No Plausible Speculationsthis new paper from the Royal Society on “The mystery of extreme non-coding conservation” that has been found across many genomes. Years ago an evolution professor told me, in defending the claim that evolution is falsifiable, that if functionally unconstrained yet highly similar DNA sequences were found in different species, then evolution would be false. A few years later that is exactly what was discovered. In fact, the DNA sequences were extremely similar and even identical in different species, and when they were altogether removed from mice it made no detectable difference. Hundreds of tests showed no significant difference between mice with and without long stretches of these DNA sequences. Did the professor agree that evolution was false? Not at all. For the fact of evolution goes far deeper than scientific findings and failed predictions. Nonetheless, ten years later, the mystery of extreme DNA conservation remains.
As the paper explains, there is currently “no known mechanism or function that would account for this level of conservation at the observed evolutionary distances.” This failure forces us to draw upon the typical explanatory mechanisms. The evolution of these extremely conserved sequences must have been abrupt and rapid, occurring in “short bursts.”
And since some of these sequences are found across a wide range of different species, the sequences, and whatever selective forces preserved them, must have been present very early in evolutionary history. On the other hand many of these sequences point to evolution’s nemesis, lineage-specific biology.
Some of these sequences are extremely conserved within lineages, but not across lineages. This forces us to conclude that the ancestral sequence first somehow arose in the common ancestor, later evolved independently in the different lineages which arose, became completely different in those different lineages, and then finally each of these different sequences, in the respective lineages, somehow became essentially unchangeable.
As is typical of the evolution genre, all of this is expressed in teleological terms. Here is a paragraph from the paper that is loaded with evolution’s Aristotelian tendencies:
Lowe et al. proposed that, within vertebrates, there have been three distinct periods of CNE [conserved non-coding element] recruitment around specific groups of genes. They suggest that this pattern is the result of regulatory innovations, which led to important phenotypic changes during vertebrate evolution. Prior to the divergence of mammals from reptiles and birds, it appears that CNEs were preferentially recruited near TFs and their developmental targets. This was followed by a gradual decline in recruitment near these genes, accompanied by [a recruitment] increase near proteins involved in extracellular signalling, and then [a recruitment] increase in placental mammals near genes responsible for post-translational modification and intracellular signalling. An analysis of CNE gain in the primate and rodent lineage has found that CNEs are either recruited near genes which have not previously been associated with CNEs, or are added near genes which are already flanked by CNEs. The interpretation was that the first set of genes is enriched in functions pertaining to nervous system development, whereas the latter contains genes involved in transcriptional regulation and anatomical development.
This example of teleological language also illustrates how the commitment to a theory can lead to a loss of parsimony. That is, in order to accommodate new and contradictory findings, additional explanations must be added to the theory. It becomes more complicated and less parsimonious. Here is how the paper summarizes these findings of extreme sequence conservation:
… despite 10 years of research, there has been virtually no progress towards answering the question of the origin of these patterns of extreme conservation. A number of hypotheses have been proposed, but most rely on modes of DNA : protein interactions that have never been observed and seem dubious at best. As a consequence, not only do we still lack a plausible mechanism for the conservation of CNEs—we lack even plausible speculations.
Reasonable speculation and even solutions to extreme sequence conservation may come in the future. But today’s science once again highlights the unique status of evolution.