CHAPTER 22 INTRODUCTION
22-1
CORE CONCEPTS
22.1 Species are reproductively isolated from one another.
22.2 Reproductive isolation is caused by barriers to reproduction before or after egg fertilization.
22.3 Speciation underlies the diversity of life on Earth.
22.4 Speciation can occur with or without natural selection.
Imagine for a moment a world without speciation, the process that produces new and distinct forms of life. Life would have originated and natural selection would have done its job of winnowing advantageous mutations from disadvantageous ones, but the planet would be inhabited by a single kind of generally adapted organism. Instead of the staggering biological diversity we see around us—current estimates are that between 10 and 100 million species call Earth home—there would be just a single life-form. From a biological perspective, the planet would be a decidedly dull place. Evolution is as much about adaptation, the result of natural selection, as it is about speciation, the engine that generates that breathtaking biodiversity.