One of the canonical publications of the modern evolutionary synthesis. Mayr discussed the different ways different investigators identify species, and he characterized these different approaches as different species concepts. He also argued strongly for what came to be called a Biological Species Concept (BSC)—that a species consists of populations of organisms that can reproduce with one another, and that are reproductively isolated from other such populations.

Condensed and extensively revised as Populations, species and evolution. Cambridge: Belknap Press of Harvard University Press, 1970.

An interpretive history of what Mayr calls “ultimate” explanations in biology, reflecting Mayr’s expertise in systematics, evolution, and genetics.