When I first read about the "hypothesis" of Intelligent Design, I figured it would go away soon enough. Clearly, I underestimated the degree to which a vocal segment of society will prioritize their personal beliefs in a Designer over all the evidence that suggests there is no design in nature (and yes, I know people talk about natural "designs" all the time in a casual way, just like we use the term natural "selection," when both are mere metaphors that save a lot of verbiage). Advocates of "ID" are constantly claiming it is science (it helps justify putting it in a public school science classroom), so I'm going to take the intelligent design argument seriously enough to put it in scientific terms. What would a theory of Intelligent Design look like, and what testable predictions would it make?
1. If natural structures are the result of a design process rather than undirected evolution, they should be consistently optimal. An entity intelligent enough to design any of the complex creatures we see around us (and powerful enough to put those designs into use) would be expected to do a good job, and the designs should be really good. As a corollary, we shouldn't see a lot of designs that mere humans like us could easily improve.
2. Good designs should not be limited to one taxonomic group. If it's a good design - one that solves a problem well - it should be widely used.
3. We shouldn't see a lot of history in an intelligently designed world. There's no reason a designer would need to provide continuity between the organisms it designed, especially if the intermediate forms between current organisms (what we call common ancestors in evolution) did not perform as well as the ones we see today. (If you like, you could call this prediction a more general version of #2).
4. An intelligent designer would try to make its different designs compatible. In other words, an designer with any intelligence would never design parasites. Why would such a creative and intelligent entity go to all the trouble to design an fancy multicellular creature, then toss in some single-celled bacteria (or even viruses) that could take that creature down?
Now I'm sure some of the folks on the ID side of things will find fault with these predictions; they may say that I'm limiting the mystery of how a Designer would have worked, or its purpose in creating living things, etc. But I'm trying to do science here, and if ID is to be a scientific hypothesis, it has to have specific testable predictions, just like all the great scientific explanations.
So how well does the evidence from nature match these predictions? Not very well. Starting with prediction #1 (designs should be consistently optimal), there's lots of non-optimal design out there. Using our own bodies as an example, humans have this terrible design in our throat, where we cross swallowed food with inhaled air and occasionally choke to death because of it. We also have chronic back difficulties from standing on two legs with a spine that works just fine for animals on four. Childbirth in humans is much more difficult than other mammals, because moms push our babies' big, brainy heads through a small opening in the pelvis (caesarian sections work in difficult deliveries because removing a baby through the big abdominal opening is far easier than the natural route for birth). And finally, anyone who has studied the hormonal mechanism for regulating blood pressure ends up with a big facepalm; the kidneys respond to an decrease in blood pressure by modifying a hormone made by the liver, which is then modified again in the lungs and kidneys, which then both goes back to the adrenal glands to make another hormone which tells the kidneys to increase salt retention in the urine, while all the while the third version of the liver hormone goes to the brain, which makes another hormone to tell the kidneys to absorb more water from the urine. In simpler terms, the kidneys use the liver, lungs, brain, and adrenal glands as intermediaries in the process of the kidney telling itself to absorb more salt and water from the urine. NOBODY would call that a good design. In each case, I'd say we humble humans could have done much better. And those are just a few examples from one species (us).
So how about prediction #2 (good designs should be used all over the place)? There are a lot of species out there that could really benefit from some traits from other groups. Gliding snakes and squirrels are pretty cool for snakes and squirrels,
but they are awful compared to true fliers like birds or bats (feathers would be helpful). We mammals would be much better at long aerobic efforts if we had the flow-through lungs that birds do. Those birds would benefit from better recognition of their chicks than just "it hatched in my nest" (you did know that cuckoos are nest parasites, right)? And there are the marine mammals, which do remarkably well given the fact that they have to routinely return to the ocean's surface to breathe ("I could sure use some gills, Shamu!"). Examples of good "designs" limited to one group of organisms abound here.
Now for prediction #3 (we should not see history). I'll just come out and say it: history pervades the record of life on Earth. Extinction has eliminated most of the intermediate ancestors between the species we see today, but we know they were there from fossils. It seems very strange that a designer would respect the ancestry of its various groups so carefully ("no nursing for you - you're a bird!" or "yes, whale, I know you have fins, but they still have to have all the fingers inside"). It also seems strange that a designer would put out an Archaeopteryx - a stepping-stone species between dinosaurs and birds that could never compete against a modern bird, but could make it without any true birds around yet. Why design a cobbled-together intermediate when you could just do real birds?
I can imagine someone arguing that a designer was learning as it went, and that the various groups of organisms were somehow the "rough drafts" along the way (no doubt arguing that we humans are the polished final draft). But that means that this intelligent designer didn't just publish the best designs, but all the "rough drafts." Does that make any sense?
And finally, prediction #4 (different designs should be compatible). I'll leave predators and prey aside, and just point out that there are roughly four species of parasite on earth for every non-parasite species (and plants may justly feel that most of those animals are also parasites as well - at least the ones that aren't helping them with pollination). While parasites certainly have ecological roles and big effects, it's hard to see those roles as necessary ones. Ultimately, why would any intelligent designer handicap its designs with so many effective parasites?
So there you have it. Some realistic and rational predictions from intelligent design, none of which match the actual world we have. I'm sure that ID advocates will argue that I'm setting up a straw man, and trot out their own prediction that they see "complex specified information" or "irreducible complexity" as predictions of the model. You should realize, however, that ID is not the only theory that predicts "CSI" (evolution would do the same), so it hardly qualifies as a prediction that discriminates ID from other explanations. As for "irreducible complexity," every time the ID folks argue something is "irreducibly complex," patient biologists point out the "reducibility" to simpler forms, at which point the ID movement abandons the case and picks out something else. Camera eyes, bacterial flagellae...it's like a big game of evidentiary whack-a-mole. I'm just going to ask you to judge my predictions on their own merits, and remind you that there's this other theory that would predict that organisms would often be bolted together in funny ways, that ancestry would be central to what features those organisms would have, and that struggling to survive would put organisms at cross-purposes all the time. I bet you've even heard of it...