Comparing blueprints

i) A number of "evangelicals" or post-evanglicals are bailing on the historicity of Adam. The tipping-point is said to be genetic evidence for evolution. Supposedly, that cinches the argument for human evolution. 

One of the oddities about this appeal is that long before comparative genomics came on the scene, both naturalistic and theistic evolutionists assured us that the fossil record and comparative anatomy established evolution beyond reasonable dispute.

Now, however, we're told that it's really genetics which supplies the decisive evidence. Does that mean the fossil evidence and anatomical evidence was oversold? Were they stalling for time until better evidence came along?

ii) I'd like to discuss one alleged line of genetic evidence for human evolution. And that's how much DNA we share with the great apes, especially chimpanzees.

From what I've read, that's puzzling on its own grounds. What's puzzling to scientists is how chimps and humans can be so alike at the genotype level, but so unalike at the phenotype level. The more DNA we have in common, the harder it is to explain our radical dissimilarities (e.g. behavior). 

iii) However, let's assume, for the sake of argument, that genetics directs morphology.  

Say humans and great apes have similar faces. Say we discover that this is based on similar DNA. 

Does the genetic basis add anything to the alleged evidence for common descent? I don't see how.

If we think physical appearance is due to genetics, then we'd expect similar appearance to be due to similar genes–wouldn't we? If the DNA code programs for the expression of particular traits, then a common effect would be the result of a common cause. The morphology is encoded in the DNA. Am I missing something?

If so, then I don't see how that contributes any additional evidence to the alleged evidence for common descent based on comparative anatomy–for surely the operating assumption has always been that comparable anatomy had an underlying genetic cause. It was never a question of discovering whether there was such a correlation, but discovering what it was. 

Compare two sedans of the same year, make, and model–only one is a 2-door sedan while the other is a 4-door sedan. They are quite similar except for the number of doors, and certain correspondence adjustments. 

Suppose we discovered the blueprints for the 2-door and 4-door models. We'd see that this difference goes back to the original blueprints.

But would that really add any new evidence to our comparison? Didn't we assume all along that one model had two doors while the other model had four doors because they were built according to respective blueprints which specified that difference? The same comparison operates at a different level, that's all. You're comparing blueprints rather than cars. Blueprints of cars. 

If comparative anatomy is evidence for common descent, then comparative genomics doesn't really add anything to the evidence, because we already knew that the genotype dictated the phenotype. We just didn't know the details of how that happens. 

Conversely, if comparative anatomy is explicable apart from common descent (e.g. common design, convergent evolution), then comparative genomics would simply underwrite the alternative explanation.  

iv) Admittedly, there are other, more specific genetic arguments that are deployed against the historicity of Adam. There's the claim that one breeding pair can't produce that much diversity. There's the claim that a chromosome 2 fusion site between chimps and humans indicates a common ancestor. 

I've discussed that before, as well as linking to other discussions.