Conceptually, the prior information encoded by evolutionary plausibility is represented in this cartoon by the rainbow road, where ascending corresponds to improving fitness and descending corresponds to lowering fitness. Moving in any direction (for example, via random or brute force mutagenesis) would most likely decrease fitness or have a high chance of being a detrimental change (represented by the green ball). However, if evolutionary plausibility is an efficient prior (Fig. 1b ), then movement that is constrained to the plausible regime (for example, when guided by a language model) substantially increases the chance of improving fitness (represented by the red ball).
