Monday, October 01, 2007

Mutational speed limit?   posted by Razib @ 10/01/2007 04:18:00 PM

Beyond A 'Speed Limit' On Mutations, Species Risk Extinction:
Harvard University scientists have identified a virtual "speed limit" on the rate of molecular evolution in organisms, and the magic number appears to be 6 mutations per genome per generation -- a level beyond which species run the strong risk of extinction as their genomes lose stability.

By modeling the stability of proteins required for an organism's survival, Imbler Shakhnovich and his colleagues have discovered this essential thermodynamic limit on a species's rate of evolution. Their discovery, published recently in the Proceedings of the National Academy of Sciences, draws a crucial connection between the physical properties of genetic material and the survival fitness of an entire organism.

This sort of biophysical process seems plausible to me. Natural selection needs some lack of fidelity during replication to have variation to work with, but evolutionary process seems like a moot point if noise overwhelms the informational signal. Here is an interesting "big picture" implication:
The Shakhnovich speed limit also offers an explanation for observed differences in genome sizes between organisms with genome error correction -- such as bacteria, mammals, birds, and reptiles -- and those without, such as RNA viruses: In more complex organisms, cells have evolved correction systems to detect and fix errors in DNA replication. These systems drastically reduce the number of mutations per replication, increasing the mutational stability of the genome and allowing more intricate and delicate biological systems to develop without the risk of interruptive mutations.

"It's an interesting corollary because it suggests that there is a fundamental tradeoff between evolutionary security and adaptive flexibility: Larger, more complex organisms have to have error correction to protect organismic viability, but this means the rate of evolution slows down significantly," Shakhnovich says. "As organisms become more complex, they have more to lose and can't be as radically experimental with their genomes as some viruses and bacteria.

The paper will be out on PNAS this week.

Related: Haldane's Dilemma: should we worry?.