What Does Common Descent Explain?A scientific claim needs to make testable predictions, but it also needs to be useful, to help explain the world we see.
The Distribution of EyesThere are various sorts of eyes in the natural world, such as the compound eye of the insect, the camera eye of vertebrates eye and the alternative camera eye of octopi and squids. According to common descent, the eyes that a creature gets depends on their ancestry. If you are descended from the first organism to evolve a rudimentary compound eye, you get a compound eye. Thus, while fish and squid live in the same environments, fish have retinas wired one way, squid have retina wired the other. One is, presumably, better in that environment, but who gets what is determined by their evolutionary history, not their current needs.
Creationists sometimes say that the designer equipped each "kind" with the eye most suited to its environment. The squid, the shark and the whale "kinds" need eyes that work well in water, so we would predict them to have the same eyes. Advocates of common design might argue that one type of eye is advantages in a more specific niche, perhaps for predators at great depths. That is a reasonable argument, as long as they follow it though. The creationist scientist should be able to state what niches a certain eye type is best suited for; not just the two camera eyes, but the compound eye and all other eyes. He should be able to give some reasoning as to why the eye works best in that niche. And then he can show how the "kinds" in that niche (or at least originally in that niche when created) have the optimum eyes.
As far as I am aware, no one has ever attempted that. Why not? Because creationist scientists know that creation science fails the prediction.
Vitamin C PseudogeneMost mammals can synthesise their own vitamin C, so cats, for instance, survive very well on a diet that excludes fruit. This is not true of humans, and two centuries ago sailors would regularly die a very painful death of scurvy (over half the crew might die on a long voyage). Curiously, humans do have the gene that codes for vitamin C synthesis, but it is broken (it is therefore described as a pseudogene). This same pseudogene has been found in chimpanzees, orangutans, and macaques, as well as guinea pigs. The common descent explanation is that a common ancestor of humans, chimpanzees, orangutans, and macaques had a mutation that destroyed the functionality of the gene, but as this ancestor ate so much fruit, it made no odds to the ancestor, and spread through the species. Thus today, all four species have the same error in the pseudogene (and this happened independantly for the guinea pig).
How does creationism explain this?
VestigialismCertain features can be identified as having a primary use in one species and not in another. The wings of an ostrich are useless for flying, but are clearly still wings. Sure, they have some other purpose, but not a purpose you need wings for. The cassowary has virtually nothing in the ways of wings, not enough to be useful for anything - but there is something there still. Some cave dwelling animals have the vestiges of eyes, but skin grows over them, preventing light, should there be any, from actually getting to the eye.
Common descent says that in the ancestor, these were fully functional features - the ancestor of the ostrich could fly, the ancestors of the cave dwellers lived on the surface, and found the eyes to be useful. The modern organism lives in an environment or manner that makes the feature useless, but it still remains.
Occasionally humans are born with tails, whales and snakes with legs. Common descent says this is a throw back to our respective ancestors. If humans were created without tails, where do these tails come from?
More..There is plenty more evidence, and much of it is presented on Talk.Origins. The interested reader may like to read this exchange between Talk.Origins and True.Origins:
Here are other interesting sites: