Microevolution and macroevolution generally are recognized as distinct phenomena that reflect two extremes of the evolutionary hierarchy. “Microevolution” denotes the evolutionary processes that operate at the genetic level (mutation, recombination, selection, random drift) among breeding pairs and their progeny (at least for mammals). These processes occur within the lifetimes and geographic ranges of individual animals: within a few months and within less than 1 km2 in the case of rodents, up to a few decades and a few hundreds of square kilometers in the case of large herbivores. Expanded over a few generations (at most tens or hundreds of years), microevolution causes shifts in gene frequencies of populations that may or may not manifest themselves as phenotypic changes.

“Macroevolution,” on the other hand, refers to the processes that cause new species to arise and interact, an enterprise that seems quite lengthy in human terms, taking place over the geological expanse of thousands of years, in some cases, though usually over hundreds of thousands to millions of years. The geographic coverage encompasses the entire range of a species during its lifespan and can vary from a few tens or hundreds of square kilometers to an entire continent or more. The motor of macroevolution still is not clear: Some argue for cumulative microevolutionary changes, whereas others claim that macroevolution is decoupled from microevolution and is driven by processes that become operative only at the species level (e.g., species selection or the “effect hypothesis”); see the literature cited by Barnosky (1987), Vrba and Eldredge (1984), and Vrba and Gould (1986).