Isochores result from mutation not selection

Mammalian genomes are made up of large regions of distinct base composition known as isochores1, but it has remained uncertain whether this mosaic structure is caused by regional variation in mutational biases2,3,4 or by natural selection1. To resolve this controversy, we compared the rate and pattern of nucleotide substitutions in two globin pseudogenes, Ψα1 and Ψη, which reside in isochores of very different base composition in the genomes of humans and other primates. Our findings indicate that isochores result from mutational biases, rather than from selection on individual nucleotides.