As I mentioned before, I do have great reservations about invoking Common Design (CD) to explain similar biological features. Yet, I also noted that “I simply cannot subscribe to the notion that a designer would always reinvent the wheel every time a machine is invented.” Previously, I used the example of G-proteins to make this very point. I commented:

When you understand the potential utility of something like the G protein, you would have to ask why any designer would use it in only one situation? Clearly, such switches could be useful in a variety of situations and a variety of contexts, controlling the maze of complex interactions that exists within a cell. In other words, an understanding of the G protein’s role inside the cell leads us to predict common design from a teleological perspective. If the cell was designed, it would make no sense to design a completely novel switch every single time a switch is used. Instead, you would seek to use the same basic switch wherever it would function well. And this is exactly what we see in the cell, where G proteins are extensively used to control such things as protein synthesis, signal transduction, vesicle trafficking, cell division, and the reorganization of the cytoskeleton. A variety of different types of G proteins, along with different GEFs and GAPs, are plugged into a variety of circuits.

When you consider all the ways G-proteins are used to regulate activity in the cell, it seems rather odd to insist that an intelligent designer would have picked only one process and felt obligated to come up with a completely novel switch for every other process. For example, for which of the myriad of processes would we deploy the G-protein and why this one and none of the others?

Yet I should clarify that I do not propose Common Design as a true explanation or conclusion; I propose it merely as a possibility that is worthy of further consideration. If design theorists are to invoke Common Design, it is incumbent upon them to begin the process of developing the concept in a more rigorous fashion. As I mentioned before, this explanation has history of being used in a purely ad hoc manner, where any similarity can arbitrarily be explained as “common design.” Also, the arbitrary use of this concept prevents it from becoming part of a positive research program.

In my essay on G-proteins, I note that we need some form of criteria that would be used to tentatively score something as ‘common design.’ Let me repeat two possible criteria and add a third:

1) CD must exist in an overall design that is modular, such the common design can be viewed as the ‘same solution’ that is plugged into a ‘different context.’ [I would also add that that such modularity is detected by individual components interacting through a distinct interface.]

2) The “solution” should be seen in many different contexts. For example, if the putative CD is seen in only two situations, then the objection about designing different solutions carries more weight.

3) The CD must be explainable according a rational design. That is, there should be a good engineering/programming-type explanation (that may or may not include evolution) to prevent us from invoking CD in an arbitrary and ad hoc manner.

In light of these criteria, I would suggest that the molecular threaders and molecular switches do meet them and thus the explanation of CD is plausible and non-arbitrary. This is not to say that all G-proteins, for example, are related solely through common design. For example, I originally noted that “a designer might be expected to select something like the G protein not only because it could be plugged into many contexts, but because it contained a certain level of evolvability, where it was wells-suited to co-evolve with the developing circuit..” Thus, a smaller set of G-proteins may have been used in the orginal design, only to be expanded through gene duplication and exploited by evolution.

Not all propositions of common design satisfy these criteria. For example, consider the comparison between humans and chimpanzees. One could explain the shared anatomy with common design, but the shared anatomical features are not modular units that are pieced together; they are the emergent properties of a developmental routine, where all body “parts” are seamlessly fit together to form a functioning whole. This would mean the “common design” is the entire body plan itself rather than a modular unit.

Furthermore, what is the engineering/programming-type explanation for the shared body plans? With G-proteins, for example, we can propose that they have been re-used in situations that call for a molecular switch. This is a respectable engineering-type hypothesis. But what would the engineering-type explanation for the shared body plan of the chimp and human?

Thus, by invoking these criteria as a guide, we can take the first steps in trying to formulate the hypothesis of common design in a non-arbitrary fashion.