Common design

Common design is a creationist explanation for the many observed similarities between species, conventionally explained by common descent. This usage of the term originates with Jonathan Sarfati in "Argument: Common design points to common ancestry", chapter 6 of his book Refuting Evolution 2.

The claim is that similarities in anatomy and DNA sequences simply reflect the fact that the organisms had the same designer.

Responses

 * 1) This creationist argument is a misunderstanding of the evidence for evolution. That evidence is not about mere similarity, but rather a very specific pattern of similarity: the twin-nested hierarchy.  The vast majority of possible patterns of similarity would not be consistent with common descent.  For example, bats with feathers would be hard to explain via common descent since feathers evolved after the last common ancestor of birds and bats.
 * 2) Most anti-evolutionists believe the common designer is all-knowing and all-powerful. But consider that a great deal of the reason why common design in human creations exists is that humans are finite beings with finite abilities, finite resources, and finite time.  For this reason humans almost never create from "scratch" but modify previously-existing designs.  The omnipotent designer envisioned by most creationists could have created each species from scratch, using radically different design philosophies. In its attribution to the designer of the same limitations which affect human designers, this argument is a transparent case of self projection as god.
 * 3) In evolution, the theory of descent clearly requires a particular pattern of similarity. Intelligent design, however, has an omnipotent creator which is capable of anything, so there is no reason to assume that each organism would have to be very similar to others at all. In fact, if they were all specially created for some particular purpose, there's no reason any of them would be similar.
 * 4) The patterns of similarities and differences in question, particularly in genetics, appear to extend even to areas in which there are many different independent structures that would all do the same things or that do not actually do anything. For instance, many proteins can have different amino acid sequences without this affecting the way they fold and hence what they do.  However, species thought to be more closely related by common descent have sequences more like each other than they do with species that are more distantly related, which may have different sequences that do the same things.  A common designer would have no reason not to simply use the exact same sequence over and over, or at least would not have any reason to create a pattern of differences that belies a specific cladistic ancestry that just so happens to be consistent with all the other apparent patterns of ancestry found elsewhere.
 * 5) To take one example of close similarity and the claim of "common design": the human body is most similar to the bodies of chimps and other apes. If that similarity is due to "common design", then that can be either because (a) the laws of nature and the properties of the raw materials constrained the designer(s) in what they could produce; or (b) the designer(s) had common goals in mind. (Option (b) could suggest that, in order to act according to the goals of our designer(s), we ought to "act like apes".)
 * 6) This is an elaboration of point #4 but it's worth mentioning that Sarfati's argument of the shared proteins in nurse sharks and camels, as evidence of common design over ancestry, unwittingly falsifies common design. To elaborate, in the case of common ancestry we expect to see similar DNA sequences, such as the similarity in the chimp and human cytochrome c proteins. According to Creationists humans and chimpanzees are both different "Kinds" and would just assert common design as the explanation. Fair enough, but when we look at the DNA sequences in those shared by the shark and the camel, also two different "Kinds", we see different DNA sequences, as would be expected in the case of convergent evolution, but completely inconsistent with what pattern one would expect from a common designer. If common design were the case, we should either see identical DNA sequences in all proteins or difference sequences for each "Kind", not a pattern that shows common ancestry where evolution needs it and convergence where it would otherwise falsify common ancestry.

Fallacies contained in this claim

 * Unfalsifiability (everything fits a common designer)
 * Special pleading (evidence is ignored)
 * Begging the question (one must believe that organisms had a designer at all in order to quibble over whether they had the same designer)