How can we decide [says Behe] whether Darwinian natural selection can account for the amazing complexity that exists at the molecular level? Darwin himself set the standard when he acknowledged, "If it could be demonstrated that any complex organ existed which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down." Some systems seem very difficult to form by such successive modifications -- I call them irreducibly complex.That's really the crux of his argument. As an argument it's poor, and it sets up a false alternative: either Darwin or Behe's Creator. But we don't even need to point out the logical error he's committing, because as we see Behe fails even to get his argument off the ground:
Irreducibly complex systems appear very unlikely to be produced by numerous, successive, slight modifications of prior systems, [says Behe] because any precursor that was missing a crucial part could not function. Natural selection can only choose among systems that are already working, so the existence in nature of irreducibly complex biological systems poses a powerful challenge to Darwinian theory.Blah, blah, blah, blah, blah. H. Allen Orr responds to this nonsense rather too politely:
Behe's colossal mistake is that, in rejecting these possibilities, he concludes that no Darwinian solution remains. But one does. It is this: An irreducibly complex system can be built gradually by adding parts that, while initially just advantageous, become - because of later changes - essential. The logic is very simple. Some part (A) initially does some job (and not very well, perhaps). Another part (B) later gets added because it helps A. This new part isn't essential, it merely improves things. But later on, A (or something else) may change in such a way that B now becomes indispensable. This process continues as further parts get folded into the system. And at the end of the day, many parts may all be requiredOrr is too polite because Darwin himself explained this process with regard to the human eye. The eye, he conceded, might at first sight be considered too complex to have been formed by natural selection. However,
Science has proved Darwin right on this point as on every other. As James Watson explains, evolution is not a Theory, it is a Law. As the youngsters say on such matters, "Deal with it."if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real.
This is Part 2 of a three-part series concluding tomorrow. Part 1 is here. Part 3 is here.
[UPDATE: Links fixed. Please explore the links before commenting -- you will find many of your questions are already answered there.]