Elliott Sober's book, The Nature of Selection, discusses the philosophical underpinnings of evolutionary explanation in relation to other sciences. I turn to it once in a while when I need to sharpen a definition, and today ran across this passage (p. 50-51):
The source laws of physical theory have the austere beauty of a desert landscape. Just four types of force are recognized, and some scientists hope to make this list even shorter (Davies 1979). By contrast, the theory of natural selection exhibits the lush foliage of a tropical rain forest. The physical circumstances that can generate fitness differences are many. Perhaps someday these will be regimented and reduced in number. But at present evolutionary theory offers a multiplicity of models suggesting a thousand avenues whereby the morphology, physiology, and behavior of organisms can be related to the environment in such a way that a selection process is set in motion.