A core concept of evolutionary theory is Darwin's prediction that the origin of derived phenotypes are accompanied by periods of increased variability. These "Zones of Variability" are transient in time and tree space. The book will establish an Extended Model of Variability by exploring how biologists perceive the relationship between microevolutionary variation and macroevolutionary patterns. It will support three primary predictions, drawing on examples from animal phylogeny. The book will conclude with an examination of the model's implications - from systematics to paleontology to developmental biology and beyond.