One of the densest Creationist tropes has to be ‘Common Design’. It is proposed as a direct competitor to Common Descent – template mediated copying of DNA – as an explanation for the high sequence similarity of two DNA segments. But what is actually held in common? If we look at a particular transposon sequence, and find it is in A and B but not C, and another that is in A but not B, etc, we can generally organise a set of such markers into a ‘tree’ structure, much as would be predicted by Common Descent. But no, we are assured that these apparent markers are in fact part of the ‘design’. If A is a whale, B a pig and C a deer, there is something that is vital for the function of both whale and pig but is definitely not required in deer. Instead, a sequence which, in whale and pig, sits either side of the insertion, runs uninterrupted in the deer. That, too, is functional, supposedly, even though the insert would give a product which was the A/B one with a gap and possibly a frameshift, if it were transcribed.
But this is held to be the case even if the sequence, with and without transposon, is never transcribed. A sequence that does nothing, and organises hierarchically exactly as would be expected of common descent, is nonetheless functional … because?
On observation, there must be some genome pairs that are highly similar because they are commonly descended. We can see it happening. But there are, on this notion, supposed to be identically-patterened runs of similarity that are not due to common descent, but instead result from a completely different cause – some entity bolting together genomes, or parts thereof, from scratch, I guess, and choosing to repeat a known pattern – up to a point – in a manner that fools our most adept molecular taxonomists into seeing descent.
There must be a line in a taxonomy where the one shades into the other – on one side, sequence commonality is all Common Descent; on the other, Common Design. Where does this discontinuity reside? Species, genus, family, order? Is it a gradual transition, gene by gene, or all at once? How could you tell? Why does it not show up in computer analyses of blind datasets?
If I were to provide 3 genomes shorn of all differences, it would be impossible to tell which were commonly descended and which commonly designed from the data. But there must logically be a transition of causes, were I to take the genomes from sufficiently distant species and this idea were true. What persuades us to adopt this causal explanation in preference to that which explains the pattern better: Common Descent?