This book brings together the results from one of the most significant long-term studies of birds in the late twentieth century, testing our understanding of evolution in natural populations. Combining genetics, behaviour, ecology, and a landmark data set, it will be essential reading for everyone with an interest in evolutionary ecology. The Lesser Snow Goose is a migratory Arctic breeder which occurs in two genetically distinct forms. Most have white plumage, but some are dark, allowing rapid scoring and study of a highly visible heritable trait. Initial chapters describe the natural history, recent evolutionary history, and current patterns of gene flow in the species. A subsequent chapter presents a detailed demographic model integrating both fecundity and survival components of fitness, which is used as an analytical framework throughout later chapters. Two chapters detail the effects of annual and age-specific variation in fitness components. The authors then focus on the microevolutionary status of the population, using data gathered over 26 years on 40,000 individually marked adult geese, 45,000 nests, and 110,000 goslings at one colony in northern Manitoba. The heart of the book analyses the working of natural selection on plumage colour, seasonal timing of breeding, clutch size, egg size, and body size, using the components of fitness approach to quantifying selection in stages throughout the animal life cycle. The results are an important advance in understanding the evolutionary process in vertebrates.