Life on earth is characterized by three striking phenomena that demand explanation: adaptation-the marvelous fit between organism and environment; diversity-the great variety of organisms; and complexity-the enormous intricacy of their internal structure. Natural selection explains adaptation. But what explains diversity and complexity? Daniel W. McShea and Robert N. Brandon argue that there exists in evolution a spontaneous tendency toward increased diversity and complexity, one that acts whether natural selection is present or not. They call this tendency a biological law-the Zero-Force...