The biologicaI species concept claims that species consist of natural populations and that species are real and objective. They are not man-made subjective abstraction. According to this concept, the members of a species are a reproductive community. The species is also an ecological unit. It interacts as a unit with other species with which it shares the resources of the environment. The species is also a gene pool. These aspects of the biological species concept are made clear in the famous definition of the Harvard evolutionist Ernst Mayr. According to Mayr, "Species are groups of interbreeding natural populations that are reproductively isolated from other such groups". The biological species is not only a distinct unit at any given time, but it also has the evolutionary capacity to change continuously over long periods of time, measured in millions of years.