Natural selection – the force that drives evolution – acts not only on whole organisms and individual genes, but also on gene networks, according to a new study appearing in Nature this week.The finding suggests that natural selection is both more powerful and more complex than scientists recognized. Researchers found that a close relative of brewer's yeast, Saccharomyces kudriavzevii, exists in two very different states: one that can efficiently digest the sugar galactose and one that cannot. Galactose is a natural sugar found in milk and many fruits and legumes. The variant found in Portugal that consumes galactose uses a network of six genes to convert the sugar into energy. What is surprising is the fact that a variant found in Japan that cannot process galactose has nevertheless preserved a non-functional version of the network of galactose genes for millions of years.