Talmudic Question #26

Given that hardly anything in biology is more central yet more elusive than the concept of species, can you conceive of a way to portray evolutionary history that is not based on the species concept?

Comments

[John S. Wilkins]

Elio: the myth here is that there is a unit of biology, "species". But it's not a myth that actual species exist as objects. M. musculus exists; it simply doesn't happen to fill a prior box of biological categories.

This allows me to add to the giant literature, I trust. Others, such as Jody Hey, have denied the reality of any species as well as the species category, but I do not go so far. I'm no conventionalist, I'm a phenomenalist, and of course a phenomenon requires some set of assays to individuate. So if you have a number of such techniques that tend to coincide, it's a species.

John: Mayr is simply historically wrong about essentialism. I'm sorry, and I can't back it up here, but essentialism is another myth (two myths. Amongst our mythology... I'll come in again). There never was any such conception of species, at any time. Instead I would argue that the default view of species was:

generative power + similarity of form

Mayr makes it identity of form and ignores, in some cases in his 1982 by simply snipping the rest of a passage, the generational aspect of species. I call this a Generative Conception of Species, and it was in play from Epicurus right up until the arrival of genetics.

I believe that there is a Conceptual Delicatessen, which biologists use to assemble the "species concept" that suits their group of organisms, and then they can debate over the best club sandwich for their purposes. This is why, for example, the "species concept" of microbiologists differs radically from the "species concept" of vertebrate zoologists. The one won't do duty for the other. This indicates to me that "species" is an evolved way of being a lineage, just like cell walls, endosymbiosis events, tetrapodality, and so on. We pick the club sandwich that best suits the way these critters have evolved to cluster together. Awful metaphor, I know, but there it is.

[Robert Murray]

Whatever may be the dislikes of a number of modern biologists for what they see as the meaning of species, when it comes down to talking to each other about life on earth it is useful and saves time to have a good number of species in the mind. So, alike as a myth, a concept or a general description it is useful and will continuue to be of use in planning how to approach a "new" phylogeny. In the case of procaryotic life there is a rule book that defines how to approach nomenclature ( a species is at least referable to a Type Species even if the boundaries will and do become fuzzy and unclear) and if a whole new approach is found it will have to have its rule book because, inevitably, there will be choices. This is exemplified now by the extraordinary differences in DNA sequences between demonstrably very close relatives in both humans and bacteria. So it is now that choices are made of semi-and very concerved genetic components for a useful bar-code approach recognizing species and relationships. Everything seems to be useful in phylogeny down to protein sequences and indels in them.
All along the way we have had to use what works as aids to understanding just as nature tries all the variations it makes and if one works it is kept. AND, if by chance it ceases to work it seems to keep useful bits of it in case they will be useful sometime. I bet any new phylogeny will be an elegant system with "bits left over".

[John Trawick]

John Wilkins certainly raises an interesting point and there is support for this concept. In Ernst Mayr's book "What Evolution Is" Mayr describes how one of Darwin's major contributions was to "remove typology and essentialism" from biology. That is, evolutionary change through time is so gradual that setting up boundaries between species and their descendants is rather arbitrary. We see distinct species largely because of the incompleteness of the fossil record and the fact that only a handful of survivors live in the present day.

Dawkins deals with the same concept in "The Ancestor's Tale". The illustration that Dawkins gives is clever: imagine going backwards in time 1000 yrs, mating, and then your progeny journeys backwards another 1000 yrs, mates, and that progeny... and so on. No one would notice changes as they did this but eventually the humans would morph into prehumans, early mammals, amphibians, etc.

Speaking of the elusive species concept, what about bacteria? Microbiol taxonomists occasionally seem to dump one concept for another, in grad school we were taught that there are ~2400 species of Salmonella, now condensed to 1 or 2. Same bugs, different ways of talking about them.

[elio]

John,

Yours is an elegant way of disposing of this Talmudic Question, by declaring that the species concept is a myth. I have no standing in this matter, but wonder if others will agree that - to put words in your mouth - the species concept is just a will'o the wisp. Does that take care of the gigantic literature on "what is a species?"

Elio

[John S. Wilkins]

I think that it is a myth that the concept of species is central to biology. It is not in any way a theoretical concept - all the basic models and principles of biology are neutral with respect to the existence of species. It is not even necessary to the doing of phylogenetic systematics - each analysis is done on a specimen, not a species. The concept of species is useful, no doubt about that, but it is useful in conventional ways: for field identification, museum taxonomy, and reference texts. But that in no way makes species central to biology.

On the other hand, and this is what I think confuses many, it is true that each individual species is usually considered to be a real thing. There is a fact of the matter whether, for example, a particular organism is a member of Mus musculus or not, and this leads people to think that the category of species, or more properly the rank, is real. But I hold that species are phenomenal objects, not categorial or theoretical, which no more means there is a rank than the reality of particular mountains means that there is an objective rank of "mountain".

In the case of asexuals, or facultative sexuals, the notion of a species is, as widely admitted, hard to define, but it seems to me that to say there is an asexual species is effectively to say what Eigen did about viral species, that they cluster their genome and phenotype in a particularly salient manner. Consequently, asexual species are no more disputed than sexual species, as they either do or don't cluster int he requisite way.