A thoughtful poster on FRDB posted the following:
It seems to me, as an interested layman (I have a B.S. but it's in computer science), that the most interesting hypothesis of intelligent design is: "Things that have a common designer will also have a common design." This is interesting to me because we should actually be able to make predictions about actual things in nature based on it, and test them against what we find. I've read it many times in ID and creationist discussions, so I don't think it should be controversial. What I want to do is set up a few scenarios where it should be able to make predictions, and see what predictions it will actually make.
So let's start with something pretty much everybody does: breathing. You inhale, you exhale. Lungs, it turns out, are a pretty remarkable medium of transmitting oxygen from the air around us to the bloodstream. And lungs are shared by pretty much all terrestrial vertebrates that I'm aware of. We also know that fish, for the most part, don't have lungs. They have gills, which are efficient machines for extracting oxygen from the water. Myriad kinds of fish and aquatic animals have gills to breathe underwater. So a design hypothesis would be: terrestrial vertebrates have commonly designed lungs, and aquatic vertebrates have commonly designed gills.
But then...you've got troublemakers. Like the whales and dolphins. These are animals that are strictly aquatic - they have no ability to go about and feed themselves on land. Yet they have lungs, just like the terrestrial mammals. This is such a wacky design for an aquatic creature that whales actually have to think about when to breathe, unlike you or I. Or fish, for that matter. Whales are magnificent creatures, and great predators in many cases, but we're talking about common design - unless it's stretched so far as to be meaningless, it makes no sense to me to say that common design should include using the land design out in the ocean. From a standpoint of "common design" it would be like creating a submarine with an engine designed like a car's, where it periodically has to surface, uncover the exhaust manifold and let some air in. This is unacceptable from a design standpoint - yet, if submarines were animals that had evolved by natural selection from cars, we might expect to see it if an air-based cooling system was powerfully encoded in their genetics.
There is much convergent evolution between the poster's thoughts and mine. I've noticed that bat and bird wings have adapted the same five digit hands in different ways to create a wing. How could common design explain that? Michael Shermer noticed long ago that fish swim by moving their tails from side to side, while whales and dolphins swim by moving their tails up and down, as land mammals do. How does common design explain this? It can't, unless the concept of "common design" is stretched so far as to be meaningless, as the poster said.