Spec evo, short for speculative evolution, is a subgenre within science fiction that focuses heavily on the biology, ecology, or evolutionary history of living organisms.
It can further be divided into at least three subsubgenres. Some spec evo covers the future evolution of life on Earth, sometimes telling grand epics that cover billions of years as humans or descendants of other animals colonize other planets. Common themes include culture, religion, and extinction.
Other spec evo covers alternate universes wherein history on Earth ran differently. The most common scenarios involve dinosaurs surviving the Cretaceous-Paleogene extinction event, or synapsid reptiles remaining dominant after the Permian-Triassic extinction event.
Other spec evo covers the biology of life not from Earth. Some of the more detailed publications cover biochemistry and cellular structure. Others cover organ systems. Others cover environmental adaptations and ecological relationships. Some cover them all.
This last type of speculative evolution is also referred to as astrobiology, xenobiology, or exobiology. Astrobiology is also the term used for serious speculation among scientists about life on other planets, while xenobiology and exobiology tend to be the dominant terms for the purely fictional side of things.
All of these forms of literature are often accompanied by artwork.
More about spec evo, including links to books, movies, and webpages, can be found at FloraAndFaunaOfTheUniverse.com