A lot of people have said something like "species are the units of evolution". What does this even mean? So far as I can tell, nobody has really fleshed this out.
What, to begin, are the units of evolution? It depends a lot on what theory is being employed. If you are talking about population genetics, then the basic unit is, of course, the allele and the locus. That is, alternative genes (a concept that is itself rather problematic) at a given point or position on the genome. If you are talking about development, then the unit is the organism, as it also is when you are talking about ecological interactions, although "species" is used as the term for a class of ecologically exchangeable organisms; that is, organisms that play the same role in the local ecosystem. Since organisms are pretty well all different (which is the point about population genetics), for the purpose of trophic webs (the food webs of ecology), conspecifics are treated as being interchangeable.
Then you have the larger units of evolution: populations, and the particular revision of that concept, the "deme". A deme is largely the population that can interbreed - the term in the equations of popgen is Ne, the effective, or reproductive, number of individuals. But nonbreeders also play a role in many species in contributing to the fitness of their kin, by helping raises them, or finding food. While "population" is itself somewhat fuzzy - the more migration there is between two (sexual) populations the more they start to look like a single population - it is a theoretical object.
But species? I am aware of no theory that requires them. Having made that outrageous claim, I had better explain what I mean before I am attacked by roving bands of disaffected taxonomists.
Sure, ecologists and conservation biologists use species, but what they are really doing is using field guides as a surrogate for the ecological roles that individuals who are more or less normal - the "wild type" - play in an ecosystem. Likewise, medical and biological researchers do the same thing with their model organisms. Mus musculus, or the common mouse, is used as a study organism because it is assumed that they share the same properties (developmental cycles, phenotypes). But in practice they use "strains" that are specially bred to see the effects of gene knockouts. The "objects" here are the genetic strains and the organisms.
Systematists use species because they describe them. The explanations of species being species they give are manifold. The notion of a "gene pool" or "metapopulation" is one such explanation. But the theories used, the explanations, are not theories of species. They are theories of gene exchange, reproduction, fitness, adaptation, and so on. Species are being explained. They do no work in explaining.
So, there are two ways we might go if species aren't theoretical objects. One is that we may deny that species exist, and a lot of people do this. I call them species deniers, because they deny that species exist, although the usual term is species conventionalists, or nominalists. A version of species denial is to replace the term species with some "neutral term". Deme was one of these, but as Polly Winsor has shown, it got subverted by population geneticists for the meaning given above. Other examples include Operational Taxonomic Units, Least Inclusive Taxonomic Units, Evolutionary Significant Groups, and so on. In each case, the term species came or is coming back into use.
Why is species so durable? The answer comes from not taking the term and concept as a theoretical term. Species is a useful term because species are real phenomena. That is, they are things that call for explanation. In technical philosophical terminology, they are explicanda. The theories of biology explain why there are species. Not all the same theories for all species, of course. Biology is not that neat. Some species are explained the way the textbooks say - through the acquisition of reproductive isolating mechanisms formed in geographical isolation. Some aren't. There are species formed by hybridisation, by sexual selection, and of course asexual or mostly asexual species that are formed, as I argue elsewhere (isn't that pretentious?) by adaptation to niches.
If there is no general theoretical account of species, why do we have this category? Well, it might be because we tend to name things that look similar to us. This is what species deniers think: it's all about us and our cognitive dispositions, not the things themselves. I don't agree with this. I think there are some general features of species that licence us calling them all species. So here's the claim:
Species are phenomenal objects. They are salient not because of our perceptual tendencies but because they do exist. They're a bit like mountains. Each instance is caused by definite processes, but they aren't all caused by the same ones. We identify mountains, because they're there. We explain them with theories of tectonics, vulcanism, or even (if they are dunes) wind.
Species are clusters of genomes, phenotypes, and organismic lineages. We explain them because they need explaining. A species is (roughly) where the lineages of genes, genomes, parent-child relationships, haplotypes, and ecological roles all tend to coincide. Not all of these need coincide in every case, but so long as most of them do, they are species, and we must give an account of them. And we can and do.
[Thanks to Gal Kober, a graduate student at Boston, for pushing me on this.]
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Huh? I thought the problem is that we have too many general theoretical accounts of species.
More seriously, is the species the unit of evolution for palaeontologists?
Bob
We define species so we can differentiate between intra- and inter-species dynamics (ie, those in which there is gene flow between individuals, and those in which there is not gene flow between individuals).
As a (molecular) biologist, I would like to read your work on species concepts. I find that the common species concept (whatever that is) is into a lot of trouble when trying to describe organisms for which horizontal gene transfers is not uncommon (i.e. bacteria).
You mention the gene concept. Some of my lab mates and I used to get into discusions about that during graduate schoool and never came to an agreement.
But even mountains are subjective. Where one person might see a single mountain, another might see three distinct peaks or ranges. Each one of those peaks or ranges has a slightly different cause than the other. Just how different does a population have to be, to be another species? Slightly different, moderately different, or unable to mate, or unable to adapt to the same niche?
Jeff: What John is saying that there's no single simple answer or deciding criterion. We humans expect that because we have evolved to work that way (that is, many times it works) so we model reality in a tentative and simplistic way and the model works many times because it's the best we can do with very incomplete information (by the way, I think that's also the reason why we humans are so prone to be inconsistent and self deluded and to believe in things for which we don't have any reasonable proof). So, we apply the same concept to things that seem very alike to us, mountains or species, when in reality, with enough information, we might find out that they are so different in significant points that we have to proceed in their analysis in a case by case, or at least cathegory by cathegory, basis.
Hmmm, upon further reflection, a thought experiment for more objective species determination: if you had a hypothetical machine that could precisely map representative samples of all genomes on the planet and plot them in a "genome space" somehow, you might see them clustered together in groups corresponding to "species", and would be able to use statistical techniques, std deviation, etc to determine which species an individual genome belonged to. Sounds good in theory, anyway.
Ribozyme: Oh, I definitely believe it is a meaningful categorization, at least to us humans. Just as long as we don't have to rely on baraminlogy ;)
Bob: We have plenty of explanations in theoretical terms of species. Each "definition" of species is such a case. But no theory requires that species exist. Paleontologists typically use a morphological set of criteria for diagnosing species, but this is not a theoretical claim either. In fact you can do paleontological taxonomy simply by using individuals as terminal taxa.
RPM: This definition fails in too many cases, even for the prototypical notion of species in vertebrates. There is gene flow between many "species". Ducks, for example, regularly introgress. The more examples we look for, the more we find. In botany, species concepts are far more complex. Gamete broadcasters too often hybridise and introgress for that definition to be that useful. Either you need to restrict the use of "species" to a certain class of mammals (and exclude horses, big cats, canines and so on) or you have to admit that this definition is only a rough guide.
Jeff: this is precisely my view, but it, too, has problems as a definition. Clusters are real but vague. Any choice of metric (say, a Hamming Distance threshold) in a hyperspace cluster view of genomes is arbitrary, although the centres of density are real. This is what is true also of any sorites heap like a mountain (or baldness, etc.). Statistical techniques will identify the peak, but not the inclusion criterion.
Ribozyme: email me and I'll send you the proof copies of my microbial species article.
A mindgame I've tried on students is to ask "how many species of human are there?"
Take, for example, someone bearing a balanced translocation. There is a considerable reproductive barrier in place there, although not a complete one. Other partial reproductive barriers include incompatible Rhesus factors, and some interesting HLA / NK receptor interactions. In each case there is a reproductive barrier which really shouldn't be simply described in terms of "infertility" - after all, an exclusively Rh+ population will have no fertility issues, and nor will a population who are all homozygous for a balanced translocation.
In other animals, these kind of partial reproductive barriers are sometimes ignored (or simply not known about), sometimes called "subspecies" (e.g. Mus musculus domesticus/musculus/molossinus/castaneus), and sometimes defined as separate species (e.g. lions and tigers).
If we define speciation in sexually reproducing species in terms of reproductive isolation, then every infertile patient is, in a way, a new species containing one individual.
And that is exactly the criticism of the absolute form of the reproductive isolation conceptions since the 1960s. More recently, Coyne and Orr have revised it to be mostly isolated, which goes to bolster my claims in the subsequent post.
In the 1960s, when reproductive isolating mechanisms were being revised and investigated, the terminology subtly shifted to "reduced likelihood" or "lowered fitness" forms of isolating mechanisms.
But surely if you shift the definition towards "reduced likelihood" or "lowered fitness" it [i]magnifies[/i] that issue rather than circumventing it?
Under such a definition, Rhesus alleles, balanced translocations etc. would seem to qualify. Not to mention various potential pre-fertilisation mechanisms of partial isolation. Doubt anybody's checked, but I would not be at all surprised if (say) Inuits and Australian aborigines are somewhat less likely to find each other attractive. And what is that if not a partial pre-fertilisation reproductive barrier?
The chief problem I think we have with species is that human need to sort everything in to nice, nifty boxes. Sure, it's not hard to get the basic ideas of descent with modification, with all those nifty pictures of little primates turning into people, with each step a nice, clearly-defined animal.
I was as guilty as anybody in trying to make species into fixed "units", clearly defined entities like "C. latrans" and "C. lupus". It was only when I really began to understand that speciation isn't an event, but a process, and one that doesn't end in sharp boundaries, but can remain quite fuzzy, that I feel began to understand evolution itself.
I don't think there is a grand unified theory of what a species is. There is simply too much diversity in organisms. I am a fish taxonomist and am quite convinced that the species I work with are real entities. I have done some serious hobby stuff with succulents, cacti and Amaryllis. It is clear to me that a plant taxonomist is a different breed of cat. The realities they deal with are somewhat different from mine.
Speciation is indeed a process, and often a reticulated one which does not respond well to cladistic analysis. I don't think I could have recognized it, if it is happening, in the fishes I studied, so I focus on recognizing the event after the process has settled down.
Things do happen, and I've done quite a bit of hybridization, and attempted hybridization, to see what hybrids would look like. There are counter examples, but generally isolating mechanisms, among the species I recognize, are effective in nature.
I think species are evolutionarily important because speciation is the cause of diversity. There is no such process as geniusization, familyization, etc.
Jim, don't misundertake me, as Cheech and Chong once said. I do not deny that species are real objects. Speciation is a real set of processes. It's just that I don't think they are all of the same kind. Diversification of biotypes is real, but not in itself enough to require species as a rank or level of organisation. In fact, I think each group of organisms has its own kind of specieshood.
On thinking about it some more, I am even more impressed with strong genetic isolation as a characteristic of animal species. It is my impression that hybridization is more common among bony fish species than among other groups of vertebrates. There is an extensive literature on fish hybrids, and one could be confused by this volume. But the reason there is such a volume is that hybridization in nature is very rare, and thus worth an note if you collect a hybrid. There are some situations where hybrid swarms are long lived, hybrid swamping of a species, etc., which are paid attention to because they are the exception that proves (tests) the rule. So the general fact of the matter is that in fishes, species jealously guard their genetic integrity. I think this is a very important point in any thinking about the nature of species. Why do species so strongly resist hybridization? What does it all mean?
I think it is very clear that a species of killifish and a species of cactus are two diferent things but with some shared attributes. Never mind bacteria species.