1. Genetic load
4. Pseudogenes and broken genes are junk
Parrington discusses pseudogenes at several places in the book. For example, he mentions on page 72 that both Richard Dawkins and Ken Miller have used the existence of pseudogenes as an argument against intelligent design. But, as usual, he immediately alerts his readers to other possible explanations ...
However, using the uselessness of so much of the genome for such a purpose is also risky, for what if the so-called junk DNA turns out to have an important function, but one that hasn't yet been identified.This is a really silly argument. We know what genes look like and we know what broken genes look like. There are about 20,000 former protein-coding pseudogenes in the human genome. Some of them arose recently following a gene duplication or insertion of a cDNA copy. Some of them are ancient and similar pseudogenes are found at the same locations in other species. They accumulate mutations at a rate consistent with neutral theory and random genetic drift. (This is a demonstrated fact.)
It's ridiculous to suggest that a significant proportion of those pseudogenes might have an unknown important function. That doesn't rule out a few exceptions but, as a general rule, if it looks like a broken gene and acts like a broken gene, then chances are pretty high that it's a broken gene.
As usual, Parrington doesn't address the big picture. Instead he resorts to the standard ploy of junk DNA proponents by emphasizing the exceptions. He devotes more that two full pages (pages 143-144) to evidence that some pseudogenes have acquired a secondary function.
The potential pitfalls of writing off elements in the genome as useless or parasitical has been demonstrated by a recent consideration of the role of pseudgogenes. ... recent studies are forcing a reappraisal of the functional role of these 'duds."Do you think his readers understand that even if every single broken gene acquired a new function that would still only account for less than 2% of the genome?
There's a whole chapter dedicated to "The Jumping Genes" (Chapter 8). Parrington notes that 45% of our genome is composed of transposons (page 119). What are they doing in our genome? They could just be parasites (selfish DNA), which he equates with junk. However, Parrrington prefers the idea that they serve as sources of new regulatory elements and they are important in controlling responses to environmental pressures. They are also important in evolution.
As usual, there's no discussion about what fraction of the genome is functional in this way but the reader is left with the impression that most of that 45% may not be junk or parasites.
Most Sandwalk readers know that almost all of the transposon-related sequences are bits and pieces of transposons that haven't bee active for millions of years. They are pseudogenes. They look like broken transposon genes, they act like broken genes, and they evolve like broken transposons. It's safe to assume that's what they are. This is junk DNA and it makes up almost half of our genome.
John Parrington never mentions this nasty little fact. He leaves his readers with the impression that 45% of our genome consists of active transposons jumping around in our genome. I assume that this is what he believes to be true. He has not read the literature.
Chapter 9 is about epigenetics. (You knew it was coming, didn't you?) Apparently, epigentic changes can make the genome more amenable to transposition. This opens up possible functional roles for transposons.
As we've seen, stress may enhance transposition and, intriguingly, this seems to be linked to changes in the chromatin state of the genome, which permits repressed transposons to become active. It would be very interesting if such a mechanism constituted a way for the environment to make a lasting, genetic mark. This would be in line with recent suggestions that an important mechanism of evolution is 'genome resetting'—the periodic reorganization of the genome by newly mobile DNA elements, which establishes new genetic programs in embryo development. New evidence suggests that such a mechanism may be a key route whereby new species arise, and may have played an important role in the evolution of humans from apes. This is very different from the traditional view of evolution being driven by the gradual accumulation of mutations.It was at this point, on page 139, that I realized I was dealing with a scientist who was in way over his head.
Parrington returns to this argument several times in his book. For example, in Chapter 10 ("Code, Non-code, Garbage, and Junk") he says ....
These sequences [transpsons] are assumed to be useless, and therefore their rate of mutation is taken to taken to represent a 'neutral' reference; however, as John Mattick and his colleague Marcel Dinger of the Garvan Institute have pointed out, a flaw in such reasoning is 'the questionable proposition that transposable elements, which provide the major source of evolutionary plasticity and novelty, are largely non-functional. In fact, as we saw in Chapter 8, there is increasing evidence that while transposons may start off as molecular parasites, they can also play a role in the creation of new regulatory elements, non-coding RNAs, and other such important functional components of the genome. It is this that has led John Stamatoyannopoulos to conclude that 'far from being an evolutionary dustbin, transposable elements appear to be active and lively members of the genomic regulatory community, deserving of the same level of scrutiny applied to other genic or regulatory features. In fact, the emerging role for transposition in creating new regulatory mechanisms in the genome challenges the very idea that we can divide the genome into 'useful' and 'junk' coomponents.Keep in mind that active transposons represent only a tiny percentage of the human genome. About 50% of the genome consists of transposon flotsam and jetsam—bits and pieces of broken transposons. It looks like junk to me.
Why do all opponents of junk DNA argue this way without putting their cards on the table? Why don't they give us numbers? How much of the genome consists of transposon sequences that have a biological function? Is it 50%, 20%, 5%?