HELEN
Jeff, I can read your impatience with my reply in your response, and I think if you think about it you will understand what I was referring to.
We have bred, in the last few hundred years, an enormous variety of dogs. And yet, with all the breeding, intentional and mutt-caused, there has never been any variation away from canine. Same with horses and equine, cattle and bovine, cats and feline, etc. That is what I was referring to. We can breed for variation but the basic kind stays.
Then you asked an interesting question about whether or not variation can be exhausted. I think so, yes. I cannot tell you how many generations it would take (and it might well vary significantly with different species), but we see this happening in the wild as well as in breeding programs. There comes a time when it seems the ?over-speciation? problem emerges and a dead end is reached. This is often what we seem to be seeing in endangered species. The one I refer to the most often here is the spotted owl of the old-growth forests of the Pacific Northwest. It has become so specifically adapted to that one environment that it is not producing any offspring which have enough variation to live outside of that environment. This would bring up the question in my mind as to the real effect of natural selection ? perhaps what it deletes from the population?s genome really cannot be replaced or substituted.
In domestic breeding we can see the example in dogs quite easily. There comes a time in a breeding program where inbreeding has finished producing what it can in terms of positive variations and what we get is, instead, things like hip dysplasia in German shepherds, deafness in Dalmatians, etc.
Then you asked if, when this limit seems to be reached, we see only genetic duplicates. Not that I have seen, no, simply because there is still the sexual recombining of genetic material in each generation so that small variations exist. Dalmatians, for instance, are more prized if both eyes are surrounded by black or liver colored spots. But look at what a minor variation this is! We are down to the placement of the coloring and not any real variation in the dog itself.
As long as there is sexual reproduction, however, I think the offspring will always show individual markings at the least. This is why we can identify zebras by their stripe patterns, the penguins of South Africa by their ?bib freckles? and so on. But the more speciated and inbred the population, the closer to clones the offspring seem to get, yes. What follows seems to be, from what we have seen, only harmful mutations and very minor variations in physical appearance, such as the placement of stripes or spots.
Finally, you asked if there was actual evidence that small genetic changes could not accumulate to large phenotype changes, thus changing one sort of thing, through large amounts of time, into another.
Genetically, we have no indication at all that this can actually happen. What we see in bacteria and other one-celled asexually reproducing organisms is primarily variation through mutations at ?hot spots.? These hot spots, as they are called, show more rapid rates of mutation than other areas of the genome. However the mutations tend to go back and forth, and are not linear. Nor do they build on one another to produce anything new. You mentioned thousands of generations. We have been working with E.coli for over a hundred years now. That amounts to more than 2 ½ million generations linearly (not counting ?cousins? in a horizontal counting), and there has been NO building of mutations one upon another to produce anything even slightly departing from what we know and identify as E.coli. Now, if we can?t do this in that many generations, how on earth are you going to get from fish to man when the generation times are greater ? up to fourteen or fifteen years in the case of man and probably not a lot less in terms of his supposed ancestors!?
In multicellular animals we also do not see mutations building on one another to produce anything at all. So it is not a matter of needing to prove that it can?t happen as simply showing that this scenario of mutations building on one another has never been seen to happen even with 20 minute generation span prokaryotes! There is NO empirical evidence for it happening at all. So I would have to ask YOU to show me where it has been shown to happen empirically (in other words, not as a matter of ?interpretation? of the fossil record.).
In the meantime, please keep in mind that a simple mutation is the least of what is needed for any sort of decent change that is not simply the crippling of some genetic expression that already exists. The mutation must be integrated successfully into an already functioning genome, the correct timing mechanisms for expression must be in place, the cell must know what to do with the new protein, and there must exist supporting structures of some kind for that protein to integrate into.
Mutations can, singly, cripple enormous numbers of genetic expressions if they are in the more sensitive places. That is because it is quite rare for one gene to determine one trait (I am not aware of it happening at all, but that does not mean it doesn?t). Traits are normally determined by an interaction among various genes and the timing involved. So while you can cripple or change a trait due to one gene change, you cannot supply a new trait from just one gene change. It is far, far more complicated than that. However, evolutionarily, it is necessary that any individual changes be maintained in a population, either by being unexpressed and somehow surviving sexual recombining, or by being favourably expressed and thus being selected for.
Quite simply, it is a process without any empirical genetic evidence that it is possible.
Yet evolution depends on it.
