All species are transitional

When does bay become ocean? When do foothills become mountains? When does a boy become a man?As the genetic similarity between two populations declines from 100% to 99.9999% to 99.9998% and so forth, at what point should the two populations be deemed separate species?These are rhetorical questions. Any chosen dividing line is arbitrary.--Percy

We're using different definitions of arbitrary. When I used the term arbitrary to describe the choice of criteria by which species boundaries are drawn, I did not mean random or capricious. I meant that the criteria are a matter of opinion and are open to debate. No matter how well a given dividing point can be substantiated with reason and evidence, it is still arbitrary because reason and evidence can be offered for other dividing points. It is the rare circumstance when criteria are completely objective. Your preference for "totally isolated" as the criteria is your opinion (one shared, by the way, by almost no one in a position of influence within biology).I think you were assuming that if we chose "totally isolated" as our criteria that it would greatly reduce the ambiguity of classification of organisms into species categories, but you'd be wrong. This isn't the main point so I won't dwell on why this is so, but we can spend some time on it if you like. Whether or not any of the analogies work for you, species change is a continuum. This must be so because the unit of change can be no larger than the difference between parent and child, which is very tiny. For this reason, all species change occurs in very, very tiny steps that only accumulate into large changes over the course of many generations.--Percy

Notice that I highlighted in red the part where I said that by arbitrary I did not mean random or capricious. I said it was a matter of opinion, explaining that different reasoned and evidence-based arguments could be advanced for different dividing lines between species. I don't think anyone would ever seriously suggest color as the criteria, and it clearly fits under the heading of random and capricious. No one is saying that either one is arbitrary. The point being made is that whatever criteria you develop, whether based upon morphology or genetics or a combination or something else, they are open to discussion and debate. It isn't possible to develop any hard and fast objective criteria that would remove ambiguity. That's why I asked the rhetorical questions at the beginning. When do foothills become mountains is the same type of question as when do two populations become sufficiently different to be considered different species. How much different is enough different? We can get into more detail if you don't believe such questions do not have firm objective answers.And the whole reason for this is that species change occurs in tiny immeasurable steps. It simply isn't possible to point to a step and say, "Before this step it was X, after this step it was Y."--Percy

No, we don't want something arbitrary. We want something hard and fast and specific and unambiguous and objective, but reality forces arbitrary criteria upon us. In this case, the continuous nature of species change means we can't unambiguously identify the dividing line between species. There will always be arbitrary aspects governing where we place those dividing lines. And again, by arbitrary I do not mean random and capricious. I mean sensible well thought out and supported criteria about which there is no unanimity and about which people disagree. What happens many times is that people recognize that no objective dividing line can be established, but they recognize that one is needed, for purposes of classification if nothing else, so they establish an arbitrary dividing line. Of course it is. Do people agree with you that permanently separating two identical populations in space or time so that they can no longer experience gene flow makes them two different species? No, they don't. Your choice of this criteria is therefore arbitrary because you can advance no better reasons for using it than other people can advance for other criteria. (In reality, gene pool isolation is a very poor criteria, and we can get into the reasons for that if you're interested.) Can't make any sense of this.--Percy

Okay, I think I'm getting closer to understanding your view. You believe that valid criteria must be arbitrary, and that for this reason gene pool isolation is a poor criteria.First, about criteria. No, we don't want them to be arbitrary. We want them to be as concise and objective as possible. Unfortunately, the real world forces arbitrary criteria upon us. For example, is this color red? For this discussion, assume the wavelength for red is 700 nanometers. If the wavelength of the text measures at 701 nanometers, is that red? How about 700.1? How about 700.01? At some point an arbitrary decision about precision must be made. Maybe we decide that everyone should invest in expensive wavelength analysis equipment and that a color should only be considered red if it measures with a wavelength between 699.9999 and 700.0001 nanometers. Or maybe we decide that accuracy isn't important and that any color between 680 and 720 nanometers should be considered red. These arbitrary decisions are not want we want. They are forced upon us by real world considerations.Second, about gene pool isolation as a basis for establishing criteria for species categorization. While it would serve us poorly as the sole criteria, it *is* arbitrary. The different opinions about it represented in this thread are proof that it falls prey to the same exigencies of reality as other criteria.I'm still getting the sense that your goal is to remove ambiguity from the species classification system. This isn't possible, as everyone has been trying to tell you.--Percy

Yes, it does. As I've explained a couple times already, you're using the wrong definition of arbitrary. The definition I'm using, and probably that everyone else in this thread is using because it's the one that applies to this situation, is "based upon individual judgment." Because the judgement of where the dividing point between species differs between qualified individuals, that dividing point is arbitrary. Then it sounds like we agree, it's just hard to tell sometimes. We can't unambiguously fix a point where one species becomes another, and that is because of the continuous nature of species change. I don't think so, but I suppose it depends upon how you define it. Point me to your definition of "gene pool isolation". Since there's no such thing as perfect isolation, you'll have to make an arbitrary (please apply the proper definition for this context) decision about how much isolation is required to qualify under your definition.--Percy

Okay, I recall this example from earlier in the thread, but it doesn't deserve much consideration. Yes, under a total gene pool isolation criteria that there could be speciation events, but this example is highly contrived since it has a species being produced through the extinction of an intermediate population. Such speciation events are only artifacts of your classification system and have nothing to do with species change. This is one reason why total gene pool isolation isn't a useful basis for classification anyway. There are other reasons, and we can discuss them if you're interested.Species classificaton with multiple criteria, gene flow being only one of them, would make much more sense. Morphological difference is another important species differentiator, as is relative fertility. Range and habits are others. Why do you feel the need for a species classification system based on limited criteria. As I've said a couple times now, and I think others have said the same, it still feels as if you're trying to eliminate the ambiguity in species classification systems, and this isn't possible. We can improve and refine our systems and reduce ambiguity, but eliminating them isn't possible.--Percy

Yeah, explaining discrete classification into species when change is seamless isn't easy. Use Randman as your guinea pig. If you come up with an explanation he understands then you've got a winner.--Percy

Good point! In these discussions we keep forgetting about the plant kingdom.I just looked up polyploidy (answers.com), and in plants it isn't clear that the process produces a new species, unless your criteria is soley genetic and you stipulate that different numbers of chromosome copies equates to different species.We also often forget about asexual reproduction, and in animals it says the result of polyploidy is usually sterile and must reproduce asexually. We keep mentioning the reproduction problem when we describe species change as continuous, because an animal with too large a change will likely find no mutually fertile mates and die without reproducing, but of course this isn't a problem with asexual reproduction.--Percy

I suppose that if a population all at once produces a subgroup whose members all have the same or compatible polyploid change, then you can get a new species in a single generation. But this is just the kind of unlikely event that we rule out for creating new animal species. How does polyploidy produced a mutually fertile population in a single generation?--Percy

Mammuthus and Wounded King,Okay, that explains what I thought was unlikely. I didn't see how simultaneous identical polyploidy producing a mutually fertile population was likely enough to ever happen, but with asexual reproduction a population can be produced from a single individual. Thanks for the information!--Percy