"'Unit of selection' is a polysemic expression, meaning interactor, replicator/reproducer or manifestor of adaptation/type-1 agent in today's biological research. This Element presents each of these concepts and demonstrates the necessity of their isolation"--

Introduction; 1. What is a Unit of Selection and How Can We Identify It? The Disambiguating and the Unitary Projects; 2. How the Expression 'Units of Selection' Acquired Its Polysemic Meaning or Why the Disambiguating Project Started; 3. Two Sources of Misunderstanding in Past and Today's Debates about Units; 4. The Framework of the Evolutionary Transitions in Individuality: A Challenge to the Disambiguating Project; Conclusion; Glossary; References.