What are Species?

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ralphrmartin
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What are Species?

Post by ralphrmartin »

We have often debated "What are species?" on this forum, and the following forthcoming book addresses this issue:
https://www.crcpress.com/What-are-Speci ... 1498714549

The table of contents sounds promising, and it argues that the concept of species should be scrapped. I doubt if it will meet mainstream approval, however...

(It's pretty pricey, but maybe you can persuade your local library to get it!)
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Re: What are Species?

Post by KarlR »

From the little info available in your link, I can't see this ever gaining any traction. I certainly don't think it would be an improvement on the current system. Personally, I think a scrapping of the subspecies rank in favour of reinstating the much more useful varietal rank would be more beneficial for the hobbyist as well as the taxonomist.

Coupled with a more liberal understanding of what constitutes a species, I think this would be more useful than both the current «lumper-oriented» view and the nebulous view seemingly argued by the book you link to.

Clearly defined species and varieties certainly make it easier to say what's what than having a 500-word description for every species to describe it in all its variety.
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Re: What are Species?

Post by DaveW »

I think botany only really adopted subspecies rather than the previous use of variety in order to fall in line with zoology. However variety is still a legal rank in botany I believe, it has simply become a fashion to change them all to subspecies, and botanists are just as subject to fashion as any other sector of society.

The old saying used to be "a species is what a competent botanist says is a species". However after repeated opposing classifications of the Cactaceae, some would say competent botanists are just as rare as Unicorns. :grin:

http://www.bbc.co.uk/schools/gcsebitesi ... rev5.shtml

https://evolution.berkeley.edu/evolibra ... cle/evo_41

Really I suppose it depends what a classification is used for. If it is to be used for identification in the field a species has to be visually different from another. With modern DNA Sequencing however in the laboratory it can be a plant that differs in it's DNA from another, although it may look virtually identical in the field having regard for the usual range of variation in a population. You really have to decide what type of classification is needed, since you can classify using any feature if you simply want to place plants into neat little Pidgeon Holes.

When it comes to genera the same applies, since a morphological classification based on similarity is perhaps more use in the field than a monophyletic one using DNA which will often place similar looking plants in different lines of evolution, Rebutia and Aylostera for instance, their similarity being merely convergence through inhabiting similar habitats.

I doubt anybody will ever come up with the definitive classification of the Cactaceae, be that at generic or specific level. In the end it really comes down to how useful it is to the user. Also more than one classification can be in use in the world at any one time, so there is no "correct" classification. Yet you still see people writing "you must now call it so and so" as though the latest name is obligatory. As long as the name is validly published you are free to use whatever classification you want, even part of one classification and part of another.
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Re: What are Species?

Post by KarlR »

The rank of variety still exists, but the rank of subspecies is recommended. I think this is a problem regarding cacti because it seems that the guidelines for what exactly constitutes a subspecies is even more unclear than the criteria for a species. It's a rank somewhere in between species and variety, but just how to use it varies greatly from person to person in deciding on the rank of new forms.

Turbinicarpus is a huge mess at the moment. T. schmiedickeanus has over 10 published subspecies if I recall correctly. Does that make sense? Does it help the taxonomist or the hobbyist in differentiating between forms? Wouldn't dividing some or all of these subspecies into smaller species units with varieties make more sense? I realise this is more or less the essence of the splitter vs. lumper debate, but I fail to see how substituting the varietal rank for subspecies and creating big species complexes makes it any easier for the taxonomist or the hobbyist.

Granted, in terms of the CITES checklist I understand that it's useful to operate with only a few species when untrained personnel are supposed to check for endangered species. The downside is obviously that you risk lumping good species that are highly threatened into big species complexes that are not. This approach can also lead to issues in DNA testing if plants assumed to belong to a certain species in fact should be regarded as separate species. Several Echinocereus forms previously considered to be part of a huge E. triglochidiatus complex now seem to be good species based on DNA.

For me, by far the most logical position is to go back to an almost Backebergian view until thorough studies based on morphology and DNA show that lumping is in order. Joél Lodè's work shows that, if we are to go by genetic studies, we should definitely take a step back from the lumper's view. I'm sure many would lament the fact that we'd get lots of new (or old..) genera and the same for species, but then at least we'd have more sensible units to deal with.

In any case, humans like to establish order out of chaos and to systematise things. I don't think the view suggested in Ralph's link will help with that. Certainly not for cacti.

Edit: I didn't mean to criticise you Ralph. I realised that what I wrote might seem a bit like it. It was an interesting link and I like to talk about this stuff!
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Re: What are Species?

Post by ralphrmartin »

No offence taken! :grin: And to get my own back - discussing varieties and subspecies is a red herring. They are just lower levels of the same broken assumption. :grin: :grin:

My own position is that people love to classify things, and and like the world to fit simple predictable patterns. Unfortunately, nature isn't always as simple as we would like. At the end of the day, there are populations of organisms exchanging genes, mainly by reproducing with organisms with similar genomes, but sometimes with ones with rather more distantly related ("hybrids"), or even via horizontal gene transfer (in bacteria). Sometimes these organisms reproducing with one another are widespread, sometimes very local populations. Sometimes they are quite uniform, sometimes more variable. It is quite clear that there is no single suitable definition of a species (that works without exceptions), so I have a lot of sympathy with the author of this book who is trying to get biologists to move on, and admit that what worked reasonably well as a first cut idea (hence its use for so long) is perhaps due for replacement with something a bit more sophisticated.

Much of the tension seen on this list is due to different people wanting labels for different purposes. Botanists tend to be lumpers as they want to see the big pictures in relationships. Gardeners tend to be splitters as they want more names to ensure they can get precisely the plant they want (the hairier one with deeper red flowers). This will always be true whether the organisms are being lumped or split into "species" or any other concept. While the "subspecies / variety" and "cultivar" approaches go some way of providing this, they are not entirely compatible alternatives which also need making more uniform, without regard to whether the organism arose with human help (by breeding, selection from a natural population, selection of a natural population, etc.

Perhaps as more and more complete genome sequencing is done, we will just be able to refer to plants by a numerical code derived from the DNA sequence, done in such a way that plants with similar genomes get similar numerical codes. However, there is still the issue of the degree to which each gene is expressed, etc...
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Re: What are Species?

Post by KarlR »

You're right that nature isn't simple, and there will likely never be a wholly satisfying system of classification that fits all the branches of the Tree of Life. The concept of a species in zoology doesn't really translate perfectly to biology, and within the major branches there are differences between families in how readily new species develop and forms diversify. There is obviously also a great deal of variation between different taxonomists on how to treat the various families of organisms. What is deemed good enough to constitute a cactus species might not be good enough to constitute a species in the rose family.

The binomial system has largely worked very well so far, and to do away with it completely in favour of a species-less, pluralistic, phylogenetic view seems to be a step backwards to me. It sounds like an extreme form of lumping, and I just don't see how that makes any sense to anyone. Let's take Acanthocalycium as an example and consider it as a monophyletic group. If you do away with the species concept (as proposed by the author), do you consider this whole group to be one single form? For Escobaria vivipara you would get one name if you were lucky. How would the variability of this species be described? Would we go back to a pre-Linnaean system of long and colourful descriptions of organisms in order to categorise something? Would we even have separate forms at lower ranks than genera? How would that be any more sophisticated than the current system? It sounds incredibly unwieldy to me.

Personally I don't think subspecies or varieties are a red herring. They are perfectly good ranks in a well developed system of classification, which help to illustrate the diversity inherent in a group of very closely related organisms. My opinion is that doing away with subspecies in favour of variety, and taking a splitter's view is the better option than taking the lumper's view. You get more names to deal with but at least you get names. If the goal is to better reflect the variability in nature, isn't it better to split things up in smaller units which are more easily describable and identifiable, than larger units which are the opposite?
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Re: What are Species?

Post by DaveW »

The advantage of splitting I suppose it is easier to move the forms around from one genus to another, or if genera, from one line of evolution to another if found to be non related by DNA Sequencing. The "lumpers" seem to want to solve their problem by simply using collection numbers for everything they lump under a species rather than giving them the rank of variety or form. Or often treating them as cultivars, whereas I personally think a cultivar should only be used for something produced in cultivation, not a normally occurring wild form.

I still think monotypic genera are a better resting place for plants that don't comfortably fit into existing genera, as in the case of Rimacactus for Eriosyce laui or Digitostigma for Astrophytum caput-medusae. Botanists these days seem too anxious to fit anomalous plants into existing genera rather than parking them into a monotypic genus until their exact relationships are determined.
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Re: What are Species?

Post by ralphrmartin »

KarlR wrote:Personally I don't think subspecies or varieties are a red herring. They are perfectly good ranks in a well developed system of classification,... If the goal is to better reflect the variability in nature, isn't it better to split things up in smaller units which are more easily describable and identifiable, than larger units which are the opposite?
I guess i didn't explain myself clearly enough. I meant that subspecies and varieties have just the same problem as species, in terms of there being no unambiguous single definition that works without exception. In other words, it is no more meaningful to keep those than to cling to the idea of species.

I entirely agree that whatever system is used, it is better to split things up into smaller units, but we should also try to merge them where meaningful into larger units. Ideally this would reflect evolutionary origins, but it is not always possible without overlap (the obvious case being hybrids). A multi-level system has the advantage of letting people who want to concentrate on the big picture do so with larger units, and people who wish to note fine distinctions to do so too.

Lumping always seems to me to be a bad idea, as it is simply throwing away information which might be useful later (or, alternatively, being to lazy to record it in the first place).
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KarlR
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Re: What are Species?

Post by KarlR »

ralphrmartin wrote:I guess i didn't explain myself clearly enough. I meant that subspecies and varieties have just the same problem as species, in terms of there being no unambiguous single definition that works without exception. In other words, it is no more meaningful to keep those than to cling to the idea of species.
I don't think it's possible to find such a system. Even with DNA, there'll always be a question about which markers to use for which organism in a given branch/family, and between organisms belonging to different families - and even within the same families.

Would the the same genetic markers be equally applicable to all members of the Cactaceae? Would morphology still have to play a part?

I don't really see why we should even need a uniform system which works in every case across all living organisms. Perhaps one system for plants and one for animals etc. is better. Perhaps even more detailed systems for individual plant families is necessary. Like a key to a genus, you could have individual «species concept keys» for whole families in which the species concept varies from family to family.
ralphrmartin wrote:I entirely agree that whatever system is used, it is better to split things up into smaller units, but we should also try to merge them where meaningful into larger units. Ideally this would reflect evolutionary origins, but it is not always possible without overlap (the obvious case being hybrids). A multi-level system has the advantage of letting people who want to concentrate on the big picture do so with larger units, and people who wish to note fine distinctions to do so too.
I think we are in agreement on the merits of lumping and splitting, though what that multi-level system is I'm not sure. Do you mean one system for the taxonomist and one for the hobbyist? As for having a system based on evolutionary origins, I agree that it would be nice in theory, but in practice it isn't a viable alternative all the while the evolutionary history of the Cactaceae is basically unknown.

Hybrids are certainly tricky, and some form of change to the classification scheme might be called for to account for them. Alternatively, one might recognise that, within Cactaceae at least, a lot of species probably have hybrid origins and ask whether or not it matters if these are called anything other than the standard binomial. E.g. Denmoza rhodacantha is a probable candidate for having a hybrid origin. But is it an issue that we don't put an «x» in there somewhere to signify that? Turbinicarpus Xmombergeri is a stabilised hybrid as far as I know, so doesn't it make as much sense to simply call it a good species as it does to call it a hybrid? Perhaps it would clarify things to say that a stabilised hybrid population in nature should be treated as a species. It would help with such situations as Ariocarpus confusus.

In the end, the species concept is an artificial construction in order to attempt to classify the natural world. It isn't perfect, but I don't think it needs to be. Perhaps we will see a classification uniting genetics and morphology which will be more accurate than the current one, but I think the species concept is more than adequate to survive.
ralphrmartin wrote:Lumping always seems to me to be a bad idea, as it is simply throwing away information which might be useful later (or, alternatively, being to lazy to record it in the first place).
This I completely agree with! :grin:
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Re: What are Species?

Post by DaveW »

It was always suggested that some present genera could have arisen from hybridisation in the distant past, so could some species.

I suppose you have to define a hybrid in terms of evolution anyway? To me the simple way would be a cross between any two plants not normally or having previously occurred in habitat together, or within pollination range of their pollinators.

That would then confine traditional hybrids to those being produced in cultivation ether by deliberate cross pollination, or accidentally by open pollination through growing non related species together in a collection.

From what I have read DNA seems to indicate Echinocactus grusonii is probably a cross between an Echinocactus and a Ferocactus and Astrophytum ornatum may long ago have been an Echinocactus/Ferocactus crossed with an Astrophytum, hence hybrid vigour meaning it grows faster and taller than the other Astrophytum's?

Interesting to speculate if Leuchtenbergia is some form of ancient cross since it seems to be related to Ferocactus and what about Digitostigma, an Astrophytum crossed with what? Or are they both simply mutations, another way species and genera are created?
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