The Role of Structural Criteria in Transitions Theory: A Focus on Organisms

The Major Evolutionary Transitions theory of Szathmary and Maynard Smith is famous for its contribution to the understanding of complex wholes in biology. Typical for Major Evolutionary Transitions theory is the select use of functional criteria, notably, cooperation, competition reduction and reproduction as part of a larger unit. When using such functional criteria, any group of attached cells can be viewed as multicellular, such as a plant or the slug-shaped aggregation of cells of a slime mould. In addition, one could also have used structural criteria to arrive at the conclusion that the cells in the slug of a slime mould are attached without plasma strands, while the cells of a plant are attached and connected through plasma strands. A theory which in addition to functional criteria also uses structural criteria for the identification of major transitions is the Operator Theory. Using the Operator Theory one can, for example, conclude that the slug of a slime mould represents a pluricellular organisation because its cells are not connected through plasma strands, while the cells of a plant are connected through plasma strands and for this reason represent a multicellular organism. In this chapter, the relationships between the Major Evolutionary Transitions theory and the Operator Theory are studied with a focus on transitions that lead to organisms.