Minimalistic examples of the effect of history and environment.
Left: simplified fitness landscapes, which only show accessible paths that increase fitness. Darker shading indicates increased fitness. Right: some possible realized paths from the wild type (black dot). Gray lines represent genomes; colored shapes indicate mutations. (a) Environmental effects on non-robust mutations. Two possible mutations: one is fit in environment A, but not in B, and the other is fit in environment B, but not in A. Upon environmental change, the unfit mutation will eventually revert to the wild type and the other beneficial mutation will fixate. (b) Historical effect—contingency: From the wild type, there are 2 options to increase fitness. While the first step will depend on random processes, the next step is defined by the history, since there is only 1 option to increase fitness from there. The fitness maximum for this example (orange/red) will not be reached if the first step taken is the green one. (c) Historical effect—gateway mutations: The fitness maximum (green triangle) is only accessible via a neutral mutation (large blue square) that has to happen first. Other neutral mutations might happen before the potentiating mutation is found and opens up the path toward the fitness optimum.