OK, I’m going to retract what I just said, after thinking about this more carefully.
On the one hand, it seems to me that Eric is arguing that common ancestry requires intelligence, which is weird. (I guess this is Josh’s main complaint with ID theorists in the first place - assuming that all MI is a signature of intelligence.)
On the other hand, Josh says this:
Yes, common ancestry explains the high initial amount of MI. But, it is unclear to me what are the relative increases/decreases of (C1 \cap C2) and (C1 \cap C2 \cap G). As I said in my initial long post in this thread, this is crucial in determining whether (C1 \cap C2 \setminus G) can actually increase - you need the second term to decrease faster than the first.
In other words, you have not explained how the circles move relative to each other. There is something in here which is not captured by the Venn diagrams - something more than just common ancestry. Common selection pressure might be. Can you clarify this, Josh?