Traditionally, the genome has been viewed as a collection of DNA molecules that vary in composition between individuals and species, and variations that generate phenotypic differences have been assumed to occur in a more or less random manner. More recently, this view has been challenged by evidence that genomes are in fact reservoirs of adaptive phenotypic plasticity. This adaptive genome concept, where mutations that convey adaptive benefits are likely to occur at greater than random frequencies (Caporale 1999, 2000, 2003) represents a synthesis of ideas and evidence from several subfields and has its genesis in work by pioneers such as Dhobzhansky (1937), Dawkins (1976), McClintock (1984) and Trifinov (1989).

The human brain underwent explosive growth after we split from our chimp cousins, but the pace of evolutionary change among the thousands of genes expressed in brain tissue has since slowed, says a new study in PLOS Biology.

The researchers involved speculate that the higher complexity of the biochemical network in the brain places strong constraints on the ability of most brain-related genes to change.

"We found that genes expressed in the human brain have in fact slowed down in their evolution, contrary to some earlier reports," says study author Chung-I Wu, at the University of Chicago. "The more complex the brain, it seems, the more difficult it becomes for brain genes to change. Calibrated against the genomic average, brain-expressed genes in humans appear to have evolved more slowly than in chimpanzees."