Structure in evolutionary transitions in individuality: Mechanics, geometry, and topology

Abstract

In the history of life on earth, Evolutionary Transitions in Individuality (ETIs) have played a transformative role, increasing complexity and creating new hierarchies of organization. Examples of ETIs–in which previously independent individuals form groups and forgo their autonomy in the creation of new, higher-level individuals–include the formation of chromosomes from unlinked replicators, multicellular organisms from single cells, and eusocial colonies from solitary insects. In this thesis, I address the fundamental role of structure in two aspects of ETIs: the mechanics and geometry in the evolution of increased size in nascent multicellular clusters, and the role of interaction topology in the evolution of specialization. The first step in the evolution of multicellularity–an example of an ETI–is thought to be the formation of large clusters. In the first part of my thesis, we show that the size of experimentally evolved snowflake yeast clusters is limited by the accumulation of internal stress during growth. We then show that snowflake yeast mitigate this challenge and increase their size by reducing intercellular contacts within the cluster. This is achieved by a geometric, cell-level change that further work reveals is among the most efficient routes to increased size, suggesting that physically-imposed geometric constraints may guide the evolution of increased size in nascent multicellular clusters. Additionally, we evolved snowflake yeast under various physical selection protocols to study the role of the environment on their evolutionary trajectory. In the second part of my thesis, I turn to a universal hallmark of ETIs, the evolution of functionally-specialized individuals–that is, members of the group that exclusively perform a specific function. Such specialization often results in an increase of fitness for the group, but renders specialists dependent on the rest of the group for survival, thus completing the shift in the level of individuality from the individual to the collective. To explore the role of interaction topology, we created an individual based model in which individuals interact via specified networks. We found that certain sparse topologies allow complementary specialists to be linked, thus favoring the evolution of specialization for a much wider range of individual investment-return profiles than previously thought.