Davison, Dinah R., and Richard E. Michod. “10 Phenotypic Plasticity and Evolutionary Transitions in Individuality.” Phenotypic Plasticity & Evolution (2021): 241.
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Evolutionary transitions in individuality occur when the unit of selection and adaptation, the evolutionary individual, changes from one level of organization to another (Buss 1987; Maynard Smith and Szathmáry 1995; Michod 1999; Hanschen et al. 2017a). Examples involve some of the key events in the history of life, including the evolution of the cellular genome from groups of cooperative genes, the evolution of eukaryotic cells from bacterial and archaeal cells, the evolution of multicellular organisms from unicellular ancestors, and the evolution of eusocial societies from solitary ancestors. These transitions in individuality are rare evolutionary events, having occurred dozens of times, as compared to the millions of speciation events that have occurred throughout the history of life. Although rare, these evolutionary transitions have bestowed on life one of its most familiar characteristics, its hierarchical organization, in which biological individuals—genomes, cells, multicellular organisms, eusocial societies—are comprised of groups of cooperating individuals from lower levels. (Page No. 274 of the PDF file downloaded after clicking on “URL1” or “URL2”).