These are the biologists who really care about species concepts. They do not, however, all agree on which concept of species is the best. In other words, organisms are considered to belong to the same species if they can potentially reproduce with each other and have offspring that are healthy and fertile. Otherwise, they are considered to belong to different species. In other words, if you have a group of organisms that all share a derived character state and that can not be subdivided into smaller groups based on shared derived character states then you have a species.
Different species have different derived character states from each other.
Both these concepts of species try to identify the groups that will evolve as independent evolutionary units. There are potential problems with each concept that are solved by the other concept. The phylogenetic species concept, since it is based on characteristics other than reproduction, provides a basis for classifying such species. The biological species concept lumps into a single species some groups that are very ecologically and structurally different and that are apparently evolving separately. All forms can apparently reproduce with all other forms, but the different forms are clearly distinct from each other and evolving independently.