intelligent design, right and wrong

The Great Dane / Chihuahua cross boggles the mind, don't it? Talk about reproductive barrier...Remember, the biological species concept merely states that there is some reproductive barrier that exists between populations that prevents them from interbreeding in the wild when given the opportunity. So - theoretically at least - chihuahuas and great danes are different species 'cause I can't IMAGINE them backcrossing to some medium sized dog.

This is getting interesting.You're not incorrect when you state that gene flow connects all the breeds of dogs. My guess would be that this is one of the key reasons why nobody's bothered to try and split dogs into separate species. That and the fact that the various kennel clubs would be up in arms immediately. You're not entirely correct, however, either. Let me try and clarify. If we look at the various breeds of dogs as distinct populations, and the generic "dog" as a metapopulation, it's pretty easy to tell that the extremes (great dane and chihuahua) are clearly connected by a chain of intermediate populations. If we lined up all these dog populations in order from smallest to largest, there is no question that adjacent populations would be able to freely interbreed (all other things being equal). Hence, there is gene flow from one end of the chain to the other. IOW great danes have chihuahua genes and vice versa, even though they likely couldn't mate directly. To borrow a creationist expression, "they're all still dogs".However, and here's where it gets a bit sticky, when the degree of gene flow between two populations becomes "insignificant", they can justifiably be claimed to represent distinct species if the divergence between them is great enough. The trick, of course, is to determine when "significant" becomes "insignificant" (i.e., where to draw the line). As with so much of nature, the problem really defies crystal clear definition. If all our populations of the metapopulation "dog" were bent around in a huge circle where the great danes ended up standing next to chihuahuas, and given the rather obvious pre-zygotic barrier between them, any "splitter" taxonomist encountering them would be justified in claiming they were distinct species. Even if our taxonomist could trace the relationship through all our intermediate dog populations, s/he could still argue the case that the degree of gene flow between our extreme populations was "insignificant", and hence still claim distinct species. A purist "lumper", OTOH, would counter that unless there was a break in the chain (i.e., a disruption of the gene flow between the populations through extinction of an intermediate form or population or whatever), there would be no justification for proclaiming great danes and chihuahuas different species. A hypothetical paleontologist from the far future comparing the fossils of both would definitely conclude that s/he had related but "no doubt" distinct species (or possibly even genera).Just to make things even more complicated, there are examples where interruption of gene flow ISN'T required for speciation.Irwin, DE, 2002 "Phylogeographic breaks without geographic barriers to gene flow", Evolution, 56(12):2383—2394

quote:

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The spatial distribution of genetic markers can be useful both in estimating patterns of gene flow and in reconstructing biogeographic history, particularly when gene genealogies can be estimated. Genealogies based on nonrecombining genetic units such as mitochondrial and chloroplast DNA often consist of geographically separated clades that come into contact in narrow regions. Such phylogeographic breaks are usually assumed to be the result of long-term barriers to gene flow. Here I show that deep phylogeographic breaks can form within a continuously distributed species even when there are no barriers to gene flow. The likelihood of observing phylogeographic breaks increases as the average individual dispersal distance and population size decrease. Those molecular markers that are most likely to show evidence of real geographic barriers are also most likely to show phylogeographic breaks that formed without any barrier to gene flow. These results might provide an explanation as to why some species, such as the greenish warblers (Phylloscopus trochiloides), have phylogeographic breaks in mitochondrial or chloroplast DNA that do not coincide with sudden changes in other traits.

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Irwin has another article, on-line, that explains in great detail the issue of gene flow and ring species: Ring species as bridges between microevolution and speciation.Let me know if the above actually made any sense to anyone.

Not always. This was Mayr's (and others, although Mayr called it something else) original definition, and it's a pretty good one. However, the point of Irwin's article, for example, was that reproductive isolation (phylogeographical breaks - a really stupid term IMO) between populations can arise that AREN'T relatable to interrupted gene flow - i.e., no physical geographic separation. We also see this in species flocks, and some of the other examples of what amounts to sympatric speciation. OTOH, the Ensatina complex DOES appear to have been caused by just that mechanism.

Sorry, moose. Won't happen again (yeah, right. )

Hi Para,I don't want to seem like I'm piling on you with everyone else (although I really am ). Brad has been around evcforum probably since near its inception. Both Moose and Percy claim - although I can find little support for it - that they can occasionally discern valid, highly erudite bits in Brad's posts. Being that they are the forum equivalent of supreme beings, far be it from me to question that assertion. OTOH, he rarely derails active threads - most of the time he lights off one post, then disappears from the thread if no one responds. Often, the threads in question are what could be described as "dead" or long-ended discussions that he resurrects, again, for one post if no response is forthcoming. My observation here is that Brad-baiting is actively discouraged by the Los Jefes Supremos. If you want to beat up someone only slight less incoherent, you are more than welcome to jump in with Syamasu on any given thread. Doesn't even matter if you have no knowledge of the subject matter - neither does he...If it helps, think of Brad as a sort of forum mascot. Or as the kind of harmless, a bit eccentric uncle that no one in the family ever talks about in polite company. Puts it into perspective. No disrespect intended, Brad.FWIW, I think Crash was a bit harsh. You had no way of knowing. Almost all of us have responded to Brad at one point or another. You're just lucky he didn't answer you with one of his famous 5,000 word essays.