Fig. 1.
Interpreting the evolution of intermediate phenotypes along the C4 evolutionary trajectory. (A) The intermediate phenotype (C3–C4 intermediate) evolved once, prior to the divergence of lineages 2 and 3, and so the extant intermediate represents the ancestral condition of lineage 3, the C4 clade. (B) The C3–C4 intermediate and the C4 phenotype each evolved once after the divergence of lineages 2 and 3, and so the extant intermediate does not represent the ancestral condition of lineage 3 (C4 clade) rather the C3 phenotype represents the ancestral condition of both lineages 2 and 3. (C) The C4 phenotype evolved prior to the divergence of lineages 2 and 3 and the C3–C4 intermediate evolved after the divergence of lineage 2; in this scenario, the C4 phenotype represents the ancestral condition of the C3–C4 intermediate. Each of these scenarios only requires two evolutionary steps, though (A) is the most common interpretation in the literature.