Saturday, 28 January 2012

The Red Queen and the Court Jester

Greater blind mole rat
What is the primary driving force behind evolution? If an animal is well suited to its environment, and able to reproduce effectively, then why does it change over time? Why, in short, don't humans still live up trees, where monkeys are still perfectly happy today?

There are two broad types of explanation for what drives evolution: they're called the Red Queen and the Court Jester. It's important to note that there's no reason why they can't both be responsible. Indeed, it would be astonishing if they weren't. The debate, then, hinges on which is more important, and when.

The Red Queen hypothesis, first formalised in the 1970s, is named for the character in Through the Looking Glass, who had to run as fast as she could, just to stay still. The idea is that no animal (or other living organism) is alone, and, if you don't evolve, the creatures around you will, and you'll be doomed. One species may be very good at eating a particular type of plant, for instance, but if another species gets slightly better at doing the same thing, if the first species doesn't improve, it will be out-competed and go extinct. You always have to be better than your competitors, and they have to try to be better than you, leading to a constant drive for change - towards better adaptation to your environment and chosen food source, whatever that may be.

It's not just competitors for the same food that you have to worry about, either. It's in the interests of your food not to be eaten, so it, too, is constantly evolving ways to stop you eating it. If some food is harder to eat than others, then soon, that's all that will be left, and then the local animals have to evolve some way of eating it, or they will starve.

Another common example of the Red Queen hypothesis applies to parasites and their hosts. A parasite doesn't want to kill the animal it lives in, because it needs it to survive, but the host has no such qualms towards the parasite. Because being riddled with parasites is a bad thing, host animals that evolve better ways of fighting them off are more likely to survive. But parasites that evolve better ways of defending themselves are also more likely to survive, leading to a never-ending arms race that pushes evolution ever onwards.

This may even explain why male mammals exist. If all an animal needed to do was make exact copies of itself, it would only need females, in order to give birth. But, by mating with males, genes are constantly being reshuffled each generation, trying out different combinations, in order to frustrate parasites, which would much prefer, like a computer virus, that everyone was running an identical copy of Windows. No two animals being identical provides the raw material on which evolution can work.

This has never been the only explanation for what's going on, but it was only around ten years ago that the term "Court Jester hypothesis" was coined to describe the alternative. It's not named after any jester in particular, but just a term that sounds good in contrast to "Red Queen". The idea here is that the main driving force behind evolutionary change is random events, outside the biological realm.

That's because, while an animal may be very well suited to its environment, that's not much use if the environment changes. Change is imposed from outside, for whatever reason, and if you don't evolve to fit the new circumstances, then you will die out, and something else will take your place. Perhaps the most dramatic example is the asteroid strike that wiped out the dinosaurs. The strike itself, and the consequent widespread destruction, left the world wide open for the mammals, and they rapidly evolved to take advantage of their good fortune, taking over environments that they'd never previously had a chance to colonise.

A more recent example might be the Ice Ages. Here again, we have dramatic changes to the world that have nothing to do with biology, yet which will have forced animals to adapt or die. It's not just the advance of the massive ice sheets themselves; with the whole world cooling, swathes of tundra and pine forest shifted south, ahead of the glaciers. Animals previously living in a Mediterranean climate, for instance, suddenly found things much colder. True, they could flee south, but only if seas or mountains weren't in the way, and, even then, they'd have to be displacing something, and probably moving into an environment that wasn't identical with their old one - the vagaries of geography being what they are.

The cold need not even affect the animals directly; if you don't mind the cold, but your food does, and dies off where you live, you're still going to have to either move or adapt. Through much of the Ice Ages, animals survived by finding "refugia", isolated regions where they could cling on until the warm weather returned, but where they would be isolated from their kin, unable to mix their genes, and perhaps even forming a new species or subspecies as a result.

Of course, both hypotheses are correct, at least in general. Animals do not live in isolation, and the non-biological environment does throw the occasional surprise into the mix. Which predominates may depend, not just on what group of animals we're talking about, but also the time scale we're looking at. For instance, the Red Queen may be more important for small evolutionary changes, where even a few generations can make a difference, while the Court Jester may have more of a role to play when we're looking at larger, bur rarer, events.

It is within this context that we have to look at a newly published paper on the evolution of the blind mole rats. When I previously discussed this animal, I stated there are almost certainly more species than we know about, partly because they're quite good at hiding underground, and partly because they tend to look fairly similar. Yarin Hadid of the University of Haifa, and co-workers, examined genetic samples from 41 blind mole rats, many of which they suspected might belong to previously unidentified species. The evolutionary tree they uncovered look like this:

   Nehring's      Lesser blind   
blind  mole rat    mole rat   
      |               |    "S. vasvarii"       
      |               |          |    
      -----------------          |    Palestinian blind
             |                   |      mole rat, etc.
             A                   |            ^
             |                   |            |    Greater blind
             ---------------------            |      mole rat,
                       |                      |        etc.
                       B                      |         ^
                       |                      |         |
                       ------------------------         |
                                  |                     |
                                  C                     |
                                  |                     |

Looking at the full tree in more detail, we can see a number of interesting features. The Palestinian blind mole rat (Spalax ehrenbergi) had, fairly recently, been identified as actually representing at least four different species, inhabiting places such as the Golan Heights, Galilee, and the Judean Mountains. This analysis confirmed that, but also shows that there are at least four others, and that, at least at the genetic level, they are pretty distinct from one another. So far, none of these new species (assuming they hold up to scrutiny) have yet been named, something that can be a tortuous process.

The existing status of the greater blind mole rat (Spalax microphthalmus) and its kin looks secure, but the picture for the others looks far more confused. There's at least one new species in there (Spalax vasvarii, which, so far as I can tell, was previously thought to be a subspecies of Nehring's blind mole rat), and probably more, but there's little consistent pattern to how they differ. That highlights a problem that can arise with evolutionary trees - real world nature isn't always that neat.

You might think that, when two species diverge, that's an end of the matter, and they go their separate ways from then on. And that's what evolutionary trees like the one above show, and what the computer programs that construct them are designed to find. But, in reality, two species can interbreed again some time after they separate, perhaps merging again for a short while before they re-split. That leaves a complicated genetic pattern, with one species having obtained some of its genes from the other, without having evolved them itself. That seems to be what has happened here, probably more than once, and it's quite likely impossible to ever disentangle it, making it difficult to decide what should, and should not, count as a full species.

So what does this have to do with the Red Queen and the Court Jester? Well, the researchers went on to try and time the various splits in the tree, assuming a constant rate of evolution within the group, and using the oldest known fossil that can be confidently placed at point "C" in the diagram above to calibrate the dates. They argue that when they did this, a pattern emerged. Rather than new species appearing at random points in time, they tended to do so at 100,000 year intervals, matching times when Earth's orbital eccentricity reaches a maximum, with attendant effects on the global climate. Evidence that is, for the Court Jester hypothesis.

The argument runs that, since blind mole rats are steppe-living animals, the alternate advances of coniferous forest and dry deserts could both have affected their survival, presumably by giving them less of their preferred food. But, personally, I'm not quite so convinced. Certainly, the evidence shows that the different species diversified very rapidly, over time scales of less than 200,000 years, but how strong the claimed pattern is, I'm not so sure. It's consistent with the Court Jester being behind it all, yes, but I don't know how strong it is as proof.

Be that as it may, there are still some interesting dates that pop out of the figures, and show that the Court Jester has probably been at play at least a few times, whether or not he's always the main culprit. For example, take split "A" on the chart, that between the lesser and Nehring's blind mole rats. The analysis dates that to 2.7 million years ago, around the time that the Bosporus and Dardanelles were last flooded. Significantly, today, the lesser blind mole rat (S. leucodon) lives only in the Balkans, while Nehring's blind mole rat (S. nehringi) lives only in Anatolia, on the other side of the straits. So the purely geological events linking the Black Sea to the Mediterranean may have led to the creation of two different species from a single ancestor that wandered happily between Greece and Anatolian Turkey.

Similarly, point "B", at 3.1 million years ago, and point "C" at 4.7 million years ago, correspond roughly with the creation of the Taurus and East Anatolian mountains respectively, opening up new highland habitats and creating rain shadows elsewhere that changed the local climate. And, if point "A" corresponds to the last time the Bosporus and Dardanelles opened, point "D", at 7.6 million years ago, may correspond to the first time that happened - although, in this case, the effects would have been obscured during the time that they were closed again. Heading even further back, the study suggests that the very first blind mole rats should have evolved around 20 million years ago, which is, indeed, the age of their earliest known fossils, and agrees with earlier studies.

The Court Jester may, or may not, have played his pranks using the Ice Ages, but he has plenty of other tools at his disposal.

[Picture by Vivan755, from Wikimedia Commons. Cladogram adapted from Hadid et al. 2012.]

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