Transitive and non-transitive models. Both models could, in principle, account for the declining rate of fitness improvement relative to the ancestor. In competition with the ancestor, both models yield identical results, and were parameterized to match the data in Figure 1 (see Additional file 1). However, the two models make different predictions about fitness measured relative to clones isolated from later generations. (A) Transitive model. (B) Transitive model, ln-transformed. (C) Non-transitive model. (D) Non-transitive model, ln-transformed.