The Biological Species as a Gene-Flow Community. Species Essentialism Does Not Imply Species Universalism

We defend a realistic attitude towards biological species. We argue that two species are not different species because they differ in intrinsic features, be they phenotypic or genomic, but because they are separated with regard to gene flow. There are no intrinsic species essences. However, there are relational ones. We argue that bearing a gene flow relation to conspecifics may serve as the essence of a species. Our view of the species as a Gene-Flow Community differs from Mayr’s definition of the species as a reproductive community. In a reproductive community, each organism is able to successfully reproduce with each other. However, there are species in which geographically distant organisms lost their ability to successfully reproduce, due to strong genetic adaptations to the respective local environmental conditions. Despite this loss of the ability to mutually reproduce, they are still bound by gene flow via continuous intermediate populations. This replacement of Mayr’s notion of an interbreeding potential as a criterion for species membership has important implications for the treatment of populations in allopatry and sympatry