> But it remains true that there are large amounts of non-coding junk DNA which is under no selection pressure

For people who may not understand how we know this -- there are "conserved" sections of DNA which don't change much over time. Very similar in mice and humans for example, because it performs important regulatory work, and if it doesn't, the animal dies.

There are other large sections where it can disappear and nothing of consequence seems to happen. And we know that, because some people have micro-deletions or other variants in the region and they are completely benign.

We will eventually identify a better classification than "intron" and "exon" to sort through the "junk" from "critical junk" but we are really only starting to untangle the situation.