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.

28 Comments:

Blogger Craig Pennington said...

This was a great post. Speciation is an interesting subject and the evidence for sympatric speciation (I am currently reading Mayr What evolution is, otherwise I probably wouldn't know the term) is interesting. Thanks.

4:23 AM  
Anonymous Bob O'H said...

"He said that two populations can be considered to be truly sympatric when mating is random with respect to birthplace."

That just artifically defines half the problem away!

One of the areas where sympatric speciation has been thought to occur is in insects, where there is selection for specialisation for larvae to eat different host plants. So they are born on different hosts, but as adults will be sympatric. BUT, for symaptric speciation to work, they have to have assortative mating, i.e the adults will mate with individuals who were brought up on the same host (there are lots of models showing that this has to happen). So, mating isn't random with respect to birth place, but that's precisely what's driving the speciation.

I guess that the definition could be refined (and perhaps it is: I haven't read Gavrilet's book), but it still won't pin it down, because you have to work out how fine the grain of the environment has to be to apply the definition.

Apart from that, great blog. Another two papers I should read!

Bob

5:15 AM  
Anonymous wolfwalker said...

I don't understand. Fourteen species of Galapagos finches, all clearly descended from the same ancestor-species, individuals generally restricted to their birth-island, divergence has been documented in populations of finches living on the same island, and sympatric speciation is still open to question?

For that matter, the bird world is full of what look like examples of sympatric speciation. The one that comes most readily to mind is the Downy and Hairy woodpeckers: two birds that are clearly different species, but even expert birders can have trouble telling them apart by sight.

5:56 AM  
Anonymous Jon Nickles said...

Very interesting read, and I'm just your average Joe interested in evolution / science in general. However, I wanted to ask a question, minor nitpicking I suppose. You mentioned a fellow researcher and his better definition of sympatric. I noticed it included "random" and I know how ID/creationists love to point to "random" evolution and claim that it is therefor godless,immoral,foolish or what have you. I was wondering if the word indifferent could be substituted in it's place and the meaning would still hold the same?

Otherwise great read. I love this stuff, makes me wish i had gone a different route with my carreer choice.

6:00 AM  
Anonymous Anonymous said...

Hi -- QA's mom here.

When QA was about 6, he asked me (very seriously) how you would know you were adult. Is that what diploma's were for?

I distinctly remember replying that "it should only be so easy."

So here he is, 25+ years later, still pursuing the same basic question.

Scary . . . but true.

6:48 AM  
Anonymous Anonymous said...

". . . little bit corse-grained" should be "coarse-grained."

7:04 AM  
Anonymous Karl said...

Great! Someone who works with "speciation". I have what I consider to be a fundamental question. I have asked it in several places and all I get is a brush-off, like it's not even worth responding to.
Preliminary questions: What is the current definition of "species"? How do you know when two groups are separate species? I always get: "reproductively isolated", even if it's only geographically. Or, if they can physically interbreed, the progeny will be sterile. Now that's a 50 year old answer. I heard that in my 10th grade biology class, and it didn't make sense then and it certainly doesn't make sense now that we can do DNA analysis. Milford Wolpoff some time ago was in town giving his usual presentation about "Neanderthals in the Closet". He says that it is still "reproductive isolation".
So, the question is: IF that is still the definition, I have two opposite situations:
1) Two "breeds" of the same species that are physically unable to interbreed. Example: I saw a TV ad for the Westminster dog show. It showed two dogs, a Pomeranian and a St. Bernard, side by side. It appears to me that, physically, there is no way they could interbreed! Therefore they are separate species.?
2) Two animals of separate "species" that have interbred and produced multiple generations of descendants. Example - Tigons.

7:52 AM  
Blogger egbooth said...

Great post! Thanks a lot!

Although, it seems that any talk about sympatric speciation should mention the work done by Jeffrey Feder at the University of Notre Dame. He does his research on true fruit flies and it appears that they speciate sympatrically. He just gave an interesting seminar at my school, UC-Davis last week.

You can check out his website at: http://www.nd.edu/~biology/JeffreyFeder.shtml

7:59 AM  
Anonymous wolfwalker said...

karl asked: "What is the current definition of "species"? How do you know when two groups are separate species? I always get: "reproductively isolated", even if it's only geographically. Or, if they can physically interbreed, the progeny will be sterile. Now that's a 50 year old answer."

It's still the best answer you're likely to get, not because it's very good but because no one can think of anything better. The current incarnation of the reproductive species concept usually goes something like this: two populations are the same species if under normal, natural conditions, they can interbreed and produce fertile offspring. Domestic animals don't count, neither do breedings in captivity or crossbreeds that are sterile. I don't know the definition for species of asexually-reproducing organisms; I suppose they're defined based on morphology and/or genetics, just as fossil species are.

8:15 AM  
Anonymous Christopher Letzelter said...

Karl: the definition of speciation that you're proposing is inaccurate. There is "disagreement" in the scientific community as to what constitutes a species, as shown by the two examples you cite. Basically, there probably isn't a single cookie-cutter definition of species that will apply to all organisms, which is what makes this field of study so interesting - and which, I think, only bolsters the validity of neo-Darwinian evolutionary theory.

For a little more in-depth discussion of this, please read:
http://www.talkorigins.org/faqs/faq-speciation.html

Christopher Letzelter

8:31 AM  
Blogger TQA said...

Karl:

Those are good questions, and they are difficult to answer.

Let's start with the definition of species. The "classic" definition is the one you mentioned, which is known in the trade as the "Biological Species Concept" (or BSC). This is the definition that was proposed by Ernst Mayr, and it is the one that appears in most science textbooks. There's plenty of disagreement about how well that definition works when it meets up with reality. The FAQ that Mr. Letzelter cited is a good place to get more info on that.

The BSC is more or less the definition used by textbooks, primarily because it is old, well known, and few of the other definitions out there are any better (although I have high hopes that my friend John Wilkins will get that taken care of one of these days).

Most of the researchers I know basically think that the BSC is a pretty good definition, but it doesn't quite work for the group that they work on. This response doesn't seem to depend much on what group they work on, either.

Generally speaking, it seems like there can be some limited interbreeding between populations even after they have diverged so much that the people who are experts on those organisms are uncomfortable calling them the same species.

In group that I work on, there are several cases where two species can interbreed and produce hybrids, but only the female hybrids will be fertile. There are other cases where the females are fertile and 90% of the males are infertile, and there are cases where 90% of the male hybrids don't survive, and 90% of the female hybrids are infertile.

In general, the inability to reproduce doesn't happen all at once, and it isn't an "all or nothing" barrier. Everyone agrees that when reproductive isolation is absolutely complete the groups are separate species. Almost everyone agrees that if there are absolutely no barriers to reproduction, the two populations are the same species. The disputes tend to happen in the grey area.

Now, to finally answer your specific questions:
A pomeranian can't interbreed directly with a St. Bernard, but it can with a terrier. The terrier can breed with a smallish collie, which can breed with a retriever, and so on. Genes from the pomeranian can't make it directly into the St. Bernard population, but they potentially can indirectly get there, and there isn't any natural barrier keeping that from happening. If we were to set up some sort of island, populate it with a wide range of dog breeds, and let them breed freely, I'd guess that the big differences would be pretty much gone in under 20 generations - probably in under 10.

Tigons don't seem to naturally occur. If they did, it would almost certainly be so rare that pretty much everyone would still consider lions and tigers to be separate species. That's in the grey area I mentioned before, but only just.

9:23 AM  
Blogger Dave said...

Mike,

Thank for the very interesting post. Speciation is a fascinating topic.

9:32 AM  
Anonymous Karl said...

I thank the three of you for your replies. It seems to me that the collective sense of what you are saying is that "species" is a human construct (corresponding to the biblical "kinds") and that nature does not make such differences. As a matter of fact, isn't that what a large part of the argument is about in human evolution. Which fossils represent separate species - the difference between "lumpers" and "splitters".
Also, TQA, isn't what you described for the dogs an example of a Ring species? And aren't the opposite ends of the chain considered to be separate species?

10:11 AM  
Anonymous wolfwalker said...

karl said: "It seems to me that the collective sense of what you are saying is that "species" is a human construct (corresponding to the biblical "kinds") and that nature does not make such differences."

Yes and no. The vast, vast majority of "species" are defined by reality, not by human fiat. These problems and fuzzy edges show up in a very small minority of cases. So obviously there's _something_ in Nature that acts to define species. Otherwise, the fuzzy ones would be a lot more common. I always saw this as a confirmation in its own way of evolutionary theory: most "species" are well-defined, but a few are currently in some stage of the speciation process, and those are the ones that show these fuzzy edges.

tqa wrote: "If we were to set up some sort of island, populate it with a wide range of dog breeds, and let them breed freely, I'd guess that the big differences would be pretty much gone in under 20 generations - probably in under 10."

Good guess. That's exactly what happens when you let a population of dogs interbreed freely: the extremes vanish quickly and the population converges on a 'basic dog' phenotype. Search on "pariah dog" and see what you find.

3:49 PM  
Anonymous Michael said...

Great Post. I'm curious if there is any mechanism proposed (in words of one syllablr) that will allow a single population to form 2 or more species. I can understand genetic drift in isolated populations.

4:10 PM  
Anonymous Karl said...

wolfwalker said..
Yes and no. The vast, vast majority of "species" are defined by reality, not by human fiat. These problems and fuzzy edges show up in a very small minority of cases. So obviously there's _something_ in Nature that acts to define species. Otherwise, the fuzzy ones would be a lot more common. I always saw this as a confirmation in its own way of evolutionary theory: most "species" are well-defined, but a few are currently in some stage of the speciation process, and those are the ones that show these fuzzy edges.

I'm sorry - I'm struggling with that.
(Before I go on - some personal notes so you won't dismiss me as a Creationist. I am a devout evolutionist. I have been working hard for the last four weeks trying to get the churches in this city to participate in Ev Sunday.) OK - on species. I am flailing in the dark here.
Is "same species" a transitive relation? i.e. If A is the same species as B, and B the same as C, must A be the same as C? This relates to ring species. Specifically - gulls. There is a ring, that apparently started in Siberia (I think), from which offspring migrated East or West around the pole until descendents met in Great Britain. Those descendents territories overlap, yet they do not interbreed. They are separate species. If species has any real meaning, how can this be? This seems to be very fuzzy, but it is by no means unique. Help me out here because the implications of this are (I think) enormous - not for Evolution, just for the meaning of "species"

4:51 PM  
Anonymous Anonymous said...

wolfwalker said...-"I don't understand. Fourteen species of Galapagos finches, all clearly descended from the same ancestor-species, individuals generally restricted to their birth-island, divergence has been documented in populations of finches living on the same island, and sympatric speciation is still open to question?"
Well, remember, that's because there was never any NET evolution among the finches. If you can't deny the evidence make up new terms as you go along.;-)

a panda refugee

5:15 PM  
Blogger mudchuck said...

Just talking off the top of my head, wouldn't one mechanism for sympatric speciation be if you had a mutation that conferred a great relative advantage as long as the mutation was rare, but that would become less and less advantageous as it spread? This kind of thing might happen when the mutation would allow the individuals with that mutation to exploit some new ecological niche, but make them less competitive in the niche that the species originally occupies. Then any further changes that allowed the emergent species to discriminate against each other when breeding would be mutually advantageous.

5:18 PM  
Anonymous a panda refugee said...

Karl,
In addition to those other resources try here for an explanation as to why the species concept is jinky difficult to use or understand.

http://lancelet.blogspot.com/2006/01/
species-is-as-species-does-part-iii.html

a panda refugee

5:24 PM  
Anonymous wolfwalker said...

karl wrote: "I'm sorry - I'm struggling with that."

No need to apologize -- it's a hard thing to get your mind around. It was for me, at any rate.

"Is "same species" a transitive relation? i.e. If A is the same species as B, and B the same as C, must A be the same as C?"

It depends on how rigid you want your concept of "species" to be. You can try to tell Nature what the truth is, or you can let Nature tell you.

Here's an analogy, which I think is a decent one. Consider the rainbow - the spectrum of visible light. If your school days were anything like mine, you were taught that there are seven colors in the spectrum: Red, Orange, Yellow, Green, Blue, Indigo, Violet. But there's no single place in the red-orange region of the spectrum where you can point and say "the color on this side of this line is red, while on that side it's orange." It's a smooth transition, red to orangeish-red to reddish-orange to orange. There's no one place where it starts being orange or stops being red, but eventually you realize that, well, it's not red anymore, it's orange. That's what happens with ring species, which is why another name for that phenomenon is "spectrum species."

Now picture what happens if you pass light through a colored filter -- say, one that filters out only light in the red-orange range of the spectrum. The filtered light contains red and orange, but no red-orange mixes. That's how you get well-defined species: you start with the broad red-to-orange range of forms, then natural selection filters out the middle part of the range, and what's left is the red and the orange and nothing in between.

Spectrum species like the Herring/Lesser Black-backed Gull are species that haven't been completely filtered yet. You can't really say they're one species, but you can't really say they're two, any more than you can say _this_ part of the red-orange range is red and only red, and _that_ part is orange and only orange.

Most species have been thoroughly filtered, so they're well-defined and recognizable "things" in their own right. Nobody's going to mistake a horse for a zebra or a lion for a tiger. Only a few species are still undergoing the filtering process, making it hard to tell if they're one thing or two. In such cases, all you can do is look at how much filtering has taken place, and arbitrarily call them one species (if there's still lots of red-orange) or two (if most of the red-orange is gone). In those cases only, it's a judgement call.

Does that make it any clearer?

6:16 PM  
Anonymous Karl said...

To: a panda refugee
The Lancelet article is excellent. He supports my feeling that "species was meant to embody the original created kinds". He also cites Darwin as saying "species as we knew them, were not real".
And To: wolfwalker
Your analogy to a spectrum helps a lot.
Thank you, and the others who wrote.

A final question: Please tell me why so many of the correspondents on these blogs insist on pseudonyms?

6:40 PM  
Anonymous jackd said...

Greetings, TQA. John Wilkins recommended your page, and he's absolutely right. I greatly admire how clearly you explain the sympatric/allopatric distinction.

Karl, you've generated some excellent discussion (as thoughtful questioning tends to do), and I'd like to add something from my layman's perspective that hasn't come up yet.

Some pretty smart people have worked on the question of whether species are "real" or primarily human constructs. One of the ways they attacked the question was to look at very distinct human cultures to see if they identify species the same way as we from the Western science tradition do. Basically, yes. If a range of animals is important to a culture (e.g. for food) then that culture will partition them the same way.

Stephen J. Gould did an essay on this, but I don't remember which collection it's in.

4:45 AM  
Blogger TomK said...

Great postings, guys.

I am a complete, but admiring, amateur at evolutionary biology. Whenever reading some new piece, I'm impressed with the level of detail to which you've examined these issues.

I do have a question for you, and let me apologize in advance if it is a trivial one. I've never seen it addressed.

With regard to speciation rates (as being the product of mutation rates & fixation rates):

It seems to me that fixation rates (and possibly even mutation rates, to a much lower degree) may well be proportional to the variability of an environment. Low environmental variability = stable populations = low speciation rates. High environmental variability = unstable populations = high speciation rates.

One nice thing about this theory is that it seems to be fairly easily tested. There are several locations that fall into each category.

Stable environments:
Oxygen concentration of the atmosphere (post Cambrian).
Temperature in the oceans.
Temperatures on land in the tropics.
Salinity of oceans & lakes.
Light content in the deep ocean and underground.
Predators in isolated locations.
Oxygen concentration in blood.


Variable environments:
Oxygen concentration (immediately pre-Cambrian).
Temperature on land, especially non-tropical, or in hot pools.
Salinity of ocean where great river dump.
Light on land and in shallow water.
Predators when new access opens (e.g., connection of N America to S America).
Oxygen concentration in muscle.
___

I'm sure you guys could think of hundreds more.

My point is that, in stable environments, it would seem very difficult to displace an entrenched species. And that, even if a new advantageous mutation developed, it might not succeed simply by virtue of numbers. In a variable environment, there would seem to be many more opportunities to succeed. And more nitches for such a mutation to hide while while growing to a sufficient critical population to succeed.

If this idea is correct, then it would predict that speciation rates in (for example) stable saline environments (oceans or lakes) would be lower than in areas of variable saline environments (such as adjacent to the Amazon, Nile or Mississippi River deltas).

Would you have any idea if this is so? Is this just the trivial stuff of Speciation 101?

tomk

8:24 AM  
Anonymous a panda refugee said...

Karl asked about pseudonyms,
I don't necessarily insist on a pseudonym, but I am a fairly obnoxious commentator where I do most of my online heckling and I didn't want that obnoxiousness to get in the way of what I thought was a good suggestion for you.

a panda refugee

2:36 PM  
Blogger Impatient Patient said...

I love it and I am not your mother- wish I understood it more though. And your mother is HILARIOUS! Have snooped around a bit and will keep you on my desktop for some elucidation and illumination.

9:50 AM  
Blogger Matthew D Dunn said...

Great post. I'm studying speciation too, although I hadn't seen the papers in Nature. Thanks for the tip.

6:25 PM  
Anonymous Anonymous said...

Karl, Your dead wrong when you say, "species was meant to embody the original created kinds." We're talking evolution here, not creationism. There were no "original created kinds"

Also, as long as there is genetic diversity in a "species", that species can be filtered. Filtering may cease while the environment is stable, but when the environment changes the filter shifts.

8:35 AM  
Anonymous Don Cox said...

I think you can reasonably say that for Linnaeus, the species corresponded to the original created kinds. ____ The usual definition of species seems to work quite well for vertebrates and insects, at least if you allow a small amount of hybridisation, but it doesn't really work for bacteria, amoebas or hawkweeds. I think a better approach is to think in terms of plotting variable features on a graph, and seeing if the individuals plotted fall into distinct clusters. In some groups the clusters will be very distinct, with no intermediate cases, while in other groups, clusters will be ill defined.

6:43 AM  

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