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The Thorn in the Side 2°


Abstract: in this conversation between two imaginary cactophiles, some aspects of the Cactaceae taxonomy of the past 40 years, are critically reviewed. Conversation about the taxonomic abuses on the concept of species
by Davide Donati & Carlo Zanovello

Second conversation
We think there is no group of men adequate or wise enough that can act without being subject to critical inquiry or discussion. We think that the only way to avoid a mistake is to make it out, and the only way to make it out is freedom of investigation.
J. Robert OPPENHEIMER

Have you seen my dear? Your ‘little fatty’ was no a cactus fter all, even if it had spines as you told me. In reality it was an euphorbia (genus Euphorbia), now dead, unfortunately. At a first glance it was a rare E. mosaica, of which only a brown, almost empty shell remains. Maybe in this case knowing its name would not have helped saving your little plant, it was too small and yellow due to the advanced rotting, but in a different situation, I could have suggested you some actions to take in order to save at least part of it. Why do we not go back to the subject that matters you? If I recall correctly, the last time you promised to give me some examples that would clarify the subject of cactus taxonomy. Maria, I truly said that I would give you some examples that apparently show that, in order to achieve a certain goal, some strained interpretations have been made, which turned out to be scientifically incorrect. As you will see, caution is never enough. You see! You do not have the guts to bring forward your opinion and therefore to get to the bottom of the question! I’m sorry you think this, you should know me well enough! Anyway, when I say caution, I mean that there are some scientific fields and situations where distinguishing is difficult and, on the contrary, is very easy to misinterpret. So in a discussion, one should take many other aspects into account, some of which could seem far away from the core of the problem to be solved. In particular, in a discussion about the use of the concept of botanical species, it is important to examine what also other biologists mean when they talk about species. Dear Lucio, they would obviously think that a dog and a cat and a horse belong to different species and thus they are unable to interbreed, that is, they cannot have fertile offsprings. It is not that always easy! You can find a lot of exceptions to that, and this is why there is not a satisfying definition of species yet, I mean, one without problems. Just think that the Linnean binomials are used even for fossils, for which a proof of hybridisation is obviously impossible, unless one has a Time Machine, or can bring those creatures back to life. Please let us forget about extreme situations! Dear Maria, I have already told you that there are many exceptions! What you have tried to articulate earlier is the definition of biological species, which in synthesis is: group of individuals that cross between each other and not with others. OK, this very synthetic version easily shows some limits. For example, how is it possible to speak about species for bacteria that notoriously reproduce by subdivision? Lucio ! It seems to me that you are using Diogenes’ lantern to look for quibbles. Now the bacteria! Not only! Not only! There are other organisms that adopt the asexual reproduction exclusively. The Bdelloide rotifers........ The rotiwhat? The Bdelloidea rotifers are a vast group of very small creatures that has hundreds of species, all reproducing asexually, and for them the abovementioned definition would be useless. Always micro-organisms! They are very small, indeed, my dear, but it is well known since a long time that they are multicellular creatures. But if their size is a problem for you, there are many other creatures with the same reproductive mechanism even among coleopterans, and not only! But then, this so-called biological definition is totally useless!< Not exactly! I gave you these examples only to demonstrate that this point of view encounters several problems. For a large part of the zoological world it seems to work well enough, even if it requires some clarification about the meaning of “and not with others”.


– My dear Lucio, did we not start by talking about plants? It would seem that we have gone a bit far away!
– I’m getting to it! If we would rigidly apply the abovementioned definition of biological species to Botany (and not only), without clarifications, we would end up in a cataclysm that would cause the elimination of so many universally accepted species that would go well beyond the dreams of the most devoted lumper, or the worst nightmare of your splitters. Maybe only one species could remain of the entire genus Haworthia, which suggests to adopt a less rigid attitude for the botanical world (and not only), that evaluates other elements, also including the morphological aspect.
– And what are you going to tell me about the Cactaceae?
– For example, Ariocarpus retusus crosses very easily with Ariocarpus bravoanus ssp. hintonii in cultivation, and the
hybrids can interbreed too, producing fairly healthy offsprings. By this, one could quickly conclude that the two entities belong to a single species, which is present in nature with two different subspecies, and this would result in a number of new combinations at the very minimum. So, A. retusus, already fairly full, would find some newcomers.
However, an accurate visit to the environments where hintonii lives, shows that on the same hill (undulation) it is possible to find, sometimes less than 100 meters away, A. retusus too and, very important detail, no hybrids! What does it mean? Since there are no known natural hybrids between the two entities (if they exist, they must be rare, and thus with no impact), obviously there must be an isolation mechanism in place, and this could be a different flowering period (in cultivation hintonii tends to flower earlier than retusus) or a pollinator that recognises only the magenta colour of hintonii rather than the white, slightly pink colour of retusus and thus we are allowed to consider them as belonging to two distinct species.
– You are confusing me!
– What I mean, Maria, is that if the biological definition of species is a good starting point, the dull application of it can cause some problems. For this reason, in addition to the potential crossing capability that I mentioned earlier, which is a necessary (but no sufficient) condition, there are other, equally necessary criteria to meet, like the belonging to the same ecological niche or to the same adaptative zone, the presence of a specific fertilisation mechanism or of a specific way of acknowledging the potential partner, the forming of a phenotypi