Organisms are adapted to their environment through their individual characteristics, like body size and body weight. Such complex traits are usually controlled by many genes. As a result, individuals show tremendous variations and can also show subtle gradations. Researchers from the Max Planck Institute for Evolutionary Biology in Plön have now investigated how evolution alters such traits through selection. To do this, they examined the genomes of mouse lines that were selected independently of each other for extreme body size. They discovered that a number of genomic regions, or loci, have undergone changes in genes that underlie this genetically complex characteristic. They also discovered many new genes that play a role in the regulation of body weight, which can lead to obesity.