The concept that species are defined by their ability to reproduce with each other (the Biological Species Concept) was first formally proposed by Ernst Mayr in the mid-20th century.

However, it's important to note that the idea itself has roots in earlier thinkers.

* Aristotle (384-322 BC) recognized that animals could be grouped by their ability to produce fertile offspring.

* John Ray (1627-1705) and Carl Linnaeus (1707-1778) developed classifications based on shared characteristics, which were later understood to be related to reproductive compatibility.

Mayr's contribution was to explicitly define the Biological Species Concept as a clear and widely accepted definition of a species, emphasizing the role of reproductive isolation in defining distinct species.