08 February 2006

Going Different Directions in the Same Space

As many of you know, I'm a graduate student in a zoology department. When I tell kids that, most of them think I'm studying to become a zookeeper. They also usually think that's something pretty cool. When I explain that I'm really studying to be a scientist who studies how animals change, it usually turns out to be a letdown. For some reason, kids are usually happier thinking that I might get eaten by the lion or stepped on by an elephant.

Anyway, what I actually study is speciation mechanisms. What that means is that I'm trying to look at the DNA of closely related species in order to figure out why they wound up as different species. There are a lot of questions left to answer, and lots of scientists are working in this area.

I'm guessing that right about now at least some of you are thinking something along the lines of, "Hey, wait a minute! Haven't you guys been telling us that Darwin figured that out way back when?"

What Darwin and that Wallace fellow that keeps getting left out of the story both managed to figure out is that new species are made by changing old species. They also figured out that natural selection was one way to make this happen - if some organisms are different from most of the rest of their species in a way that makes them more likely to survive and reproduce, and if the differences get passed to their offspring, then in time the bulk of the population will wind up with that trait. If this happens to only one population of a species, the changes can make that population so different that they won't be able to mate with the other populations. When that happens, you have two species where you used to have one.

That view is a little simplified, and we don't think that it is the only process that leads to the formation of new species, but it's more or less accurate. The reason that scientists are still gainfully employed working on the question of how you get new species is that the view is also a little bit corse-grained. It's like looking at a picture on a really old, low-resolution monitor. You can see things well enough to see what it is a picture of, but you don't see a lot of the details - and details can be very important.

Another way to look at this might be to use the way we grow up as an example. We know that there are children, we know that there are adults, and we know that adults happen when children grow up. You can look at a ten year old and a forty year old and know that one is a kid and one is an adult. Figuring out exactly when the change from child to adult took place is a lot harder, and so is figuring out what "growing up" actually means.

Those of us who study speciation spend a lot of time looking at geography. In part, we do this because looking at the geography can mean that you get to go lots of cool places to do your research. Mostly, though, we look at the geography because figuring out where the two populations were living in relation to each other when they split into different species can give us some good clues about how it might have happened.

There are basically three ways that you can arrange two populations in space relative to each other. Both populations can be living in the same place (we call this "sympatric"), the two populations can be living in different places ("allopatric"), or the two populations could be arranged so that they are mostly living in different places, but with an area of overlap between them ("parapatric"). That's a bit of a simplification, and there are some fairly obscure variations that can turn out to be important when you actually start to study things, but it's good enough for our purposes.

In theory, populations can split into different species in any of those three categories. There are some scientists who have developed mathematical models of the process of speciation, and those models all seem to indicate that it doesn't matter if the populations are allopatric, parapatric, or sympatric - given the right set of circumstances they can wind up as two different species.

That's the theory. When we take the theory to the field, what we find is that it's pretty easy to find examples of allopatric and parapatric speciation, but it's really hard to find clear cut cases where two populations living in the same place have split into different species. There are two resons for this. One is that it is a little easier for populations to split when they don't live in the same place. The other is that the way these things are defined makes it very hard to prove that the species were living in the same place when they split.

The exact definition of "sympatric" has actually been a little hard to pin down. For a long time, it basically meant that the populations weren't living in different places. For a long time this was good enough, but when people actually started to look at what happens to the individuals involved, it got a bit harder to pin down. For example, if one population of insects lives in the branches of the trees on a small island, while another population lives in the low bushes, are they really living in the same place? That might sound like it's just nit-picking, but when you go to look at the population genetics you find that seemingly trivial distinctions like that can make a really big difference in how likely it is that the populations will completely separate.

One of the researchers who models speciation recently came up with a more precise definition. He said that two populations can be considered to be truly sympatric when mating is random with respect to birthplace. Now that's pretty obviously an ideal that isn't often going to be achieved, but there can be situations that at least come close. You'd think that solved the problem, right? Unfortunately, you'd be wrong.

There are a fair number of scientists who really don't like the idea of sympatric speciation. For a long time, Ernst Mayr, who was an extremely influential evolutionary biologist, argued that populations could only separate into two species if they were separated from each other by some sort of barrier. He made a relatively persuasive case for that positio and argued his case with passion for decades, so it's no surprise that there are some scientists who are skeptical of the possibility of sympatric speciation. In the past, when scientists have presented cases where it looks like two species split while living in the same place, the skeptics have demanded proof that the species were never geographically isolated from each other during their divergence. Proving a negative like that is kind of tough, so the controversy over whether or not two species can actually split while living in the same place has continued.

In this week's issue of the journal Nature, two different papers are presented that offer pretty convincing proof that species have diverged in sympatry. In both cases, two species that are clearly more closely related to each other than to any other species are found living in places that make it extrordinarily unlikely that the populations were ever geographically isolated from one another.

In one case, two species of cichlid fish are found living in a small, isolated lake in Nicaragua. The habitat within the lake is relatively uniform, and the authors demonstrated that the two species are not reproducing with each other and are physically, ecologically, and genetically different. In technical terms, that's called a "grand slam." For this to be anything other than sympatric speciation, a founding population of fish would have had to arrived not once but twice. That's unlikely enough to begin with, before you start to take into account the similarities that these two species share with each other but not with any of their relatives in other nearby lakes. When you take that into account, it becomes extrordinarily improbable that they didn't split in that lake.

The second case involves two species of palm on Lord Howe Island off Australia. Here, again, the species seem to be reproductively, physically, and genetically distinct. They also flower at different times, and the genetic work showed evidence that selection was operating to increase the divergence between these species. This is another really solid case for sympatric speciation.

These discoveries should convince all but the most unreasonably skeptical that sympatric speciation almost certainly has happened. That means that we know that species can diverge in all three of the different geographic relationships, and that they probably have done so. That, in turn, tells us that speciation almost certainly doesn't always happen through the same mechanisms.

By this point, I've probably lost half the people who started to read this, and most of the rest of you are probably wondering why I thought something this confusing and boring is actually exciting enough to be worth blogging. In part, of course, it's because I am, as my brothers will cheerfully confirm, a hopeless science geek. But if you've read this far you're probably one too (or you're my mother), so that can't be all of it. Part of it is because this is hot research in my own field, but that's not all of it either - I don't write about every cool article I read.

The reason I thought this was worth writing about - the take-home message of this post - is actually pretty simple. Evolutionary biology is a very active field, and we continue to learn new and exciting things almost every day. It is a field populated by people who are eager and driven to learn new things about the way evolution works. It is not a cult of personality centered around one man who wrote one book almost 150 years ago, as some would have you believe.
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