In a recent article entitled On the immortality of television sets: "function" in the human genome according to the evolution-free gospel of ENCODE, Graur et al. dismantle ENCODEs evidence and conclusion that 80% of the human genome is functional. However, the article by Graur et al. contains assumptions and statements that are questionable. Primarily, the authors limit their evaluation of DNAs biological functions to informational roles, sidestepping putative non-informational functions. Here, I bring forward an old hypothesis on the evolution of genome size and on the role of so called junk DNA (jDNA), which might explain C-value enigma. According to this hypothesis, the jDNA functions as a defense mechanism against insertion mutagenesis by endogenous and exogenous inserting elements such as retroviruses, thereby protecting informational DNA sequences from inactivation or alteration of their expression. Notably, this model couples the mechanisms and the selective forces responsible for the origin of jDNA with its putative protective biological function, which represents a classic case of fighting fire with fire. One of the key tenets of this theory is that in humans and many other species, jDNAs serves as a protective mechanism against insertional oncogenic transformation. As an adaptive defense mechanism, the amount of protective DNA varies from one species to another based on the rate of its origin, insertional mutagenesis activity, and evolutionary constraints on genome size.It's not a good idea to attack someone who; (a) is an expert in the field, (b) is intelligent and outspoken, and (c) has a blog. But that never stopped Claudiu Bandea before so why should it now?
Here's part of how Dan Graur responds at: A Pre-Refuted Hypothesis on the Subject of “Junk DNA”. There's more, read it all.
The first problem with this hypothesis is that big eukaryotic genomes consist mostly of very few active transposable elements and numerous dead transposable elements. So, big genomes seem to need little protection. Moreover, a positive correlation exists between genome size and number of transposable elements. In 2002, Margaret Kidwell published a paper entitled “Transposable elements and the evolution of genome size in eukaryotes.” In it, she showed that an approximately linear relationship exists between total transposable element DNA and genome size. Copy numbers per family of transposable elements were found to be low and globally constrained in small genomes, but to vary widely in large genomes. Thus, the major characteristic of large genomes is the absence of selective constraint on transposable element copy number.I can't count the number of people who have tried to explain to Claudiu Bandea that his idea is ridiculous. Hopefully, this last embarrassment will silence him.
Given that the vast majority of transposable elements are dead, the most parsimonious explanation is that the continuous accumulation of dead transposable elements is the reason for genomes becoming large. Let me spell it out: the “large” part in “large genomes” is made of transposable elements. Genome do not become large first and then protect genetic information by becoming sinks of transposable elements.
The other problem with the protection-from-mutation hypothesis is that it assumes selection on mutation to be effective. Selection on mutation is referred to in population genetics as second-order selection. The reason is that this type of selection is anticipatory. It protects against a possibility, not an actuality. Second-order selection on mutation (mutability) requires huge effective population sizes, so huge in fact that they are only found in a few bacteria and viruses. Unfortunately for the protection-from-mutation hypothesis, genome size is known to be inversely correlated with effective population size. In other words, huge genomes are found in species that have very small effective population sizes. So small, in fact, that even regular selection (first-order selection) is not very effective.
Thomas Huxley was proven right again: "The great tragedy of Science is the slaying of a beautiful hypothesis by an ugly fact." Several ugly facts in this case.
Naturally, the Intelligent Design Creationists are all over it [Another response to Darwin’s followers’ attack on the “not-much-junk-DNA” ENCODE findings].