichocereinae Buxbaum (ibid.), where on the basis of previous molecular analyses (Ritz et al. 2007; Lendel et al. umpubl. data; Nyffeler et al. umpubl. data), it is clearly demonstrated that flower characters and pollination syndromes are highly plastic and evolutionarily labile, and therefore the presence or absence of a certain syndrome is not a sign of closeness or distance of two lineages. In this sense, distinctions based on the different floral syndromes, such as those used by Backeberg (1966) to separate genera (eg Echinopsis s.s., Lobivia, Pseudolobivia) now seem devoid of meaning. Nyffeler & Eggli (2010), point out that in the subtribe Trichocereinae, the most difficult group to interpret, the macro-genus Echinopsis Zuccarini s.l. appears, plus the genera currently recognized as segregates: Acanthocalycium Backeberg (separated from Echinopsis in Anderson 2001, 2005, 2011, but not in Hunt et al. 2006), Denmoza Britton & Rose, Harrisia Britton, Samaipaticereus Cárdenas, Weberbauerocereus Backeberg, and Yungasocereus F. Ritter. In recent molecular analyses the authors (Lendel et al. 2006; Schlumpberger 2009) agree that all these taxa are very closely related with Echinopsis s.l. being widely polyphyletic. Regarding the relationships highlighted, the authors argue prematurely to draw firm conclusions, preferring to wait for additional results, more comprehensive of molecular studies (eg based on more species-dense sampling). Although choosing to not change the genus Echinopsis s.l. as currently conceived, (Anderson 2001, 2005, 2011; Hunt et al. 2006), the two authors show that both the molecular data, and the widespread occurrence of intergeneric hybrids (see Rowley 1994, 2004a, 2004b for listing), indicate that Trichocereinae has a relatively recent evolutionarily origin [ie about 7.5-6.5 Ma according to Arakaki et. al. (2011, 8380)], and that the genetic divergence between the various taxa is far lower than the difference shown by the same in morphological and floral characters. It seems in fact, that not only the floral syndromes are evolutionarily labile, but that also the growth forms appear to have changed repeatedly within this clade. In the data of Lendel et al. (2006, unpubl. data), the authors summarize their position as: “The close relationships between taxa of divergent growth forms (such as the voluminous columns of Echinopsis tarijensis and the tiny dwarf Echinopsis chamaecereus) within one and the same clade illustrates the previously formulated caveats as to ‘logical’ evolutionary pathways in character transitions in an exemplary manner” (Nyffeler & Eggli 2010). This approach is completely overturned by Schlumpberger (2012, 28: 29-31) with the option he chose, between the two possible highlighted by the results of his latest study in Echinopsis made with Susanne S. Renner (2012, 99 (8): 1335-1349). We’ll see how what seems a taxonomical “revolution” in the genus Echinopsis, is nothing more than an attempt at “restoration” of old ideas, and how the evolutionary hypothesis is more convincingly eluded by the authors.

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Dr. Schlumpberger’s monsters

In September 2012 the awaited work of Schlumpberger & Renner on Echinopsis and related genera came out (2012, 99 (8): 1335-1349); a study which currently represents the most comprehensive analysis on the taxa in question. The aim is to attempt to define the real relationships between the heterogeneous components of Echinopsis s.l. as currently conceived (Anderson 2001, 2005, 2011; Hunt et al. 2006; Nyffeler & Eggli, 2010), and related genera of the tribe Trichocereeae (Anderson 2001, 2005, 2011; Hunt et al. 2006), or subtribe Trichocereinae (Nyffeler & Eggli 2010). The macro-genus currently comprises of between 7 (Nyffeler & Eggli 2010) and 11 (Anderson 2001) ex-segregated genera. Genera initially separated by distinctions in the growth form (eg globular in Echinopsis s.s. / columnar in Trichocereus (A. Berger) Riccobono), diurnal vs nocturnal anthesis (Lobivia Britton & Rose / Echinopsis s.s.), or different pollination syndromes (eg. hummingbirds in Cleistocactus Lemaire / bats in Espostoa Britton & Rose). The number of species included starts from 77 in Hunt et al. (2006), to 129 in Anderson (2001), as well as various heterotypic subspecies. These molecular analyses consider the sequencing of 3866 nucleotides of cp DNA representing 144 species and subspecies in Echinopsis, including the type species of all relevant generic names, as well as representatives of all genera in recent years assigned to the Trichocereeae tribe, again including relevant generic type species, in addition to the outgroup. The data concerning chromosome counts, pollination syndromes and growth habits of the taxa in question, were traced on the phylogeny. PCR amplified 3 noncoding cp DNA regions, using 3 published standard primers: the trnS-G intergenic spacer (Hamilton 1999), the trnL region (Taberlet et al. 1991), and the rpl16 intron (Asmussen 1999). Phylogenetic inferences based on the maximum likelihood (ML), obtained by the alignments of 3866 nucleotides of cp DNA, for the 144 taxa, appear on a phylogram (ibid., 1342-1343). The absence of statistically supported topological contradictions is defined as > 70% maximum likelihood bootstrap support (the numbers at nodes in the phylogram). The tree is completed by the data on chromosome numbers, pollination syndromes and growth habits. The researchers used this data to address the following questions (ibid. 1336):

1) Is Echinopsis s.l. monophyletic?

2) What is the level of variation in growth habit and pollination syndromes, characters that have been used to define clades in Echinopsis s.l.?

3) How common are ploidy changes within the Echinopsis group, and are they clustered in certain subclades, perhaps indicating a role for hybridization in the evolution of certain species groups?

Regarding the first question, phylogenetic inference based on maximum likelihood produced the aforementioned tree, from which we observe that the genus Echinopsis, as currently conceived, is highly polyphyletic. According to the authors (ibid., 1336, 1341, 1346-1347) there are two possible options, which would allow for the interpretation of the taxa in question as natural (monophyletic) clades in Hennig’s sense. a) To be monophyletic, the genus Echinopsis should include: Acanthocalycium Backeberg (already included in Hunt et al. 2006), Acanthocereus A. Berger, Cleistocactus Lemaire (including Borzicactus Riccobono and Cephalocleistocactus F. Ritter, already included in Hunt et al. 2006), Denmoza Britton & Rose, Espostoa Britton & Rose (including Vatricania Backeberg, as already in Hunt et al. 2006), Haageocereus Backeberg, Harrisia Britton, Matucana Britton & Rose, Mila Britton & Rose, Oreocereus (A. Berger) Riccobono, Oroya Britton & Rose, Pygmaeocereus H. Johnson & Backeberg, Rauhocereus Backeberg, Samaipaticereus Cárdenas, Weberbauerocereus Backeberg, Yungasocereus F. Ritter. All of them form part, with the current species of Echinopsis s.l., of a well-supported monophyletic clade (100% bootstrap support). b) The alternative is to divide Echinopsis into smaller units. This solution requires the resurrection of old generic names, and the transfer of epithets at the specific level. The authors then discuss the major clades in which Echinopsis s.l. could be divided. These are: the Echinopsis s.s. clade (100 % bootstrap support); the Echinopsis atacamensis clade (100 % bootstrap); the Harrisia clade (97 % bootstrap); the Cleistocactus s.s. clade (100 % bootstrap), including Espostoa guentheri, Samaipaticereus, Weberbauerocereus, Yungasocereus, Cephalocleistocactus, Cleistocactus, but not Borzicactus; the Reicheocactus clade (100 % bootstrap); the Oreocereus clade (99 % bootstrap), including Oreocereus, Borzicactus, Espostoa, Haageocereus, Matucana, Mila, Oroya, Pygmaeocereus and Rauhocereus; the Denmoza clade (100 % bootstrap) that includes the monotypic Denmoza rhodacantha, Echinopsis mirabilis and Acanthocalycium with Echinopsis leucantha embedded, the Trichocereus clade (73% bootstrap), the Helianthocereus clade (76% bootstrap), and the Lobivia clade (93% bootstrap). Regarding the second question, the analysis shows that species grouped according to previous distinctive characters, i.e. growth habits, floral characters and pollination syndromes, do not form clades (ibid., 1341). The authors underline, however, that the growth habits appear to be more stable characters and therefore less subject to convergence phenomena compared with floral characters and pollination modes, which are highly plastic. For the third question, contrary to the conclusion about the importance of the hybridization role in the evolution of Cactaceae (Rowley 1994; Machado 2008), also hypothesized for Echinopsis (Friedrich 1974; Font & Picca 2001; Anderson 2005) and related genera (Rowley 1994), and despite the various infrageneric hybrids found in nature, polyploidy seems infrequent in the Echinopsis alliance and hybridization may thus be of minor relevance in the evolution of this clade. In the conclusions we read: “A new generic classification of the Trichocereeae now requires finding morphological characters sufficiently conservative for distinguishing larger groups of species. Seed morphology and growth form, perhaps in combination, seem promising starting points” (Ibid., 1348). Schlumpberger opts for option b), considering it “a more practical approach” a new division of Echinopsis in small separate genera. The result is the publication in Cactaceae Systematics Inititives (2012, 28: 29-31) of 48 new combinations in the resurrected genera Acanthocalycium, Chamaecereus Britton & Rose, Leucostele Backeberg, Lobivia Britton & Rose, Reicheocactus Backeberg, Soehrensia Backeberg, in view of a possible (and probable) publication in NCL 2. At this point we would have some objections to advancing with this kind of interpretation; because it seems to us a way of bringing something back through the window which had been let out of the door (with great effort).

Objection N°1: Practicality. Schlumpberger (ibid., 29) states that, instead of considering the idea of a monophyletic genus Echinopsis, which would require the inclusion of 15 genera hitherto never incorporated before, a more practical approach is the splitting of it into separate smaller genera again. Disagreeing with this statement, we recall that one of the synonyms of Denmoza rhodacantha (Salm-Dyck) Britton & Rose is Echinopsis rhodacantha (Salm-Dyck) Förster, and that the basionym of Oreocereus hempelianus (Gürke) D. R. Hunt is Echinopsis hempeliana Gürke. Also, if it is true that a monophyletic Echinopsis requires the inclusion of 15 genera, it is also true that the division proposed by Schlumpberger requires the resurrection of at least 7 old genera (Acanthocalycium, Chamaecereus, Leucostele, Lobivia, Reicheocactus, Soehrensia and Setiechinopsis), but most importantly, it does not solve the internal relationships of the clades Cleistocactus s.s. and Oreocereus (Schlumpberger & Renner 2012, 99 (8): 1342). In fact, for consistency with the other solutions adopted, the Oreocereus clade (99% bootstrap) or, given the results of the analysis, Borzicactus (according to Kimmach), should include: Borzicactus (or Oreocereus), Espostoa, Haageocereus, Matucana, Mila, Oroya, Pygmaeocereus and Rauhocereus. The clade Cleistocactus s.s. should include at least Vatricania guentheri (100% bootstrap), if not also Cephalocleistocactus, Samaipaticereus, Weberbauerocereus, Yungasocereus (100% bootstrap). Therefore, we see that under a practical perspective, the Schlumpberger’s proposal does not solve the relationships within the group in question in a natural way.

Objection N°2: Communication, clearness, order. According to Hunt (1999, 7: 8), we think that names, even before classification, serve to communicate. But to communicate, they should have an internal coherence that links them to the reality that they want to identify. In this sense, they should indicate an order. In this context, the “new” genera proposed by Schlumpberger do not even express clearness, let alone order. In contrast to the original genera of Britton & Rose and Backeberg, which although not natural (in Hennig’s sense), did show an internal coherence based on the recognisability of one or more characters that unite the members of the generic group. For example: more or less globular - diurnal anthesis = Lobivia; globular - white, funnel-shaped flowers - nocturnal anthesis = Echinopsis s.s.; columnar - large white flowers - nocturnal anthesis = Trichocereus; etc. But if we attempt to define, in the same way, to communicate the distinctions between the genera proposed by Schlumpberger, it generates chaos. In fact, the new genus Chamaecereus Britton & Rose, includes ex-members (and characters) of Lobivia, such as Lobivia saltensis Spegazzini, or Lobivia stilowiana Backeberg. The new genus Lobivia Britton & Rose, includes ex-members (and characters) of Echinopsis, such as Echinopsis calochlora K. Schumann, or Echinopsis mamillosa Gürke. The new genus Soehrensia Backeberg, includes ex-members (and characters) of Lobivia, such as Lobivia crassicaulis R. Kiesling, or of Trichocereus, such as Trichocereus angelesiae R. Kiesling, etc. We think that the aforementioned Schlumpberger & Renner’s conclusion is at least questionable, the conclusion with which the authors wonder about the possibility of finding, (for the classification of the genera of the Trichocereeae): “morphological characters sufficiently conservative for distinguishing larger groups of species. Seed morphology and growth form, perhaps in combination, seem promising starting points” (2012, 99 (8): 1348). It does not seem to us a serious way of proceeding, changing the names of 48 taxa, and only then, to wonder which could be the characters that will identify them. Are the molecular characters not characters in all respects? So, why do they not suffice in defining the groups in question? The answer is: they are not sufficient because the chosen phylogenetic hypothesis is less approximate to something that exist in nature. Instead: choosing the option of unifying the 15 genera in Echinopsis, the definition to identify them as part of the composed genus is simple: Echinopsis with floral characters and / or pollination syndromes modified.

Objection N°3: Something approximating to the truth in nature. Among the results of molecular analysis, the phylogenetic hypothesis must be chosen, which leads to a valid estimate of something that exists in nature. In other words, the success of the evolutionary model chosen in predicting new data, requires that the fit of data to the model may lead to something approximating to the truth in nature (see also Sober 2008). What are Cleistocactus, Denmoza, Haageocereus, Oreocereus, Weberbauerocereus, etc., if they are not Echinopsis with floral characters and / or pollination syndromes modified? The hypothesis is confirmed both at the molecular level, then at the morphological one (or holomorphological, in Hennig’s sense). For years, molecular analysis revealed the close relationship between Echinopsis s.l. and the other genera within the tribe Trichocereeae, or subtribe Trichocereinae, (Nyffeler 2002, 317, 319; Lendel et al. 2006, unpubl. data in Nyffeler & Eggli 2010); until the Schlumpberger & Renner’s latest analysis (2012), which reaffirmed, even more clearly, that a large part of the genera constituting the tribe Trichocereeae, form with Echinopsis s.l. a single well supported monophyletic clade. In nature the most striking example is the monotypic Denmoza rhodacantha, a taxon otherwise attributed by various authors to Cleistocactus, Echinopsis and Oreocereus, and which, for us, is the perfect link between the current concept of Echinopsis s.l. (resulting as polyphyletic), and a new monophyletic macro-genus Echinopsis, which also includes species of Echinopsis with floral characters and / or pollination syndromes modified.

Objection N°4: Coherence. Being the molecular biology results expressed through theories, methods and techniques which describe rules, and not laws (as for example the process by which are interpreted synapomorphies, or the phylogenetic inferences assigned to ML techniques, etc), in the interpretation of the results, the researchers coherence is essential. We recall that in a similar case, the aforementioned genus Parodia, the possible options / interpretations gave rise to opposite choices to those proposed for Echinopsis. In 1999, Nyffeler in Cactaceae Consensus Initiatives, proposed to the IOS Cactaceae Working Party members, the molecular analysis results conducted using ITS (nuclear ribosomal DNA) and trnL-trnF (cp DNA) as molecular markers to investigate the relationships between the members of the subtribe Notocactinae, and especially among those internal to Parodia s.l. (i.e. Brasilicactus, Brasiliparodia, Eriocactus, Notocactus, Parodia and Wigginsia) (1999, 7: 6-8). After detecting the basal position of Brasilicactus / Brasiliparodia and Eriocactus in the group, which in the words of Nyffeler, “are not true parodias” (ibid.: 7), 3 options are proposed:

1) Include everything in Parodia s.l., including Brasilicactus / Brasiliparodia, Eriocactus, ‘Notocactus’ s.s.,and Wigginsia.

2) Recognize Brasilicactus / Brasiliparodia, Eriocactus and Parodia s.l. (including ‘Notocactus’ s.s., and Wigginsia).

3) Recognize Brasilicactus / Brasiliparodia, Eriocactus, and probably up to 5 different genera for the rest of the members from ‘Notocactus’ s.s., Parodia s.s., and Wigginsia. At that time Hunt chose the first option, arguing: “And since, in biological nomenclature, the genus is part of the name, stability is best served by reserving that category for the largest readily recognizable ‘natural’ (i.e. evolutionary or phylogenetic) units, ... This would be my main reason for preferring the more inclusive options Reto identifies “(1999, 7: 8). Philosophically we agree with Hunt, and despite the diversity of Eriocactus compared with the other members of the group, for coherence we agree also with the phylogenetic option adopted (Anceschi & Magli 2013, 7: 27-29). Schlumpberger discussed his conclusions with the NCL “team” (2011, 25: 30; 2012, 26: 7; 2012, 28: 3-4), and the result is the 48 new proposed combinations in CSI (2012, 28: 29-31). We do not see any coherence of approach in this procedure. Maybe Cleistocactus and Oreocereus should be more “protected” than Notocactus and Eriocactus? As far as we are concerned we think that time cannot be reversed, and that the indications of the real relationships between the taxa involved in the Schlumpberger & Renner’s study are rather clear. As highlighted, we prefer to opt for the solution of a monophyletic macro-genus Echinopsis, with the consequent inclusion of the genera indicated in the study of Schlumpberger & Renner, currently involved in cactusinhabitat.org (ie Cleistocactus, Denmoza, Haageocereus, Harrisia, Oreocereus, Vatricania and Weberbauerocereus). For the new names and combinations required in Echinopsis see Anceschi & Magli (2013b, 37-40).

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The interpretation of the arrow of time in the methods of defining the lower taxa (the species). Hennig’s semaphoronts

As we have seen, the temporal dimension and the reconstruction of an exact chronology of the real historical events are crucial in order to distinguish monophyletic groups in higher taxa (Hennig 1966, 238-239). The instruments at our disposal are the system based on symplesiomorphy / synapomorphy (ibid. 89), in addition to the best chances of parental relationships offered by the evolutionary models chosen in the elaboration of molecular data. But are there methods which will help us in the definition and distinction of species naturally, and if so what are they? At a biological level, the distinction between species, involves the same concept supporting the definition of species (Mayr et al. 1953), that the author summarizes: “Species can maintain themselves only if they have genetic isolating mechanisms” (Mayr 1957). But we also know how difficult it is in many cases to determine whether the populations, constituting a natural species, are really isolated from the populations of vicariant species surrounding them. At morphological level, or better holomorphological, the instrument made available by Hennig to define the lower taxa (species), is based on the concept of semaphoront (1966, 6-7, 32-33, 63, 65-67). The brick at the basis of the biological system is neither the species nor the individual but: “... the individual at a particular point of time, or even better, during a certain, theoretically infinitely small, period of its life. We will call this element of all biological systematics... the character-bearing semaphoront” (ibid., 6). The author specifies that a semaphoront is the individual during a certain, however brief, period of time, and “not at a point in time”. Adding that there are no rules to define how long the semaphoront exists as a taxonomic entity, and that this depends on the rate at which the different characters change. In the maximum extreme it can take the entire life of the individual, but in many cases, especially in organisms that undergo metamorphic or cyclomorfotic processes, it would be notably shorter. The semaphoront’s morphological characters are the synthesis of its physiological, morphological and ethological characters, and the totality of these characters is defined as the holomorphy of the semaphoront (ibid., 7). The comparative holomorphology between semaphoronts is defined as the auxiliary science of systematic (ibid., 32-33). The author continues saying that differences in form between ontogenetically related semaphoronts in the same individual are called metamorphisms, adding that in everyday language the metamorphisms are the differently shaped age stages of an individual. Citing Naef: “We comprehend ontogenesis by fixing a series of momentary pictures on ‘stages’ out of an actually infinite number. In practice we select as many as seem necessary for understanding the process.” (ibid., 33). And again, it highlights that the general tendency is to distinguish only a few stages in a metamorphosis, i.e. only if the differences that are relatively great, and if the duration of relative constancy of a character is appreciably longer than the period of transformation. Stressing that there are no general rules for determining what constitute a stage (ibid., 33). In the summary of Taxonomic Tasks in the Area of the Lower Categories, Hennig summarizes his idea: “The semaphoront (the character bearer) must be regarded as the element of systematics because, in a system in which the genetic relationships between different things that succeed one another in time are to be represented, we cannot work with elements that change with time. Accordingly the semaphoront corresponds to the individual in a certain, theoretically infinitely small, time span of its life, during which it can be considered unchangeable.” (ibid., 65). All “the character bearers” appear so connected to each other by ontogenetic and tokogenetic (sexual relations between members of the same reproductive community) relationships, from the beginning of life’s history up to the present. Even in relation to the use of this instrument, as for the use of symplesiomorphies  / synapomorphies for the definition of the higher taxa, Hennig indicates that the limits of applicability must be determined empirically in each case. Furthermore, that the comparative holomorphy can be used as an accessory science for recognizing genetic relationships that are to be presented in the taxonomic system, and apart from the chorological system that we will go into in more depth in the future, Hennig does not provide additional instruments for the definition of species (ibid., 67). We do not understand why Hennig’s successors did not give importance to the idea of the semaphoronts, which are the protagonists of the first 40 pages of Philogenetic Systematics (1966), and for the author the major instrument for defining the lower taxa. It is as if the reality of the ongoing transformation or metamorphosis of living beings through endless stages, some of which being discrete and measurable (Hennig being an entomologist), were exclusively prerogative to the insect world. In Wiley & Liebermann (2011) the term semaphoront does not appear; the closer concept is the “ontogenetic homology”, to which half a page is dedicated, summarized as follows: “The use of the concept of ontogenetic homology on the systematic level represents an attempt to study the differentiation and growth of the organism and to provide a basis for comparisons between organisms” (ibid., 116). In reality the Hennig’s semaphoronts, though not always applicable, are the only instruments that allow us to make a comparison between species in a real-time, present, one of the distinct ontogenesis of the compared taxa. In this sense, the relationships shown are phylogenetically natural (in Hennig’s sense). The semaphoronts can be used to define the relationships between species (usually of close evolutionary lines) in the following ways:

a) In an ontogenesis process in the same lineage (species), distinct growth phases can be identified, showing the relative constancy of one or more characters for quite a long time (i.e. distinct semaphoronts), phases that had previously been interpreted as distinct phyletic lines.

b) We can compare the ontogenetic processes of two lineages, and through the comparison of the semaphoronts constituting them (if recognizable), evaluate its proximity or distance.

During our research, we used Hennig’s semaphoronts on several occasions. One example of case B is Parodia calvescens (N. Gerloff & A.D. Nilson) Anceschi & Magli, which is distinguished from Parodia erinacea (Haworth) N. P. Taylor [including Parodia sellowii (Link & Otto) D. R. Hunt and Parodia turbinata Hofacker], for showing 2 separate semaphoronts in the process of ontogenesis. The first displays a delicate spination comprised of 3-6 whitish radial spines, 2-5 mm long, until reaching puberty (approximately 2 years). The second, which occurs from puberty onwards, displays old areoles that lose their spines, and the new ones that cease to produce them, leaving the taxon completely bare. Instead, in the ontogenesis process of P. erinacea, the second semaphoront does not appear, i.e., a normal evolutionary progression of the spines exists, from the juvenile phase to adulthood. On the basis of this evidence the two taxa can be recognized as two distinct lineages (Anceschi & Magli 2013, 6: 29-30). We would add that the populations constituting P. erinacea can also provide us with an example of case A. In fact, in a first phase of the ontogenesis process, the taxon assumes a discoid-globular aspect (the semaphoront known as P. erinacea / P. turbinata), then moves to a second phase indicated by a typical elongated shape (the semaphoront known as P. sellowii). The system based on symplesiomorphy / synapomorphy, and the priority given to the characters and to the molecular analyses are the methods and techniques currently used in the phylogenetic definition of the higher taxa. Concerning instead the philogenetic definition of lower taxa, where the system based on symplesiomorphy / synapomorphy can fail as a result of a lack of useful characters to be analyzed, and where we do not know how far the molecular data can be explanatory, at the specific level, we think it would be useful that accessory science, for recognizing genetic relationships that are to be presented in the taxonomic system, which Hennig defines as comparative holomorphy between semaphoronts (1966, 66-67).

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Summary and conclusions

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Updates on Taxonomy. Summary and conclusions (2010)

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Taxonomy

The names of plants

We here indicate the guidelines adopted by cactusinhabitat.org, regarding the names to be given to plants; this is a controversial argument that is being disputed by different schools, and that basically gave birth to two positions: that of the “splitters” (those who divide, and mainly capture differences), and that of the 'lumpers' (those who merge, and mainly capture similarities). Similar problems certainly do not occur only in the cactus world, but they concern every community of specialists that are devoted to the classification of living organisms. The taxonomic categories (taxa) of the Linnaean classification system (1753) that are covered here are the genus and the species, ie the categories that formally identify the name of the plant, and that are also, for this reason, most subject to nomenclature changes.

The concept of species

While the genus is the taxonomic category that includes similar species, and that should by its nature include as many species as possible, since Linnaeus times, the species have represented the minimal unit of taxonomic classification. But until now, nobody has been able to clearly define what it consists of. Darwin (1872 / It. Ed. 1967, 548, 549) was convinced that one day the systematics would no longer be haunted by the doubt if this or that form were true species, and that they would eventually get rid of the useless discussion about the meaning of the term. Far from all this, over time a more strictly morphological definition (which summed up in the same species groups of individuals that showed common morphological features, though without specific indications about their number and nature) developed into the definition that is known as the biological species concept, defined by the famous German ornithologist Ernst Mayr: “Species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups.” (1942, 120). Note that Mayr does not provide information about the appearance of these populations. Lately, the most accepted approach is basically a mixture of the two methods, that regarding cacti is expressed by David Hunt as follows: “A series of similar intergrading and interfertile populations, recognizably distinct from other such series and reproductively isolated from other such series” (Hunt et al. 2006, text: 4). We emphasize that the potential expressed in Mayr's definition does not appear in Hunt's definition, who nevertheless proceeds to say that, in theory, populations in question are genetically able to interbreed, but this does not happen, due to isolation caused by geographic or ecological barriers.

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Genera and species in the Cactaceae

Since the late Nineteenth century, specialists have developed several trends regarding the number of genera and species to be recognized in cacti. The first important monograph on the family is Gesamtbeschreibung der Kakteen (Monografia Cactacearum) by Karl Schumann (1897-99), in which the author recognizes 21 genera. Later, in the 1920s, the two North American botanists Nathaniel Lord Britton & Joseph Nelson Rose, who are considered to be the first splitters in the history of these plants, in their four volumes The Cactaceae (1919-23) divide the 21 Schumann genera into 124. This trend towards the fragmentation of the family into a greater number of genera and species is even more evident in the four volumes work by the German Friedrich Ritter, Kakteen in Sudamerika (1979-81), which only concerns the South American cacti; it reaches its climax in the work of his compatriot Curt Backeberg, who in Kakteen Lexicon (1966), identified 233 genera. In the late 1980s, a group of international experts, born as IOS Cactaceae Working Party from the Cactaceae section of the International Organization for Succulent Plant Study, aimed at a more traditional taxonomic approach. The search for a new consensus on the cactus genera and its progress were published by two members of the group, David Hunt & Nigel Taylor, in Bradleya (1986, 4: 65-78; 1990, 8: 85-107). In 2000, the group was renamed International Cactaceae Systematics Group (ICSG) (Hunt 2000, 9: 1). Another member of the ICSG, Ted Anderson, published his monograph The Cactus Family (2001) which recognized 125 genera and 1810 taxa, including species and subspecies. However, the work that represents the result of ICSG joint efforts, that is probably the most up to now comprehensive monograph on the cactus family, is The New Cactus Lexicon (Hunt et al. 2006), in two volumes, where the authors display their vision of 124 genera (the same number of Britton & Rose), and 1816 taxa comprising species (1438) and heterotypic subspecies (378) (Hunt et al. 2006, text: 5).

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A more traditional approach

What does the more traditional ICSG (the lumpers) approach consist of, compared to Backeberg, Ritter and their successors (the splitters)? What are the differences between the two schools, and what is our position? The first difference is organizational; while the works by Backeberg and Ritter resulted from individual researches and studies, ICSG works are due to the collaboration of a group. At generic and specific levels, the biggest changes proposed by the new school are generally given by the evidence emerging from molecular studies conducted in recent decades. At infraspecific level, their novelty, justified by greater taxonomic effectiveness, consists of replacing the rank of variety that was widely used in the past, with the rank of subspecies. Concerning genera, the gap between the two schools comes from putting together some series of ex-genera (that enthusiasts and collectors really love) into fewer macro-genera. Some examples: Echinopsis Zuccarini now includes Acanthocalycium Backeberg, Chamaecereus Britton & Rose, Helianthocereus Backeberg, Lobivia Britton & Rose, Pseudolobivia Backeberg, Setiechinopsis (Backeberg) De Haas, Soehrensia Backeberg and Trichocereus Riccobono. Eriosyce Philippi, in Fred Kattermann's (1994) revision and amplification, now includes Horridocactus Backeberg, Islaya Backeberg, Neoporteria Britton & Rose, Pyrrhocactus (Berger) A. Berger, Thelocephala Y. Itõ. And especially Parodia Spegazzini, in one of the options proposed by Reto Nyffeler for the genus (1999, 7: 6-8), now comprises Brasilicactus Backeberg, Brasiliparodia F.Ritter, Eriocactus Backeberg, Notocactus (K. Schumann) Frič, and Wigginsia D. M. Porter. Here the supporters of Notocactus (including the other segregates) as separate from Parodia, based essentially on the morphological diversity of seeds (Glaetzle & Prestlé 1986, 4: 79-96), are far from surrendering. Regarding species, Backeberg & Co. coined many superfluous names that are now correctly listed as synonyms. This is, in our opinion, due to the fact that many researchers in the past (except Ritter) did not devote enough time to study the plants in their places of origin, often taking the different evolutionary phases of the same taxon for new taxa, thus 'discovering' the same plant several times (see Parodia mueller-melchersii). Another reason is the desire to give new discoveries to the enthusiasts ' world, preferring, for this purpose, to capture every minimal difference rather than the similarities. Furthermore, in our experience, the populations that make up a species in habitat show a variability that, while in nature is linked by a space continuum, in cultivation appears artificially fragmented, generating some unnecessary distinctions. In agreement with Ritter, in a letter written to Krainz on April 25th, 1955 (Leuenberger 1996), we believe that whenever the classification of cacti is possible, this must be done only through careful studies in the places of origin; because, we insist, the observations made on plants in cultivation (although with certified field numbers) may often be misleading.

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Variety or subspecies?

As we have just mentioned, one of the changes proposed by the ICSG is the replacement of variety with subspecies, which thus became the only formal category recognized at infraspecific level. The decision was taken during a workshop that was held during a meeting of the group in 1994, whose summary is given by Hunt in Cactaceae Consensus Initiatives (1999d, 8: 23-28). In point number 15, it should be noted how the discussion emphasizes that the choice between variety and subspecies does not involve the concept that the rank implies, but rather the nomenclature consequences that this choice entails. In point number 17, Taylor says that subspecies, having been less used, would make the authors' work quicker and more free. In point number 19, Taylor's arguments are accepted, and the rank of subspecies is confirmed. Only later, that choice would also be substantiated by a meaning similar to the idea that, since Darwin's times, we have of the term subspecies, namely that of a geographical race, or in Hunt's words: “for significant variants, especially those which represent groups of population occupying more or less discrete areas within the overall range of a species.” (Hunt et al. 2006, text: 4).

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From the confusion of varieties to the one of subspecies

If it is true that Backeberg & Co. were responsible for many unnecessary names, the fact remains that the lumpers are currently responsible for some questionable nomenclature changes. For example, as Rob Bregman pointed out (2002, 13: 18-20), in his aforementioned revision and extension work of Eriosyce, Kattermann (1994) maintained, for the most part, the old generic names like Pyrrhocactus, Islaya, Neoporteria, etc. simply lowering them in rank, and recognizing them as infra-generic groups of Eriosyce. The case reported by Bregman is representative of a fashion that does not convince us. If we really think that Pyrrhocactus (Berger) A. Berger, and other ex genera, are not distinct from Eriosyce Philippi, then why distinguish it as an infra-generic level, creating more unnecessary taxa? For this reason, although we accept Eriosyce Philippi sensu lato (as for Echinopsis Zuccarini and Parodia Spegazzini sensu lato), because this interpretation, in the cases mentioned, is closer to what we found in nature, we consider the infrageneric distinctions to be misleading. Having to express an opinion, we now turn to the dispute on the use of variety or subspecies. According to Detlev Metzing (in Hunt 1999d, 8: 26), we would have thought that the choice of variety, being more used with cacti, would probably cause less nomenclature changes. But the observation of species in their habitats led us to conclude that none of the two ranks is needed to better understand the evolution of a natural species. We are not saying that in the range of one single species, populations with distinct morphological and geographical characteristics are not distinguishable. We'd rather say that this variability is closer to the idea of species that is obtained by observing the populations in habitats; and in order to indicate these minor variants, the use of the term form (without taxonomic value) seems more appropriate. We still consider Linnaeus's opinion to be very modern, when he says that the botanist should not take into account these slight variations. We think that still today the minimal unit of measure in the classification should be the species, and that each additional category below this rank is confusing, rather than simplifying. Unfortunately, in some cases, we suspect that the only reason that could justify the use of a formal infra-specific category is the desire to sign the so obtained 'new taxa'.

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A wide-mesh sieve

We believe that nature and evolution essentially follow simple laws; therefore, the easier it is to describe the processes that take place there, the easier that they will be understood. We also agree with Hunt (1999d, 8: 24) that the primary goal of the names, even before classifying, is to identify; therefore, it is necessary that names are the result of simple definitions. Why use a trinomial system if the binomial is sufficient to express the diversity that exists in nature? Perhaps the concept of species, as it is generally used, is too restrictive to describe reality. If we really need to reach a more stable classification, in which the taxa are not at the mercy of doubt, for example regarding the identity of the pollinator (see Pilosocereus minensis), we must broaden the range of the characters capable of defining the boundaries of a natural species. Or, so to speak, we should use a wide-mesh sieve for this filtering operation, in order to avoid the complications that occur when using a closer one. This allows us to better define the really distinct entities in nature as species, without the need to recognize further subdivisions at a lower rank.

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The identification characters of species

But which, and how many, are the characters that the taxonomists considered, in order to assign the rank of species? And which are the most important? In the mid-Eighteenth century, Linnaeus built his classification system that is still in use today, believing in a static world, where God created all species in a single solution. Species, therefore, were different or similar to each other, because of purely morphological characteristics. Over a century later, Darwin (1872), with his theory of evolution where, on the contrary, species are constantly fluctuating, shifted the attention to the fact that the close similarity between species is due to common ancestry. Thus, morphological characters are important in classification only because they reveal ancestry. It follows that the hierarchical arrangement of groups inside other groups created by Linnaeus must be a genealogical arrangement according to Darwin (1872 / It. Ed. 1967, 483, 488, 492-493). This idea about the genealogy of life is the basis for modern taxonomy or natural classification. Darwin certainly gave a direction to the taxonomists, without indicating to them how to move, but we will return to this point later. In the field of cacti, and in view of a natural classification, important studies on morphology, with particular attention to flowers and seeds, were conducted by the Austrian botanist Franz Buxbaum (1950; 1957-1960). Then, among all the morphological elements (stem, ribs, areoles, leaves, spines, roots, flowers, fruits and seeds), particular attention was paid to the study of seeds (Barthlott & Voit 1979; Barthlott & Hunt 2000; Stuppy 2002) because it was assumed that their characters were more stable and less susceptible to environmental factors (Anderson 2001, 34). Later on, Gyldorro (2002, 14: 27) stressed that in mixing and remixing the dominant genes, similar appearances may result as final products of distinct lineages; so he considers it senseless to use the character of seeds as more reliable to define the genera, and we add, the species. As Roy Mottram stated: “Seed-types are subject to convergences like any other characters…” (in Hunt & Taylor 1990, 8: 102). After the seeds, the last frontier investigated to find the fundamental character for defining species (and any other taxa above this), is the use of molecular variations. This involves the application of the molecular systematics techniques (DNA sequences) to infer relationships, almost completely avoiding the similarity due to parallel evolution, because the molecular characteristics are not subject to the same external forces to which the morphology of organisms is subject (1995 Wallace, 13: 2). The molecular data that are collected can easily be interpreted by the cladistics methods, ie through cladograms, that are schemes of evolutionary trees, on which the links between different lineages (taxa) under study appear. Let's step back and say a few words about the work by the father of cladistics, the German Willi Hennig (1950; 1966), whose first publication, that is now considered irreplaceable, was virtually unnoticed. As Gordon Rowley states: “No taxonomic revision is considered complete without a cladogram…” (1997, 4: 13). In a nutshell, the main points of Hennig's thesis can be summarized as follows: a) To identify groups of evolutionary relatives, only shared evolutionary novelties should be used. b) Only those that included all the descendants of an ancestor may be recognized as evolutionary relative groups (or clades). Hennig called monophyletic or olophyletic, a taxon (category) that includes all members of a clade (evolutionary branch); while paraphyletic is a taxon that does not include all the members of a clade; and polyphyletic is a taxon that includes different clades. It is clear that only monophyletic taxa can be defined as natural groups. These methods seem eventually to be coming to the rescue of the genealogical vision that Darwin thought the modern taxonomy should follow. But can we consider the molecular data, and their interpretation in cladograms, as 'The Data' that tip the balance in one or the other direction to define a species, a genus, or any other taxonomic category? And more, are we sure that taxonomy should correspond to phylogenetic criteria? Regarding the first question, it should be noted that the molecular data are currently almost exclusively investigated, either when the previously developed hypothesis, basing on the morphological evidence of the phenotype, leads to doubtful conclusions (suspecting possible convergent evolution), or simply to confirm already acquired morphological data. If a comprehensive study of all possible relationships between the taxa that make up the family seems impossible, it is clear that this use of molecular data can lead to very subjective conclusions. Moreover, it is not uncommon that (Rowley 1997, 4: 14) cladograms obtained with slightly different character sets, either with the same sets of characters encoded with a different method, or with the same sets of characters and the same method but with a different interpretation; all lead to different views of the story of a group of taxa, as it happened with the fission or the fusion of the genus Opuntia Miller segregates (Hunt 2007, 22: 7). When the evidence of molecular data does not support the perception of our senses, and it does not support our beliefs, it is said that the matter requires further study. Or, as it happened for example with Echinocactus grusonii Hildmann, such data were simply ignored, in order to continue considering it an Echinocactus Link & Otto (as it is commonly accepted). Indeed, to be monophyletic, Echinocactus should include Astrophytum Lemaire (Wallace 1995, 13: 7-8); since Astrophytum is easily distinguished, it is preferred not to proceed in this direction and to accept a paraphyletic genus Echinocactus, which, however, according to Hennig's method, is not a natural group. Moreover, E. grusonii, would always seem closer to Ferocactus Britton & Rose than other species of Echinocactus (Butterworth & Wallace 1999, 8: 7). An opposite case is that of Echinomastus Britton & Rose, that is recognized as monophyletic on the basis of molecular evidence (Porter 1999, 7: 5-6), and assimilated by the ICSG in Sclerocactus Britton & Rose (Hunt et al. 2006, text: 259). We are not expressing an opinion on the effectiveness of the decisions that were taken, but we emphasize that it seems to us more appropriate to coherently follow the choices made. Either Hennig's methods are followed, or the molecular-cladistic datum is to be considered as one of the many morphological or physiological data that contribute to define a taxon. This could approximate the cladistics to objectivity, which is too often replaced by abstract solutions, sometimes hybridizing the use and the results. In agreement with the basic idea that was expressed by Darwin (1872 / Ed. It. 1967, 485), and by others after him, we believe that the classification should take into account all the characters, gathering as many data as possible, without giving prevalence to any of them. For any school of thought, it is important to have clear approach, supported by a method as coherent as possible, that does not change depending on the needs. Let's take one example amongst many other. The different flower color in populations that constitute one species (even if related to a geographical location inside its range) cannot represent the distinctive element to recognize other taxa besides the species in question: this happens with Parodia werneri Hofacker and Parodia werneri ssp. pleiocephala (N. Gerloff & Königs) Hofacker, which are now treated as synonyms of Parodia crassigibba (F: Ritter) N. P. Taylor (Hunt et al. 2006, atlas: 310, tab. 310.6, 311, tab. 311.1, 311.2). We so far agree, but we do not understand how, in a similar case, could the different flower color and the northern distribution of the species make the Ferocactus covillei Britton & Rose a distinct taxon, even if at the level of subspecies, as it happens with Ferocactus emoryi ssp. covillei Hunt & Dimmitt with respect to Ferocactus emoryi (Engelmann) Orcutt (Hunt & Dimmitt 2005, 20:16; Hunt 2005, 20: 27, 29, tab. 4-7; Hunt et al. 2006, text: 120; atlas: 377, tab. 377.4, 377.5, 378, tab. 378.1). To answer the second question, namely if taxonomy must correspond to phylogenetic criteria, we would say that on one hand we are aware that the species are in transformation, and we agree with Darwin's genealogical vision of the taxonomy following Hennig's methods, but on the other hand, we realize that we still use Linnaeus's hierarchical classification system. Now, how is it possible that perpetually fluctuating elements, the species revealed by the evolution theory, are classified into Linnaeus's static categories? The reason is simple: the process of speciation as we all know is very slow; its evolution is not perceived by our senses and the time required is not measurable in terms of human lives. So, regardless of the natural evolutionary vision of the idea of species, what we every day perceive with our senses (our umwelt) is actually the same static idea of species that was in front of Linnaeus's eyes. That is why his system is still unsurpassed: it corresponds to the only possible way that we have to perceive the world around us.

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Summary and conclusions

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