Scientific American has published another short note on junk DNA [Jumping 'Junk' DNA May Fuel Mammalian Evolution]. RPM noticed that there was no reference to the actual study being quoted in the article so it wasn't possible to verify the accuracy of the reporting [Junk DNA in Scientific American]. I couldn't find it either when I looked last week but it has now appeared on the PNAS website [Thousands of human mobile element fragments undergo strong purifying selection near developmental genes]. RPM also complained about the over-use of the term "junk DNA" in the Scientific American Article. That's what I want to discuss.
The author of the Scientific American article, JR Minkle, has responded on the Scientific American website [The DNA Formerly Known as Junk]. Minkle is a science writer who has covered a lot of stories in many different fields. As far as I know Minkle has not written very much about biology before summarizing the work in the PNAS paper. There was a time when all the science in that journal was written by scientists who were experts in the field [The Demise of Scientific American]. Anyway, that's not the main point here. JR Minkle has listened to the critics and made a decision to avoid the term "junk DNA" from now on.
That's a bad decision. RPM never asked anyone to avoid the word "junk." He merely called for appropriate use. Ryan Gregory has serious doubts about the usefulness of the term as he explains in his excellent article A word about "junk DNA".. If you want to keep up with the discussion about junk DNA you need to read that article—but you don't need to agree with everything in it. :-)
Gregory has also commented on the Scientific American article by proposing a new term, Junctional DNA, to describe DNA that probably has a function but that function isn't known. According to him, this avoids the confusion between using "junk" DNA to describe DNA that we really know to be junk (pseudogenes) and DNA for which no function has been discovered so we assume it has none.
I think we don't need to go there. It's sufficient to remind people that lots of DNA outside of genes has a function and these functions have been known for decades. Thus, it is highly inappropriate to assume that all non-genic DNA is junk and no scientist should ever do this. Note that I'm avoiding the term "noncoding" DNA here. This is because to me the term "coding DNA" only refers to the coding region of a gene that encodes a protein. Thus, in my mind, there are many genes for RNAs that are not properly called coding regions so they would fall into the noncoding DNA category. Also, introns in eukaryotic genomes would be "noncoding DNA" as far as I'm concerned. I think that Ryan Gregory and others use the term "noncoding DNA" to refer to all DNA that's not part of a gene instead of all DNA that's not part of the coding region of a protein encoding gene. I'm not certain of this.
The importance of the term "junk DNA" is to highlight the fact that it has not evolved by natural selection. This is a point I made in one of my first blog postings way back in November [Bill Dembski Needs Help, Again] and again a few days later [The IDiots Don't Understand Junk DNA] [Two Kooks in a Pod].
This isn't original. Everyone knows that junk DNA poses a major threat to both Intelligent Design Creationism and adaptationism [Junk DNA Disproves Intelligent Design Creationism] [Evolution by Accident]. Read Gregory's article for the short concise version of this dispute. What it means is that junk DNA threatens the worldviews of both Dembski and Dawkins!
Science writers often get trapped into thinking like an adaptationist when it comes to junk DNA. Remember that according to the adaptationist worldview the existence of huge amounts of truly nonfunctional DNA in a genome must be a problem. It can't be explained if natural selection is a powerful driving force behind most of evolution. You can't propose that all minor changes in behavioral genes, for example, have been selected and then turn around and admit that 95% of the human genome is junk!
Adaptationists celebrate every discovery that some little bit of DNA has found a function. That's because in their heart of hearts they think that almost all of the junk DNA will eventually be found to have a function. This is one of the reasons why papers like the PNAS paper mentioned above get so much attention.
I want to keep the term "junk DNA" to refer to all functionless DNA. That includes DNA for which we have direct and indirect evidence of no function (pseudogenes, most of intron DNA, corrupted transposons etc.) and it also includes the rest of the DNA for which no function has currently been discovered and we think it's junk because it's not conserved (among other reasons). Junk DNA is not noncoding DNA and anyone who claims otherwise just doesn't know what they're talking about.
The term "junk DNA" forces people to think about the underlying causes of evolution. It makes them stop to appreciate the fact that modern organisms could have evolved with useless DNA in their genomes and the only way this could have happened is if there's a lot more to evolution than just natural selection and adaptation. It's a good term. It's an accurate term. It's a useful term. And it makes people think.
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