Multi-Level Selection of the Individual Organism

Multi-level selection (MLS) occurs when populations are structured into groups. We are interested in the effects of MLS on frequency-dependent fitness interactions, such as cooperation and conflict, during the evolution of the development of cell groups. The foundational models and concepts in MLS theory are reviewed. The concept of counterfactual fitness is discussed and how it may be used to partition selection between levels in a causal sense during the evolution of multicellularity. MLS hypotheses about the evolution of development during the transition from unicellular to multicellular life are reviewed. These models show how developmental modifiers may coevolve with group structure and create the first true group-level functions that shape reproduction of the group. These modifiers take the population from groups of cooperating cells to integrated groups of cooperating cells with organism-level functions that mediate conflict within the group and enhance the heritability, reproduction, and individuality of the cell group. After these developmental modifiers evolve, fitness at the group level is no longer the average of cell fitness. The power of individual selection and the primacy of organisms is often used to deny the need for MLS in evolutionary biology; however, the multicellular organism is a derived state and MLS theory is needed to explain its origin and evolution.