Part Five -The Patterns and Processes of Evolution 463

23. Species and Their Formation 508

23.1 What Are Species? 509 We can recognize and identify many species by their appearance 509 Species form over time 510 23.2 How Do New Species Arise? 511 Allopatric speciation requires almost complete genetic isolation 511 Sympatric speciation occurs without physical barriers 513 23.3 What Happens when Newly Formed Species Come Together? 515 Prezygotic barriers operate before fertilization 515 Postzygotic barriers operate after fertilization 516 Hybrid zones may form if reproductive isolation is incomplete 517 23.4 Why Do Rates of Speciation Vary? 518 23.5 Why Do Adaptive Radiations Occur? 520