One of the most contentious questions in evolutionary biology is, how did the Amazon become so rich in species? A new study focused on birds examines how the movements of rivers in the Amazon have contributed to that area's exceptional biological diversity. The researchers found that as small river systems change over time, they spur the evolution of new species. The findings also reveal previously unknown bird species in the Amazon that are only found in small areas next to these dynamic river systems, putting them at high risk.