I was having a discussion with someone about using genetic similarity as evidence for relatedness and this issue came up. It was suggested that genetic similarity is also compatible with ID as well as common descent so it is ambiguous as to relatedness. I agreed that it is indeed compatible with both ideas. However, I suggested that common descent predicted similarity while ID did not. Here is the logic I used:
If you compared two species with the following genes (letters represent individual genes):
A B C D E F G H I J K L - house mouse A B C D E F G M I J K L - white rat
You could conclude that the two species are probably related since they only differ by one gene and this conclusion would indeed be compatible with either common descent or ID.
However, if the 2 species had the following genetic codes:
A B C D E F G H I J K L - house mouse B D G M J L C A E I F K - white rat
Based on common descent you could conclude the two species were not related because common descent requires that the genetic sequence be similar. This is the very thing that is happening in the plant kingdom right now. Researchers are finding that plants that were once thought to be closely related cannot possibly be because there is too much difference in the genetic code. They are more similar to species in other genera.
You could not draw this conclusion with ID since the designer could have made these similar looking creatures with different genetic sequences (note that the same genes are all there, only the sequence is different). There is no requirement of ID that sequences be conserved. It would seem logical that a designer would make similar creatures with similar genetic codes, and if I were the designer, I would use modular components. But there is nothing that constrains the designer to do so. In fact, modular construction is really a very recent idea - within the last 100 or so years. Before that everything was made one at a time and each piece was unique.
So, genetic similarity may be compatible with both ID and common descent, but only common descent predicts genetic similarity.
Is this a logical fallacy? Do you agree there is a difference between evidence being compatible and evidence being predicted in the way I described above? Why or why not?
A follow up question is this: What other predictions does ID make or at least claim to make and how do they compare to the predictions that common descent makes?
(Intelligent Design please)
Whoever calls me ignorant shares my own opinion. Sorrowfully and tacitly I recognize my ignorance, when I consider how much I lack of what my mind in its craving for knowledge is sighing for. But until the end of the present exile has come and terminated this our imperfection by which "we know in part," I console myself with the consideration that this belongs to our common nature. - Francesco Petrarca