28 May 2006

How many species 2: What is a species, and Why does it matter?

One of the questions asked in the comments of the previous post in this series is quite pointed, and very much on topic for this discussion, so I'm going to take a minute or two to answer it. I'll give the answer to the example in another post, that will shortly follow this one. Karl asked:
So why are you asking that question? How is "species" defined. Does it really have a definition? Does it matter? Isn't "species" just a modern reaction to the biblical term "kinds"
Now that I've taken a few minutes to think about it, I'm starting to remember why I was dodging that question. I could write a long, rambling discourse on the topic, but in all honesty the best I can do for a definition of "species" is to paraphrase Justice Stewart's concurring opinion in Jacobellis v. Ohio: I might not ever be able to intelligently define the term, but I know it when I see it.

If you ask working biologists to give you a definition for "species," most will provide you with some version or another of Ernst Mayr's Biological Species Concept:
"species are groups of interbreeding natural populations that are reproductively isolated from other such groups."
In practice, however, most working biologists tend toward the "Justice Stewart" species concept, especially when their own preferred organisms are involved. It's a simple (and common) problem, really. Mayr's definition looks absolutely fantastic on paper, but it has this nasty habit of falling all to pieces when confronted by reality. We keep using Mayr's definition when asked to give one because (to paraphrase another famous guy) it is the worst possible definition for "species" - except for all the other ones that people have tried.

The next question, of course, is why we keep trying - why we think that it is important to try - if "species" is such a hard concept to define. Opinions differ on this, and I can only give you my own. It matters because we are trying to describe something that really exists. Some people do disagree, and believe that what we call "species" are simply human groupings, but I think they're wrong. I think that species really do have a biological existance, independent of our own intellectual activities.

The reason that we have a hard time coming up with a definition of species that works for everything is that there isn't one. Different types of organism keep themselves sorted out in groups in different ways. Mayr's definition works (at least in theory) for organims that are sexually reproducing. It goes all to hell when confronted with asexual organisms, and it gets worse when you toss plants into the mix - they do the wierdist damn things as they evolve.(Go read John Wilkins' paper on the topic, since he does a much better job explaining this.) Because evolution is an active, ongoing process, you will also find - no matter what definition works best for your organisms - some groups that are pushing the boundaries between species. Mayr's definition explicitly lumps together anything that can successfully interbreed, but what do you do when two populations successfully interbreed only 0.001% of the time in the lab, and never in nature? What do you do when they can be hybridized in the lab 100% of the time, but when they consistently refuse to interbreed with each other in the wild? What do you do when two populations will only produce sterile males when crossed, but always produce fertile females?

One of the best examples of this problem involves Lions and Tigers. Female Tigons (the hybrid offspring of a male tiger and a female lion) are fertile, and potentially could serve as a means of moving genes from tiger populations into lion populations or vice versa. This means that tigers and lions are potentially interfertile, and under a strict application of the biological species concept would be considered to be a single species. Of course, we "know" that they are not. Tigers and lions do not interbreed in the wild, are very different in appearance, and have very different behaviors. If they are not different species, the term becomes essentially meaningless.

So, to sum it up, species are real, and there are definitions for "species." The best definition will depend on the group of organisms that you are looking at, and no matter how you define "species," you will probably still find some group or another that is at a point in evolution where it doesn't quite fit the definition you're using. To sum up the summary, life is pretty damn messy.

But it does matter. When we look at living things (as our most distant human ancestors did) we see species. We can tell that living things are divided into groups, and that each individual is more similar to the other members of the group than it is to any other group. (Usually, anyway.) It matters because it is the way nature behaves, and because that is what we are trying to describe and explain.

It's a tired analogy, I know, but dealing with "species" really is a lot like dealing with the concepts of "child" and "adult." We know that there are differences between children and adults. It takes no real insight to look at the two and identify significant distinctions between the groups. The problem comes when you try to come up with definitions of "child" and "adult" that are capable of unequivocally distinguishing the two. It can't be done - there is no sharp distinction between the two there to identify. The same could be said for other age-based distinctions, like "adolescent" and "senior citizen." We recognize those as "real" groups, sharing numerous common features, even though we can only come up with arbitrary methods for distinguishing them from the neighboring class. The same is true for "species." We can see that they are real in nature, and that organisms really do form distinct groups. We just can't come up with a definition that will always allow us to distinguish a single species with very distinct populations from two species that sometimes interbreed.

15 Comments:

Anonymous Keanus said...

As a non-biologists, I think the dilemma in defining a species lies the fact that one species morphs, through mutation, natural selection, gene drift, etc, into another species very slowly (compared to the human life span). Out effort to differentiate a species from all others will always run afoul of close similarities, like those between the two great cats. From our standpoint as students of nature, making the distinction has meaning, but we have to recognize that it's an artificial construct we require for making sense of an otherwise confusing world.

10:29 AM  
Anonymous David Margolies said...

Perhaps you ought at least to mention that some biologists (e.g. Prof. Brent Mishler at UC Berkeley) reject the concept of species, or any taxonomic rank, and go for rankless taxa, the requirement for a taxon being that it is monophyletic. Under this scheme, the problem with definition that are obvious in your note simply melt away. (Most current species remain as taxa, often the most specific taxa, in the new scheme.)

John Wilkins wrote a note attacking this on the grounds that species is a perfectly valid idea for 80% (70% 85% -- I do not recall the exact percentage and have lost the link) of plants, to which I think Dr. Mishler would say "exactly".

David Margolies

10:31 AM  
Blogger Mark Frank said...

I am with the Judge...

"The idea that in order to get clear about the meaning of a general term one had to find the common element in all its applications has shackled philosophical investigation. - Ludwig Wittgenstein"

11:08 AM  
Anonymous Anonymous said...

I may not be able to tell precisely when dusk ends and actual night begins, but I sure as Hell can tell the difference between day and night.

RBH

11:17 AM  
Blogger TheBrummell said...

Thanks for tackling this question. I feel I did not adequetly respond to Karl's comments in the previous post - but that's what the blog's author is for, to write nice, long, detailed answers.

Once again, I am reminded that a word I use almost every day - "species" is far more baggage-laden and complicated than I normally think. I'm looking forward to further posts in this series.

Also, I agree: I think species really do exist independent of human perception or pattern-recognition.

1:04 PM  
Anonymous DragonScholar said...

Good job working on the definition.

I think the best way to put it is that the term species represents a continuity, not always a sharp division. So Tigers and Lions are speciated less than, say, Tigers and Penguins, but there is a level of speciation between Tigers and Lions due to fertility-of-offspring issues and a lack of in-nature interbreeding.

1:46 PM  
Anonymous Karl said...

Wellll...? First of all, I am not an IDer. I am not making this argument for purposes of building a case against evolution. Secondly, I am not a biologist. So I have no stake in the answer. My education is in Math where it is of primary importance to be precise in one's definitions. So I guess it is the general vagueness of the term that bothers me.
And, while I appreciate your taking the time to post a thoughtful reply, I still think that you haven't dealt with a fundamental problem with the definition - and that is its inconsistency. Specifically: 1) dogs. You say "Tigers and lions do not interbreed in the wild, are very different in appearance, and have very different behaviors. If they are not different species, the term becomes essentially meaningless." Yet I see the same thing between chihuahuas and St.Bernards. Why are they not separate species?
And 2) ring species: specifically the ring from Herring gulls to Lesser Black-backed gulls. Those birds are related in the same way as the two dogs are and yet the gulls are considered separate species!???
So I propose, given the recent development of genome decoding, that the definition be replaced with an objective, measurable one. e.g. two populations are separate species if the differences in their genome is greater than X%.
Finally, I think that all of this is just a semantic argument anyway. Animals are what they are. It would appear to me that lions and tigers have a common ancestor and they are now separate species because all the intermediate forms linking them have died out. But dogs, on the other hand, being a younger and artificially created type of creature, still have an entire range of types. If all dogs other than chihuahuas and St. Bernards were to die out, I would guess that they would be considered separate species too.

2:28 PM  
Anonymous Karl said...

And now that I've posted that (it took me an hour to write it), I've gone back and read the latest new comments and re-read the first ones. I think you are all correct. It is an amorphous concept. And, obviously, there can be a series of small gradations because, after all, we are talking about Evolution - where animal populations change slowly and new branches are created - as opposed to Creation where it is considered that all the existing types came into existence ex-nihilo. Maybe that is the lesson from all of this: that the indeterminacy of species is strong evidence for Evolution.

2:41 PM  
Anonymous plant person said...

"So I propose, given the recent development of genome decoding, that the definition be replaced with an objective, measurable one. e.g. two populations are separate species if the differences in their genome is greater than X%."

and now how are you going to decide on the exact level of genetic difference between groups after comparison of genetic sequences - and what region of the genome, and which genome- nuclear or plastid? And there is the problem of chosing the individuals from which to take the DNA in the first place, as this is based on morphology.

The rate of evolution of gene regions is heterogenous within lineages - and even between lineages. An example is that many recently radiated plant groups have profoundly different morphologies yet this is matched by relatively little genetic differentiation. Small changes in regulatory genes within the genotype can have profound changes in the phenotype.

Where do you draw the line as to what is a meaningful level of genetic dissimilarity when describing species? Morphological and genetic tools are used simultaneously in the process of describing and defining species. It can be a re-iterative process.

2:56 PM  
Anonymous Karl said...

Yeah, that's the problem with letting a naive amateur (me) ask questions about a very technical field. All I was basing my proposal on is the statement that (I think) I've seen that says that h.sap and p.trog share 94 (96?) % of their genes. If that is wrong or inapplicable, I bow out of that discussion.
I must say though that I really appreciate all the responses. I'm learning a lot. Thanks to all.

3:08 PM  
Blogger TQA said...

Don't be sorry. Actually, using DNA sequence similarity has been seriously proposed - it's known as "DNA Barcoding."

I think it's a bad idea, myself - I think that any value chosen as the cutoff between "species" is going to be entirely arbitrary and any correspoondance between such a cutoff and reality is going to be purely coincidental - but it has been seriously proposed.

3:21 PM  
Anonymous snaxalotl said...

it's not worth winding yourself up too much over the definition of species. Species is one of those words (like "life" or "consciousness") which were in use long before people conceived of a clear definition, and when people had far less examples in mind so that the exact distinction didn't change the categorization. Creationists need species to be exactly distinct in some single definite way because God made them distinct from the start, but biologists don't. Another way to say the same thing is that although there OBVIOUSLY ARE different species, sometimes we separate them according to different criteria, and all those distinctions are scientifically useful. "Explanatory filter" needs to be defined in a consistent way to be useful; "species" does not.

4:14 PM  
Anonymous Karl said...

TQA said:
"...I think that any value chosen as the cutoff between "species" is going to be entirely arbitrary and any correspoondance between such a cutoff and reality is going to be purely coincidental."

I read the reference to DNA barcoding. Of course the value for the cutoff will be arbitrary. But isn't what you're doing now also arbitrary?
"correspondence...reality" What is reality? Do you mean that the barcode determination will differ from the current one? Maybe that will be an improvement.
And, sorry to be a noodge, but, you didn't deal with the situation with ring species. I asked this same question some time ago on TQA and TQA (maybe a different A) talked about putting dogs on an island and letting them interbreed. He said the smallest dog could breed with the next smallest dog and so on and thus the smallest dog's genes would be in the total gene pool of all the dogs and therefore they are all one species. Isn't that also true for the ends of a ring species. Yet they are considered separate species.

5:05 PM  
Blogger TQA said...

You're right, I did skip over the ring species issue. I think the answer there, as in many of these other areas, is "it depends." I realize that isn't a very satsifying answer, especially if you're used to cut and dried situations, but it's really the best I can come up with.

You are correct, and the situation is analogous to the explanation that I gave for dogs - genes can flow from one end of the ring to the other by going through the intervening populations.

Ideally, I think the decision about whether or not the two ends of a ring species are the same species or different species should depend on how readily genes move from one to the other through the intermediate populations. If gene flow is fairly common, then the ends should probably be considered to be the same species. If genes only manage to successfully move from one end to the other rarely (say less than once every 10 generations), then I think a reasonable case could be made for calling them separate species.

In practice, I'm guessing that history has probably played a bigger role in whether or not the ends are considered to be the same or different species. If the two ends were originally described as different species, then they are more likely to remain such when later work is done.

As far as the utility of barcoding is concerned, I think it is somewhat more aribitrary to try to sort organisms into species based on a specific percentage similarity cutoff than it is to use a definition of "species" as a guideline instead of a rule, making exceptions where necessary in order to have "species" represent actual groupings found in nature.

A secondary problem with barcoding is that single-locus similarity is not necessarily a good proxy for what's happening across the entire genome, particularly in cases of recent divergence. Sequence similarity - particularly the type of mtDNA similarity they're proposing to use in barcoding - also doesn't correlate very well with physical, ecological, or any other organism-level differences.

This means that by using a barcoding-type definition, a situation is set up that could easily result in two populations that are very similar to each other being classified as different species, while two populations that are quite distinct both in appearance and ecological habits wind up the same species - just because one pair of populations has had one more mutation in a specific gene than the other has.

Barcoding definitions are both arbitrary and biologically uninformative. I'm not an unreasonable man, and I don't think that either arbitrariness or biological uselessness should be an absolute bar to a species concept. It's the combination of the two that I have problems with.

5:37 PM  
Anonymous Doug said...

I am no biologist, but I always thought that the whole point of the biological species concept was that it was not enough just to have two individuals produce fertile off-spring in the lab; they had to be interbreeding natural populations, ie, they have to be able to breed in the wild with some consistency. Which would clearly put lions and tigers as separate species (although closely linked). Of course you still have all the problems with asexual animals, plants, and ring species, but at least it clarifies some ambiguities. But perhaps I'm wrong?

3:15 AM  

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