The paper includes a proposal establishing a new clade, consisting of A. simplex, A. mckelveyana, A. chrysantha, and A. subsimplex, and this is at the heart of my concern. The authors express some difficulty discerning the newly named A. simplex from A. mckelveyana ("Differentiating Agave simplex from A. mckelveyana is more difficult." and "The morphological differences between Agave deserti subsp. deserti and subsp. simplex and A. mckelveyana are difficult to discern with most herbarium material. Additional fieldwork and a revision of the group is sorely needed to refine their species boundaries and construct better taxonomic keys."), which I find troubling. My impression is that their clade assignments are based entirely upon herbarium specimens and some small sequencing analysis toward the detriment of actual field work and consideration of flower morphology. My experience is that A. simplex and A. mckelveyana are easily discerned, as are their respective ranges, which do not overlap.
I find their inclusion of A. chrysantha in this clade, suggesting greater alliance to A. simplex and A. mckelveyana than A. palmeri and other Ditepalae agaves, quite a stretch. The fact that A. palmeri and A. chrysantha are the only naturally occurring paniculate agaves in Arizona to feature dramatically upright tepals is neither coincidence nor convergent evolution, as Ms. Hodgson once argued. A. palmeri blooms feature upright tepals for the express purpose of attracting Leptonycteris bats as pollinators. These bats do not come especially close to entering the range of A. chrysantha, yet A. chrysantha also features the same dramatically upright tepals. As such, there is no evolutionary impetus toward convergence, there is only proximity. A. palmeri and A. chrysantha have adjacent and occasionally overlapping ranges, and are primarily discernible via flower morphology, especially toward the part of A. chrysantha's range, which approaches that of A. palmeri. These taxa have historically been considered so similar, A. chrysantha was at one time formally described as a variety of A. palmeri.
The authors describe some level of sequencing analysis to link A. chrysantha with A. simplex, but fail to mention a locality for their A. chrysantha source (one would hope the type locality). This is no small point, since much of the general A. chrysantha population is broadly introgressed with A. parryi, as well as a complex nucleotide stew resulting from pre-Columbian horticultural efforts. A. simplex is a fairly close A. parryi relative, so analysis comparing A. simplex with the northern form of A. chrysantha might possibly yield affinities on that basis. Beyond that, they seem to ignore radically distinct flower morphology of A. chrysantha in their analysis, which I find far more compelling than minor DNA affinities. Finally, the authors write of some difficulty discerning A. simplex from A. mckelveyana, as referenced above, but no such difficulty discerning these taxa from A. chrysantha. That is because A. chrysantha is an entirely different agave, easily distinguished and not nearly so closely related to A. simplex as the authors suggest.
In the end, I find it difficult to justify the authors’ apparent reliance upon herbarium material when all manner of living agave is found in natural habitat, practically in the authors’ backyards. I agree, these would be difficult to differentiate from herbarium material, but it only takes a little time and effort in the field to gain enough familiarity to properly distinguish living plants and boundaries. Allow me to suggest that before assuming some mantle of authority regarding their taxonomy, the authors might explore nearby natural habitat and actually learn to discern these taxa and grasp clearly defined boundaries, rather than employ the tired old epithet of ‘additional fieldwork … is sorely needed’.
I've attached the paper to this post for your perusal. It is mercifully brief.
Simplex.pdf- AGAVE SIMPLEX, A NEW COMBINATION FOR AGAVACEAE
- (457.42 KiB) Downloaded 18 times
