 Griffiths (1999), Okasha (2002), and LaPorte (2004) have suggested a form of species essentialism which can be called ‘relational essentialism’. According to relational essentialism, certain relations among organisms, or between organisms and the environment, are necessary and sufficient for membership in a species. Such relations, argue Griffiths, Okasha, and LaPorte, are species essences. For example, they suggest that being descendent from a particular ancestor is necessary and sufficient for being a member of a species.