 While diachronic identity of an organism is understood as its persistence though time, synchronic identity is defined in terms of its individuality or distinctiveness (Sober 2000: 154). Attempts have been made to identify distinguishing characteristics such as functional autonomy, possession of mutually interdependent parts, genetic homogeneity and spatial boundaries (Clarke 2010). Others, pointing to the heterogeneous character of biological individuals, have argued that no sole feature can serve as a universal determinant of individuality (see the entry on  the biological notion of individual).  Part of the contention resides in differing frames of reference of study: Most considerations of synchronic individuality focus upon evolutionary individuals as units of selection (Gould and Lloyd 1999) that differs from how physiological individuals might be distinguished (Pradeu 2016). Regardless of their involvement in evolution, organisms exhibit various levels of physiological and morphological integration that makes their indivisibility problematic (Godfrey-Smith 2009). Immunology contributes to discussions of individuality in this latter sense by considering the role of the immune system in defining the boundaries of the organism by the functional measures of rejection or assimilation.