Illustration by Christoph Niemann
THE PAPER: Evidence of positive selection acting at the human dopamine receptor D4 gene locus
THE JOURNAL: Proceedings of the National Academy of Sciences, January 8, 2002
THE AUTHORS: Yuan-Chun Ding, Deborah L. Grady, James M. Swanson, and Robert K. Moyzis, et. al.
THE GIST: A genetic mutation that causes hyperactivity could have been crucial to human development.
BEFORE TRANSLATION: “Based on an observed bias toward nonsynonymous amino acid changes, the unusual DNA sequence organization and strong linkage disequilibrium surrounding the DRD4 7R allele, we propose that this allele originated as a rare mutational event that nevertheless increased to high frequency in human populations by positive selection.”
Scientists believe that some of the bad genes that we humans walk around with today are there for a good reason. By “bad genes,” they mean the ones that predispose young people to disabling diseases and disorders; by “good reason,” they mean an evolutionary advantage. If these genes were entirely bad, say scientists, they would not be so common.
That’s the spirit in which biologist Robert Moyzis and colleagues from the University of California at Irvine studied attention deficit hyperactivity disorder. The ailment is linked to a variation of a gene called DRD4. This gene makes a receptor for dopamine, a biochemical that in addition to conveying feelings of pleasure, also helps people focus on specific tasks. About 50 percent of kids diagnosed with ADHD have the gene variation, which is present in about 10 percent of the population worldwide. Moyzis wondered: How could a gene that makes it almost impossible for people to concentrate be so widespread? At some point during our evolution, he decided, the gene must have given its bearers a fantastic edge. But what could the advantage have been?
Moyzis and his team decided to look at the gene closely, and what they found surprised them. There are more than 10 different versions of the DRD4 gene, but by far the oddest is the one associated with ADHD. By analyzing its structure and comparing it with the other versions, they concluded that the variation must have occurred suddenly, rather than evolving over many years. What’s more, says Moyzis, “the genes adjacent to the variant DRD4 were almost exactly the same, whether in an individual from Africa, America, or Europe.” Since genes constantly rearrange themselves and mutate at a certain rate, an ancient variation should have been surrounded by many different combinations of genes among different people. Consequently, he reasoned, the gene must have sprung up sometime in our recent history.
Moyzis then plugged this information into some equations and figured the gene variation likely arose and spread some 50,000 years ago-right around the time that scientists estimate humans began migrating out of Africa. Could the gene have made its bearers so antsy they couldn’t help but pick up and move to a new continent? Moyzis believes it’s quite possible. The gene is active in the frontal lobes of the brain-our emotional control centers and home to our personalities. The frontal lobes are especially important in problem solving and learning from one’s experience. This part of the brain is also responsible for traits like spontaneity. Scientists haven’t yet figured out exactly how variations in the gene cause differences in a person’s behavior. But they do know that people who have the version associated with ADHD have a penchant for thrill seeking. What’s more, the version is rare in Asians but common in South Americans-evidence that it could have spread as people migrated into Europe and the New World.
There’s another theory as to why this gene variation became so common. Maybe the hyperactive males-wild, crazy, unpredictable hunks that they were-attracted women in large numbers, and still do today. Of course, it’s just a theory.