Local adaptation, in which populations of a species become better able to survive and thrive in their home environment than in conditions found elsewhere in the species’ range, is a widespread pattern that evolutionary biologists have long used to study the causes and consequences of natural selection. My newest paper, which is now online ahead of print in The American Naturalist, combines data across many studies of local adaptation to answer a persistent question about the history of life on Earth — has evolution been influenced more by selection arising from environmental conditions, or by interactions among living things?