In behavioral ecology, the adaptive significance and diversity of animal traits and behaviors can be understood based on natural selection and kin selection at the individual level. This is because animals are multicellular organisms composed of clonal cell populations, and the unity and functionality as individual is maintained by altruism between cells. However, in the process of evolution and establishment of “individuality” as an animal from protist through multicellular body, what kind of natural selections acted at each level of cell and multicellular body, and in what order? What is the overall picture of the interactions and transitions between natural selection processes?
  In this presentation, I focus on animals that have a bottleneck life cycle and separate germ and somatic cell lineages early in development, and will attempt to construct a picture of the interactions and transitions between multiple natural selections based on the results of previous research and considerations from the perspective of multi-level selection. In particular, I will summarize the conditions for the evolution of altruistic traits in cells and the conditions and mechanisms that prevent the spread of selfish cells, which are essential for establishing individuality, based on Hamilton's rule in kin selection, the Price equation condition for group selection, and evaluation results from simulations.
  Taking protist as the starting point and animal as the endpoint, I examined evolutionary scenarios by dividing the period between them into three stages: 1) group formation, 2) group maintenance, and 3) group transformation. I then compiled a table comparing the life cycle characteristics, main types of natural selection, degree of integration as multicellular organism, body size (number of cells), and conditions for transitioning between stages at each stage.
  The main results (hypotheses) are summarized as follows:
1) The emergence of a single-cell bottleneck in the life cycle created the conditions for multicellular level selection that overcame cellular level selection, allowing cooperative/altruistic cells to gain an advantage in clonal formation.
2) Increasing body size promoted the conditions for reproductive division of labor, i.e., germ cell/somatic differentiation. This shifted the role of somatic cell lineages from cellular productivity to multicellular survival, leading to the evolution of traits (strategies) for individual level adaptation (e.g., foraging, fighting, mating), a major theme in behavioral ecology.
3) The multicellular level selection and kin selection that operated during the establishing process of animal individuality have now transformed and integrated into individual selection, which operates on modern animal individuals.