Agave xleopoldii finally flowers

[edds]

I've always thought it a bit of an anomaly that they have made a distinction between say a pdf catalogue published by a nursery and a paper nursery catalogue. I can understand a website that is so fluid and changes regularly (though PDN do seem to keep pages of no longer available plants cached so you can retrieve them on a search) but it seems a little out of date to say it has to be printed.

I'm a splitter by nature so I'd like to keep Manfreda and Polianthes and, if necessary split Agave as well.

I can't agree with the stance behind saying that a clade that has evolved distinct physical features, that cladisticly sits embedded within a larger genus, cannot be classed as a new genus without impacting the larger genus.

[Colin Walker]

I've always thought it a bit of an anomaly that they have made a distinction between say a pdf catalogue published by a nursery and a paper nursery catalogue. I can understand a website that is so fluid and changes regularly (though PDN do seem to keep pages of no longer available plants cached so you can retrieve them on a search) but it seems a little out of date to say it has to be printed.

I'm a splitter by nature so I'd like to keep Manfreda and Polianthes and, if necessary split Agave as well.

I can't agree with the stance behind saying that a clade that has evolved distinct physical features, that cladisticly sits embedded within a larger genus, cannot be classed as a new genus without impacting the larger genus.

Ed, the first issue is clear cut. The code for the naming of cultivars states that electronic publication only is not permitted. A print off from say a website that's deposited somewhere [in say a library] does become effective publication.

To reply fully to your second issue would require a long essay or lecture so all I'm going to do is a brief response, at least for now. I really need illustrations to show clearly what I'm trying to say in words, but I don't have such images readily to hand, so here goes.

Cladistics and phylogenomics are about showing evolutionary relationships on which taxonomic decisions are now based. Such relationships are generally portrayed as molecular family trees in which the branching patterns are significant and show the closeness or otherwise of the component parts, whether species, genera, families or whatever. The components of any branch (clade) have to have a unique common origin. If a branch is clearly separated from its neighbours then distinction as a unique taxon is justified.

However, if a cluster of branches is embedded within another group it's not a distinct entity.

A quick example is the genus Monadenium which is embedded within Euphorbia and doesn't have a unique single origin. Monadenium can only be justified as a distinct genus if Euphorbia itself is split into a fair number of separate genera. The current status quo is that Euphorbia encompasses Monadenium along with a few other small segregate genera. An alternative approach would be to split Euphorbia into many genera. This may indeed happen in time.

A simpler example is the case of Calibanus & Beaucarnea. The molecular evidence here is that these two very small genera are not distinct. The two species, Calibanus hookeri & C. glassiana don't even form a unique branch in the tree for Beaucarnea because they're closely related to Beaucarnea compacta and these three species all have subglobose caudices with reduced branching at maturity. Hence Calibanus was merged with Beaucarnea which now has 13 species and this approach I adopted for the 2nd ed. of the Illustrated Handbook of Succulent Plants.

[edds]

Thanks Colin. I only studied evolutionary genetics to degree level but I understand all that you are saying. My point was that I think a branch with a common ancestor should be able to be classified as a different genus (or whatever level of classification) even if embedded within another larger genus.

I think that if a group evolves a different characteristic that would separate it from the other group then it ought to retain its classification as a separate group. Obviously if that group were polyphylletic then it wouldn't be a sound classification itself.

[Colin Walker]

Thanks Colin. I only studied evolutionary genetics to degree level but I understand all that you are saying. My point was that I think a branch with a common ancestor should be able to be classified as a different genus (or whatever level of classification) even if embedded within another larger genus.

I think that if a group evolves a different characteristic that would separate it from the other group then it ought to retain its classification as a separate group. Obviously if that group were polyphylletic then it wouldn't be a sound classification itself.

Ed, the "rules" don't allow for one "genus" to be embedded in another. That's why in my examples Monadenium and Calibanus were ditched.

The goal is to delineate a genus that is monophyletic, but Euphorbia was paraphyletic with Monadenium separated from it. Euphorbia with Monadenium merged then becomes monophyletic. Job done.

In my other example, I was fortunate in that the clade of Beaucarnea hookeri, glassiana & compacta has distinct morphology. Regrettably, though, more often than not, newly redefined units don't have discrete morphologies.

[edds]

Ed, the "rules" don't allow for one "genus" to be embedded in another.

Yeah, I know, that's the point I don't think I was making very clearly! I don't think that part of the rules are the right way to go as we start to get a lot more genetic information about species. (And this is from someone who used mitochondrial D loop sequences to do some cladistics work on Tangyikan Cichlids following on from a paper published by Stumbauer)

I think it is a big part of the problem with some of these big genera as you're either forced to split into them into lots of genera (that no one could possibly differentiate morphologically) or lump then all together (in a grouping so large it becomes useless as well). And if classification isn't useful to distinguish groups apart from each other then what is the point (other than in genetic studies of relationship to each other)?

Nice when it works out as per your examples though!

[Colin Walker]

Ed, the "rules" don't allow for one "genus" to be embedded in another.

Yeah, I know, that's the point I don't think I was making very clearly! I don't think that part of the rules are the right way to go as we start to get a lot more genetic information about species. (And this is from someone who used mitochondrial D loop sequences to do some cladistics work on Tangyikan Cichlids following on from a paper published by Stumbauer)

I think it is a big part of the problem with some of these big genera as you're either forced to split into them into lots of genera (that no one could possibly differentiate morphologically) or lump then all together (in a grouping so large it becomes useless as well). And if classification isn't useful to distinguish groups apart from each other then what is the point (other than in genetic studies of relationship to each other)?

Nice when it works out as per your examples though!

Now Ed, I'm not a geneticist but trained as a microbial biochemist (DPhil in nitrogen fixation enzymes from Azotobacter chroococcum), so please enlighten me: what are "mitochondrial D loop sequences"?

[edds]

Ed, the "rules" don't allow for one "genus" to be embedded in another.

Yeah, I know, that's the point I don't think I was making very clearly! I don't think that part of the rules are the right way to go as we start to get a lot more genetic information about species. (And this is from someone who used mitochondrial D loop sequences to do some cladistics work on Tangyikan Cichlids following on from a paper published by Stumbauer)

I think it is a big part of the problem with some of these big genera as you're either forced to split into them into lots of genera (that no one could possibly differentiate morphologically) or lump then all together (in a grouping so large it becomes useless as well). And if classification isn't useful to distinguish groups apart from each other then what is the point (other than in genetic studies of relationship to each other)?

Nice when it works out as per your examples though!

Now Ed, I'm not a geneticist but trained as a microbial biochemist (DPhil in nitrogen fixation enzymes from Azotobacter chroococcum), so please enlighten me: what are "mitochondrial D loop sequences"?

Sorry! A section of the mitochondrial DNA that has a slow rate of change and so relatively useful in determining relationships between different species.

I am so glad you're back posting on here Colin! I have missed your posts!

[Colin Walker]

Yeah, I know, that's the point I don't think I was making very clearly! I don't think that part of the rules are the right way to go as we start to get a lot more genetic information about species. (And this is from someone who used mitochondrial D loop sequences to do some cladistics work on Tangyikan Cichlids following on from a paper published by Stumbauer)

I think it is a big part of the problem with some of these big genera as you're either forced to split into them into lots of genera (that no one could possibly differentiate morphologically) or lump then all together (in a grouping so large it becomes useless as well). And if classification isn't useful to distinguish groups apart from each other then what is the point (other than in genetic studies of relationship to each other)?

Nice when it works out as per your examples though!

Now Ed, I'm not a geneticist but trained as a microbial biochemist (DPhil in nitrogen fixation enzymes from Azotobacter chroococcum), so please enlighten me: what are "mitochondrial D loop sequences"?

Sorry! A section of the mitochondrial DNA that has a slow rate of change and so relatively useful in determining relationships between different species.

I am so glad you're back posting on here Colin! I have missed your posts!

Thanks Ed, but what makes it a D loop? It's shape somehow? Presumably there are also key genes in these loops too?

[edds]

You know what, I never asked that! It was just the name of the region and I was sequencing and comparing a section of it!

It's a non-coding section of DNA that has a low rate of mutation.

It turns out D loop is short for a displacement loop where the double strand section is held apart by a third bit of DNA - I've learnt something new!

[Colin Walker]

You know what, I never asked that! It was just the name of the region and I was sequencing and comparing a section of it!

It's a non-coding section of DNA that has a low rate of mutation.

It turns out D loop is short for a displacement loop where the double strand section is held apart by a third bit of DNA - I've learnt something new!

Glad I challenged you to answer my query. Thanks for the explanation Ed, that makes sense.