Ward Belfield Watt (October 21, 1940 – October 27, 2024) was an American evolutionary biologist who was a professor of biology at Stanford University. He was known for studying evolution using biochemical, physiological, and ecological approaches. For over 50 years, Watt conducted field studies at the Rocky Mountain Biological Laboratory in Gothic, CO.[1] There he worked primarily on Colias butterflies.[2] He developed these butterflies into a model system for the study of natural selection in the wild. He was known for studying the impact of wing pigmentation polymorphism on thermoregulation and on fitness.[3] He was also known for studying the impact of natural amino acid variation in enzymes of central metabolism on insect flight performance.[4] He championed the idea that adaptation and constraint are distinct elements in evolution, prior to fitness differences.