Neutral Evolution Has Helped Shape Our Genome

Just a very brief post which I’ll write up more fully later, and today comes news via Science Daily that much of the DNA in our genome appears to be neutral, i.e. large parts of it have appear to have no directly beneficial or detrimental effects to the whole.

“For a long time, the basic belief of evolution was that all random genetic changes that manage to stick around have some selective advantage,” says Nicholas Katsanis, Ph.D., Associate Professor at Hopkins’ Institute of Genetic Medicine. “But our work adds to the case that frequently, we are what we are largely due to random changes that are completely neutral.”

“I am not at all discounting the role of natural selection, the persistence of genetic changes that confer some advantage,” Katsanis adds, “because it has been instrumental. What this study does is to reinforce and highlight the equal, and in some cases greater, importance of neutral genetic drift.”

I’ll need to read up on this some more, but this research would seem to highlight how little we really understand about the function of DNA and what makes us uniquely human. Here’s another excerpt from Science Daily, describing numts (pronounced ‘new mights‘), the genomic sequence of mitochondria

When they expanded their study across the whole human genome, they found more than 1200 such pieces of mitochondrial DNA of various lengths embedded into chromosomes. While chimps have a comparable number, mice and rats only have around 600 numts. Since they increase in frequency as species advance, it suggested there was some evolutionary purpose to keeping them around.

Strikingly, however, none of these numts contained the blueprint (an actual gene) to make a protein that does anything, nor did they seem to control the function of any nearby genes. “At best, it seems numts are a neutral part of our genome,” says Katsanis. “If anything, they may be mildly negative since long repeat sequences can be unstable or get inserted inside genes and disrupt them.”

Katsanis then goes on to describe a genetic bottleneck in primates that is estimeated to have occurred some 54 million years ago…

Their calculations revealed that most numts became embedded in our genome over a 10-million-year period centered roughly 54 million years ago — right around the time when the first primates emerged. “When new species emerge, their numbers and therefore their genetic differences are very small,” Katsanis notes. “This creates a genetic bottleneck during which any changes in the genome will either get eliminated quickly or spread to the whole population quickly.”

Katsanis proposes that numts, being “neutral,” were generally at low levels in ancient mammals, but during the primate emergence 54 million years ago, they accumulated and spread through the small early primate populations precisely because they were not detrimental enough to be eliminated. Then, as these populations expanded, numts reached stable but higher frequencies.

Fascinating stuff, the overall message of which seems to be that primate evolution is far more random than thought, and less influenced by drivers such as natural selection across the genome as a whole. The full paper, in the form of a provisional pdf, can be accessed at PLoS Genetics, by clicking the link below. (TJ)

Population Bottlenecks As A Potential Major Shaping Force Of Human Genome Architecture

image from Brainethics

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