Mung had asked me to do a thread on common descent and common design. So, anyway, I’ll get things started by stating my own thoughts on these ideas. I intend this to be an open discussion, but I also find having a starting idea tends to help get things started.
So, as I have maintained for the last decade, I believe there is no fundamental conflict between ID and common descent. That is, it is fully possible to hold to both at the same time.
In fact, I would say that ID *potentially solves* many problems that common descent would bring. For instance, if you have gaps that are unbridgeable by a traditional Darwinian mechanism, you could posit that there was a prior source of information that the organism used to bridge the gap.
That being said, I don’t myself hold to common descent. If so, how does one explain the commonalities of organisms? The common explanation of this is “common design”. That is, since the same designer was designing these organisms, then we should expect repeated motifs to be used to solve various problems, various design ideas presented with variation, etc. The question is, is there a way to distinguish common design and common descent?
Well, to start with, common descent can actually be a mechanism for implementing common design. Therefore, inferring common design doesn’t necessarily tell you much about common descent.
Additionally problematic is that common descent actually depends on having a specific model of the origin of life. To understand this, let us say that instead of being rare, life comes about rather easily, but ALWAYS in the SAME WAY (i.e., builds the same starting organism, which we’ll call X). Therefore, if you find two X’s, there is NO WAY to tell if they are related by ancestry, despite the fact that they are identical! Let us then say that X is likely to branch to Y and then Z. Again, we could easily have two Zs that share NO common ancestors!
As you can see, inferring common descent as a definite conclusion requires a number of auxiliary hypotheses. These may or may not be reasonable, but they are there. However, *rejecting* common descent can be done with fewer.
Rejecting common descent can be done in one of two ways that I can think of:
- Have a model that better fits the character space of organisms, or
- Show that certain gaps are unbridgeable genetically.
#1 I believe has been done (at least on an initial basis) by Winston Ewert, with his dependency graph model. #2 is harder because, as I mentioned, in theory there could be repositories of information that help you close the gap. However, you could show that #2 *requires* information to close the gap, and then look and see if any such information repositories are known. Obviously, we might find information repositories in the future, but the question always is about what can we do with the knowledge we do have without overly-worrying about the knowledge we don’t have.
Anyway, along these lines, I think that Stephen Meyer has done a fairly good job with #2 in Darwin’s Doubt, where he discusses the origin of the phyla.
In short, ID can work inside or outside a Common Descent framework. However, ID also allows more degrees of freedom in biological thought, and many in the ID movement feel that the evidence against common descent outweighs the evidence for it. That means that many of the similarities in organisms are the results of common design and not common descent, but there is no logical contradiction in asserting both.
Those are my thoughts – what are yours?