Fig 9.

Large population size leads to CAS loss. The upper panel shows a fraction of simulations where cas genes are retained (red) and lost (blue) as a function of the population size parameter (inverse encounter rate b⁻¹). The lower panels show typical evolution of single realizations in small (b⁻¹ = 10², left) and large (b⁻¹ = 10³, right) populations. (A) Numbers of viruses (red), CAS⁺ hosts (blue), and CAS⁻ hosts (green). (B) Fraction of immune encounters. (C) Fraction of hosts that conserve CAS.