According to a 2002 study, the "FOXP2" gene showed indications of recent positive selection. Some researchers have speculated that positive selection is crucial for the evolution of language in humans. Others, however, were unable to find a clear association between species with learned vocalizations and similar mutations in "FOXP2". A 2018 analysis of a large sample of globally distributed genomes confirmed there was no evidence of positive selection, suggesting that the original signal of positive selection may be driven by sample composition. Insertion of both human mutations into mice, whose version of "FOXP2" otherwise differs from the human and chimpanzee versions in only one additional base pair, causes changes in vocalizations as well as other behavioral changes, such as a reduction in exploratory tendencies, and a decrease in maze learning time. A reduction in dopamine levels and changes in the morphology of certain nerve cells are also observed.