A most excellent piece of work: Ryan Gregory is featured in this article on the ubiquity of junk DNA. Good explanations all around.
Darwin was certainly ignorant about genomes, as scientists would continue to be for decades after his death. But Gregory argues that genomes embody the very mix of adaptation and arbitrariness that Darwin had in mind. Over millions of years, the human genome has spontaneously gotten bigger, swelling with useless copies of genes and new transposable elements. Our ancestors tolerated all that extra baggage because it wasn’t actually all that heavy. It didn’t make them inordinately sick. Copying all that extra DNA didn’t require them to draw off energy required for other tasks. They couldn’t add an infinite amount of junk to the genome, but they could accept an awful lot. To subtract junk, meanwhile, would require swarms of proteins to chop out every single dead gene or transposable element — without chopping out an essential gene. A genome evolving away its junk would lose the race to sloppier genomes, which left more resources for fighting diseases or having children.
The blood-drenched slides that pack Gregory’s lab with their giant genomes only make sense, he argues, if we give up thinking about life as always evolving to perfection. To him, junk DNA isn’t a sign of evolution’s failure. It is, instead, evidence of its slow and slovenly triumph.
Exactly! If evolution works, this is what we expect, whereas a human genome that was 3 billion nucleotides worth of perfectly optimized functional DNA is what creationists want…and why they’re so adamant in denying the evidence.