A species tree for three species (A, B, and C) to illustrate the parameters of the MSC model, with a gene tree for five sequences (a₁ and a₂ sampled from species A, b₁ and b₂ from species B, and c₁ from species C) running inside the species tree. Within each species/population, sequences coalesce at random at the rate determined by the population size (or θ parameter), generating a gene tree with branch lengths (coalescent times), conditioned on the species tree. Note that θ_C is not estimable if there is at most one sequence from species C at each locus.