 Whether or not we consider collectives or organismic individual agents as the core entities partaking both in development and evolution, attempts to integrate the two realms have to show in each case that, in fact, it is the same unit that develops and evolves. In other words, if we want to unify development and evolution through the unit of the biological individual (being the one entity that partakes in both) this unit needs to meet criteria of both physiological (e.g., metabolic) and evolutionary individuality (see the entry on   biological individuals). Evolutionary individuals have been traditionally conceptualized as reproductive units with differential fitness and shared lineages (so-called ‘Darwinian individuals’; see Godfrey-Smith 2009) or as units of selection (‘interactors’; see Hull 1980). Unfortunately, both of these units do not always coincide (Godfrey-Smith 2013; Pradeu 2016). For example, some organisms (holobionts) form developing but no reproductive units, as they include a multitude of lineages (e.g., microbial ones). Other possible units of selection (like genes or populations) are not identical with physiological individuals. Thus, a physiological individual may not necessarily be an evolutionary unit or vice versa.