 Species concepts might be of little philosophical interest if the only organisms that needed classifying were animals. The most applicable and conceptually well-founded species concept for animals—and the most familiar species concept to those outside biology—is the biological species concept (BSC). It says that a species is a group of organisms that can breed only within that group. Even though there are some well-recognized problems in applying a single species concept to all animal life (see Dupré 2001), problem cases are usually seen as exceptions that do not fundamentally trouble the BSC (Mayr 1982; Coyne &amp; Orr 1998; however, cf. Mallet 2008). Plants introduce additional but probably surmountable problems, argue many zoologists (e.g., Mayr 1982); unfortunately, fungi might be beyond the pale by virtue of resisting any such “universal” species concept (see Petersen &amp; Hughes 1999). But since many fungi are unicellular, and exhibit all kinds of reproductive oddities, it might just be safest to accept the old assertion that unicellular and particularly asexual organisms do not and cannot form species, and thus are not exceptions to a universal species concept (e.g., Mayr 1987). These are just some of the issues that arise when venturing beyond animals in attempts to apply species concepts. In response to these sorts of issues, a range of other species concepts have been proposed, drawing on morphological, molecular, or evolutionary characteristics such as forming lineages of a certain sort (see the  SEP entry on species).