 One branch of the levels of selection debate that is particularly interesting from the point of view of levels of organization is the issue of evolutionary transitions. Here the focus is on the emergence of new levels of organization through evolutionary processes (Buss 1987: ch. 5; Griesemer 2000; Maynard Smith &amp; Szathmáry 1995; Okasha 2006). The background idea is that the complex hierarchical organization of nature that we observe today must itself be a result of evolution, and therefore requires an evolutionary explanation. For example, somehow prokaryotes evolved to eukaryotic cells, single-celled organisms evolved to multicellular organisms, individual animals evolved to colonies, and so on. In their highly influential book, Maynard Smith and Szathmáry (1995) proposed that the characteristic feature of major evolutionary transitions is that entities that were capable of replicating independently before the transitions are only capable of replicating as parts of higher-level wholes after the transitions (see also Buss 1987). For example, after a single-celled organism has evolved into a multicellular organism, the cells of the organism can no longer replicate independently of the organism as a whole. For more on levels of selection and evolutionary transitions, see Okasha (2006) and the entry  units and levels of selection.