[AndrewC]

After all these years I still don't know what the evolutionary argument for the existence of junk DNA was supposed to have been.

Really, now? It seems to me the argument was that, necessarily, "junk" was a basic component of a "random" process. In fact, most of DNA should be a complete wasteland of unused and "decaying" parts.

[jennyp]

Really, now? It seems to me the argument was that, necessarily, "junk" was a basic component of a "random" process. In fact, most of DNA should be a complete wasteland of unused and "decaying" parts.

But junk DNA would be analogous to an unused organ. Since there's some nonzero cost to keeping & reproducing an unused DNA sequence, any deletion of a truly junk sequence should be slightly favored over time. Or maybe the energy cost of any single unused stretch is negligible, so its deletion wouldn't get selected out. But regardless, there's more to evolution than just saying "randomness happens" and leaving it at that.