Taxonomy: A rose by any other name . . .

Don’t get too hung up on names. With the advent of DNA analysis the taxonomy of the plant kingdom has undergone radical changes which better reflect the evolutionary relationships of species. In fact, this has always been the fundamental basis of taxonomy (often called systematics) but now it has a more objective and scientific basis.

Back in the 1970s aloes were in the Liliaceae (lily family) but it is now known that they are not closely related at all. Coming back to the hobby in 2007 after a break of some 35 years I also found many changes in the classification of cacti as well as quite a few new species.

The taxonomy of the Cactaceae (this applies much less to other succulents) is confusing to most people. Historically, cacti have attracted fewer professional taxonomists than economically important crop plants, and although many outstanding amateur and professional botanists have devoted their careers to studying them, they often reached different conclusions. As a result some species have accumulated long lists of synonyms, often more than 10, so it’s no wonder we are all confused!

Since Darwin published On the Origin of Species in 1859, taxonomists have increasingly sought to make classification reflect evolutionary relationships rather than simply overall similarities. Before the advent of DNA analysis, plant taxonomy relied largely on comparing physical features and geographical distributions to infer relationships.

Cacti probably evolved in the Americas around 30-35 million years ago, and their rapid spread into different habitats means many species are closely related. This makes distinguishing them at the genus or species level more difficult, not helped by the fact that many cacti hybridise naturally, blurring the boundaries between species.

Cacti have a surprisingly poor fossil record. Although many species produce woody tissue (some large columnar cacti even leave behind substantial woody skeletons), their soft, succulent stems rarely fossilise. As a result, the evolutionary history of the family has had to be reconstructed largely from living plants and, more recently, DNA evidence.

Cacti have evolved to survive in arid environments, resulting in adaptations like spines, succulent stems, and reduced leaves, making it hard to distinguish true evolutionary relationships based on appearance alone. Different groups independently evolved ribs, tubercles, dense spination and compact globular bodies because these are successful adaptations to arid environments. This process, known as convergent evolution, can make unrelated species appear deceptively similar but DNA analysis has shown that some superficially similar cacti are only distantly related.

Many traditional botanists were remarkably good at recognising natural groups based on careful observation, and DNA has confirmed much of their work. Where DNA has had the greatest impact is in revealing that some apparently obvious groups are actually artificial. DNA has revolutionised taxonomy, but it has not removed all uncertainty. Different genes sometimes tell slightly different evolutionary stories, particularly in groups where hybridisation has occurred.

For example, DNA studies have shown that the traditional boundaries of Mammillaria do not always reflect evolutionary relationships. Botanists then have to decide whether to split Mammillaria, merge neighbouring genera, or move individual species. Different researchers have reached different conclusions from essentially the same evidence.

There is very little disagreement that DNA contains the most reliable evidence of evolutionary relationships. The controversy is about how to interpret the data, which genes to analyse, and how much weight to give DNA compared with morphology, geography and ecology.

So why is cactus taxonomy so confusing?

1. Cacti evolved recently, so many species are genetically very similar.
2. Many unrelated cacti evolved similar appearances (convergent evolution).
3. Hybridisation is common, blurring species boundaries.
4. The fossil record is poor, so DNA has had to do much of the heavy lifting.
5. DNA does not eliminate judgement—taxonomists still have to decide where genera and species begin and end.

Perhaps the important thing to remember is that taxonomy is not simply about finding convenient names. Its aim is to describe the evolutionary history of life as accurately as possible. As new evidence emerges, especially from DNA, classifications will continue to change. While this can be frustrating for growers, it is also a sign that our understanding of cacti is steadily improving.

How to find out more
Despite all this, it is a good idea to gain a general understanding how cacti are grouped and to appreciate their characteristics so that you can more easily identify plants, at least down to the level of genus. Some cacti scream their genus at you but others carefully hide it under a cunning cloak of disguise.

The other succulents usually reveal their identities when they flower but their general appearance can give the game away. For example, aloes and agaves look superficially similar but even without flowers it is relatively easy to distinguish between them.

Identifying an actual species is more of a challenge with both cacti and other succulents but it’s worth trying to spot the more obvious ones which are, for the most part, the more common ones.

Hybridisation is something that some growers love but which I find really rather pointless. It’s easy to cross two plants and call the resultant hybrid after your maiden aunt but what’s the benefit to mankind? All you are doing is confusing the hell out of the average collector. Growing hybrids creates a plant with added vigour and possibly a different coloured flower, but there are so many wonderful true species to grow, why bother? That’s all from me on that topic!

I consult The Plant List and use this as my guide for naming cacti and other succulents, sometime supplemented by CactiGuide and LLIFLE. I am often aware of a plant’s other names but when you find a cactus with more than 10 synonyms you know that there must be more to life than taxonomy.

As for common names, take those with a huge pinch of salt. Far more so than with garden plants, the common names, particularly of cacti, are completely meaningless and rather pointless.

What makes a succulent a cactus?
. . . . Cacti
. . . . Other succulents
Taxonomy: A rose by any other name . . .
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