Vachellia tortilis, or why there are no Acacias in Africa

I'd promised to write about marine things this week, but here's an interruption that can't be ignored as I read today that there really aren't any Acacia in Africa. (Bad news for the Acacia Rat and Acacia Tit, etc!) What's that, you say? Well, you may (or may not) have heard that for the last few years there's been some discussion among plant taxonomists about the correct taxonomy and names for the genus Acacia. And what seems to be the final final word on the matter was decided on Monday at a meeting of the International Botanical Congress over in Australia – the result? Africa no longer has any Acacia.

Vachellia tortilis will always make good sunsets, whatever you call them!

Now, this is going to cause some confusion (especially to poor ornithologists like myself), and it probably helps to understand a little about where these scientific names come from in the first place. Let's say you find an odd little plant, look around a lot and can't find anything like it. After quite a lot of asking the experts you conclude you've got a new species, so you need to give it a name. Now, ever since Carl Linnaeus back in 1735, scientific names have had two parts – one genus, one species. You can add extras designating sub-species if you like, but when you see a scientific name the important parts are the genus (always capitalised) and the species (never capitalised, even if named after a person). The aim of the name you chose is to tell you something about the plant and the first thing you have to do is identify the genus. Does your plant have any obvious relatives? If so, I'm afraid you're not free to chose the full name – you'll have to name your species with the same genus, though you can chose the species name (btw, it's generally considered bad form to name it after yourself, sorry. You could name it for a friend though – or get them to name it for you, if you feel particularly narcissistic). So in a scientific name, the genus should tell us about evolutionary relationships. And if our understanding of the evolutionary relationship changes then, bad news, the name will have to change too. Which is what has happened with the genus Acacia. In fact, we touched on the issue yesterday, when I wrote about sponges not being monophyletic, but we didn't take that discussion very far.

Still, what's happened to Acacia? Well, the genus was first described back in 1754 (lots of these details from here, but it's behind a paywall I'm afraid) and until the recent changes referred to some 1300ish species, of which 960ish live in Australia, where they're known as wattles. It's long been noticed that within this large genus, there are some specific sub-groups (called subgenera) that are more closely related to one another - like those Australian wattles (two of which made it to Madagascar, btw) - but that's OK as long as the genus refers to all the relatives. The problem came when studies started showing that some of these sub-groups were more closely related to other members of the Mimosoidae subfamily, than to other members of the genus Acacia, which is what means the group is no longer monophyletic. Now, it's easiest to explain this with a little diagram, I think. So, here's what we used to think Acacia relationships were:

 You can see how all three groupings (subgenera) within the genus Acacia are grouped together, and all the Acacias are separate from the other members of the Subfamily Mimosoideae. That's good - it's a monophyletic group and everyone could have Acacias. (You know a few of the members of tribe Ingeae, btw - this is where Albizia are found.

Now, however, recent DNA evidence has shown a different pattern (it's not quite certain which pattern, so I've given an illustrative diagram from one paper):

Now this shows the problem - no matter how you twist and turn the branches, Acacia subgenus Phyllodineae comes out as more closely related to members of the Ingeae than other two groups of Acacia. As the genus therefore describes multiple branches of the tree, but not all the descendents, we know it's polyphyletic, and we're going to have to change some names. We could, of course, resolve the problem by naming all the plants within the tribe Ingeae as members of Acacia too - then the name would only refer to one group and would again be monophyletic. But imagine the chaos that would cause - suddely all the (obviously different) Albizia and their diverse relatives within the entire tribe, would have to become Acacia. No, it's much simpler to split the Acacia genus into several different genera, each of which are monophyletic. And because Australia has by far the most species in the old genus, they've got the Acacia name for their wattles, leaving the rest of the world to find some other names.

In fact, there are some names out there that look set to be standard. Many of our well known Acacia species look set to become Vachellia, whilst others will become Senegalia. Which become which, I've not sorted out yet. But, but I do know what was Acacia mellifera will be Senegalia, whilst A. tortilis will become Vachellia, and for many species it's fairly obvious which of these two are more similar, suggesting likely names - bushy, multi-stemmed versions look set to become Senegalia, big trees follow tortilis into Vachellia. The good news, of course, is that there's no reason to change the common names - we can still have Whistling-thorn Acacias if we want, even if their scientific name becomes Vachellia drepanolobium. Unless, of course, someone decides common names should change too, something I find much harder to understand (and I still see Crowned Plovers, even if they are Vanellus) - in which case there really will be trouble for the poor Acacia Rats...

Update: You can find a (nearly) complete list of the new species names here.