Richard Zander wrote:

Well, operationally, a species needs to be described and published according to the rules, then survive the marketplace of ideas.

If by "rules" you mean codes of nomenclature, then they only apply to the name (label), not the species. It's the "marketplace of ideas" that serves as judge & jury for the definition of the species concept.

This is close to "what a community of taxonomists say it is" and rather different from "what a taxonomist says it is" (which I have also heard).

An unfortunate reality is that in many cases the relevant "community" of taxonomists consists of only one individual taxonomist. And actually, it's very often the case that only one taxonomist "said" it (in the form of describing new species, ansd asserting synonymies); the rest of the community simply chooses to follow or not.

Richard Jensen said:

If a species exists in the world, but no human is there to observe it, is it real?

That's the crux of the question. But I think a better way to think of it is: If "extra-terrestrial beings of a super-intelligence" arrived on our planet and classified all living things, would they come up with the same units as we have? Would they come up with a hierarchical classification scheme? If so, would they recognize one unit of classification in the hierarchy as being "special", moreseo than the others? Would those units correlate well with our "species"? Would they draw stick-figure cladograms as representations of evolutionary affinity? I suspect that in some cases there would be good congruency, and in other cases, not so much.

A parallel test we can do without the extra-terrestrials is to compare traditional nomenclatures created by native peoples, independently of western science, and see what the correlation is. In my experience, the correlation is reasonably good in some cases, and in other cases ... not so much.

I am not vain enough to believe that all other species exist solely because we humans are here to make observations about our surroundings. Trees falling in forests make noise whether anyone is there or not - the physical disturbances created by the falling tree, which our brain perceives as noise, still exist. Unless, of course, you believe that nothing exists until or unless it is perceived by a sentient being. If that's the case, then the light coming from a star one million light years away didn't exist until it was sensed. But if it didn't exist until it was sensed, then how did it make the journey? I find it terribly ego-centric to think that nothing else exists except in the context of human perception.

I don't think that's the issue. Nobody is claiming that organisms don't exist. The organisms would be here whether or not humans were here to observe them and (attempt to) classify them. I think we can all agree with that (at least for the purposes of this conversation).

The question is whether there is a special circumscription of organisms (across time and space) whose boundaries are more clearly defined than other broader or more restrictive circumscriptions of organisms. Another way to visualize it is as some sort of "inflection point" on the evolutionary path, where the rate of divergence, or propensity to reproduce, or whatever fundamental metric of "species-ness" you want to use takes a sharp bend, relative to an otherwise smooth line before and after the bend. This would be the "speciation event", and would be objective evidence that species actually exist as units in nature with some basis in reality other than our human subjective interpetations. There would always be some fuzziness at the bend, but the point is that demarcations prior to the bend are in the realm of subspecies and other such arbitrary clusterings defined by humans, and demarcations after the bend are in the realm of genera, families, and other such arbitrary clusterings defined by humans, but the "species" demarcation has some basis in biological reality that is independent of human subjective interpretation.

Exactly what is a "natural" unit that exists in nature independently of human interpretations? Does an atom of gold qualify? How about a planet? Unit implies something that can be individuated, most typically as a consequence of connections, however defined, between its constituent parts. You have suggested that populations may be natural units, but the very concept of population is linked to one or more of the many available species definitions. If the point is that species are not natural, then we have to know what "natural" means. Care to give it a shot?

"Natural" in this sense means "not invented by a human". The gold atom and the planet (probably) exist outside of human imagination. As mentioned above, individual organisms (probably) exist outside of human imagination. Populations, subspecies, species, genera, families ... these are all sets of individual organisms defined by humans. The question here is whether one of these kinds of sets (species) has more distinct boundaries based on properties of the organisms themselves, and is hence more "real" (independent of human imagination) than the other sets.

My assertion is that "species" do not hold a special place on the scale of hierarchically nested sets. I would argue that, as a general rule (exceptions abound), the more restrictive the set, the more objective its boundaries.

The set of organisms that includes two parents and all their offspring has fairly clear and objective boundaries. Expand that back several generations (great-great-great grandparents and all of their descendants), and the sets start to become a bit fuzzy and uneven (especially if individual organisms produce offspring through multiple mating events with different partners). Expand back further to what most of us would think of as a population, and the boundaries become even more fuzzy and arbitrary (i.e., rates of gene-flow among different populations may not be even or consistent over time, etc.) The further back you go (i.e., the more inclusive the sets become), in my perspective, the less clear the boundaries of the sets become.

This is contrasted with the "species are real units of nature" perspective, which maintains that there is a particular zone -- a sweet-spot -- an inflection point -- a circumscription boundary -- that is "natural" (independent of human imagination), and therefore "real". From this perspective, the task of taxonomists is to discover these circumscription boundaries, and define species boundaries to be congruent with them. The fundamental premise is that the boundaries are defined by some metric of "propensity to reproduce" (or something similar).

If we define a speciation event as the shift from high "propensity to reproduce" to low "propensity to reproduce", then cases where this shift is abrupt (proportional to the time between such shifts along the evolutionary path) are much easier to visualize as demarcations between species. And in such cases, it is very useful and practical to think of species as "real" units of nature, that exist in nature of human definitions, and are marked by these abrupt shifts. In cases where the shift is not so abrupt, and perhaps even continues long enough for another shift to begin (i.e., the tranisiton periods overlap), then species boundaries become much less clear, and it is much less useful to think of species as "real" entities in nature.

My contention is, and has been, that almost none of these shifts are instantaneous (i.e., there are likely to be very few cases where an individual organism would be regarded as being a different species from its parents), and as such, there will always be some fuzziness in the boundaries.

I'd love to ramble on (and on, and on, and on). But I suspect that only a very small percentage of readers have made it this far; and I've got other things to do today.

Sorry for the bandwidth.

Aloha, Rich