In my previous post, I noted that because of what we understand about the nature, origins, and cross-taxon quantitative diversity of the various sorts of non-genic DNA in large eukaryote genomes, the default assumption is that much or even most of it is not functional at the cell and organism levels. Thus, the burden of proof rests with authors who claim that a large fraction, or indeed most or all, of this DNA is functional for the organisms in which it occurs.
This should not be construed as claiming that all non-genic DNA is assumed to be non-functional. I have pointed out in various preceding posts that even those who postulated non-adaptive explanations for its existence did not rule out — and indeed, explicitly favoured — the notion that a significant portion would turn out to serve a function. You need not take my word for this, as it is not difficult to find unambiguous statements from the original authors themselves.
For example, here are Orgel and Crick (1980) who, along with Doolittle and Sapienza (1980), first proposed the concept of “selfish DNA” in detail:
It would be surprising if the host genome did not occasionally find some use for particular selfish DNA sequences, especially if there were many different sequences widely distributed over the chromosomes. One obvious use … would be for control purposes at one level or another.
Here, too, is Comings (1972), the first person to use the term “junk DNA” in print and the first to provide a substantive discussion of the topic. (The term was coined by Ohno in 1972, but Comings’s paper appeared in print first, citing Ohno as ‘in press’, and Ohno used the term only in the title).
These considerations suggest that up to 20% of the genome is actively used and the remaining 80+% is junk. But being junk doesn’t mean it is entirely useless. Common sense suggests that anything that is completely useless would be discarded. There are several possible functions for junk DNA.
The use of the terms “selfish DNA” or “junk DNA” has changed over time, and both are now often applied to all non-genic DNA, rather than to the sequences to which they originally referred (i.e., transposable elements and pseudogenes, respectively). Moreover, it seems that many authors — at least those whose studies focus primarily on protein-coding genes and DNA sequencing — believe that the assumption has been that all non-genic DNA is “junk” in the sense of totally non-functional. However, amidst any such assumptions there has always been a diversity of views on the subject, ranging from assuming that most non-genic DNA is non-functional (as in the quotes above) to expecting it all to be functional — the latter being a position held by strict adaptationists, and a large part of the motivation for proposing the alternative view of selfish DNA the first place.
As with many issues in evolution, this is a matter of relative quantity, not an exclusive dichotomy. We may reasonably expect a significant fraction of non-genic DNA to show evidence of function, and the pursuit of such evidence is a valid and important endeavour. It does not follow, however, that the pendulum must be perceived to swing from entirely functional to entirely non-functional and back again. We will undoubtedly refine our estimates of the amount of non-genic DNA that is mutualistic at the organism level, how much is commensal, and how much is best characterized as parasitic in nature.
As it stands, the evidence suggests that about 5% of the human genome is functional at the organism level. The total may be higher — as noted, Comings suggested 20% is actively utilized. It is conceivable that 50% or more of the genome is functional, perhaps in structural roles or some other higher-order capacity. It would require evidence to support this contention, however, and the question would remain as to why an onion requires 5x more of this structural or otherwise essential DNA, and why some of its close relatives can get by with half as much while others have twice the onion amount. There is nothing remarkable about onions in this sense, by the way — animal genome sizes alone cover a more than 7,000-fold range, and even among vertebrates there is a 350-fold difference. The range among single-celled protozoa is at least 30,000-fold, though even higher estimates have been presented.
The take home message is simply this. What we know about eukaryote genomes suggests that there are many mechanisms that can add non-coding DNA that do not require it to be functional. This does not in any way preclude the possibility of, or invalidate the search for, function in some, many, or possibly even most of those non-coding components. How much proves to be functional is an empirical question, and at present the indication seems to be that most non-genic DNA is non-functional. That said, non-functional is not the same as inconsequential.
Comings, D.E. 1972. The structure and function of chromatin. Advances in Human Genetics 3: 237-431.
Doolittle, W.F. and C. Sapienza. 1980. Selfish genes, the phenotype paradigm and genome evolution. Nature 284: 601-603.
Ohno, S. 1972. So much “junk” DNA in our genome. In Evolution of Genetic Systems (ed. H.H. Smith), pp. 366-370. Gordon and Breach, New York.
Orgel, L.E. and F.H.C. Crick. 1980. Selfish DNA: the ultimate parasite. Nature 284: 604-607.