Figure 2. Conceptual illustration of the relationships between fitness landscapes, trait distributions, and selection strength.

(A) Non-uniform fitness landscape and broad trait distribution. The gray distribution represents a chronological distribution of lineage trait $x$; the cyan distribution represents a retrospective distribution of lineage trait $x$; and the black dashed line represents a fitness landscape. Due to the non-uniform fitness landscape and the broad chronological distribution, there is trait fitness heterogeneity for selection to act on. The retrospective distribution therefore shifts significantly from the chronological distribution, and the selection strength is large (${\displaystyle S[X]>0}$). (B) Non-uniform fitness landscape and narrow trait distribution. Due to the lack of trait heterogeneity, there is little fitness heterogeneity for selection to act on. The retrospective distribution shifts only slightly from the chronological distribution, and the selection strength is small ($S[X]\approx 0$). (C) Uniform fitness landscape. When the fitness landscape is constant ($=\tau \mathrm{\Lambda }$) across the lineage trait state $x$, there can be no trait fitness heterogeneity regardless of whether the trait distribution itself is narrow or broad. The selection strength is therefore zero ($S[X]=0$).