Figure 1
From: Evolution of eye development in the darkness of caves: adaptation, drift, or both?
Effect of population size and environmental modifications on the substitution rate (rate of fixation of mutations in the whole population). A Relative probability of fixation of a mutation according to the population size (Ne) and its selective value (s) [24, 25]. s = 0 => neutral; s < 0 => deleterious; s > 0 => advantageous. Y axis scale: probability relative to the probability of fixation of a neutral mutation. Kimura suggested a simple rule of thumb: if |s| < < 1/4Ne selection is weak and genetic drift dictates allele fixation; otherwise, selection dominates. A slightly deleterious mutation with s = -0.001 has a high probability to reach fixation in a population of size 250 (when 4Nes = -1, relative probability = 0.58, i.e., about half the probability of fixation of a neutral mutation), whereas it has a very low probability to be maintained in a population of size 2,500 (when 4Nes = -10, relative probability = 0.00045). B Relative substitution rate of mutations according to the selective values and constraint changes on genome after a shift in cave environment. The constraints on coding sequences (rectangles) and their regulatory sequences (ovals) can be “unchanged,” lifted if a protein has no function (“neutralized”), or “changed” due to adaptation to this new environment. Y axis scale: The substitution rate relative to the substitution rate in a larger population and before the environmental shift. Neutral mutations (pink) should accumulate at the same rate; slightly deleterious mutations (green) should accumulate at a higher rate in all sequences because they behave as neutral mutations in a small population; previously highly deleterious mutations (blue) could accumulate in “neutralized” sequences; adaptive mutations (red) would accumulate in sequences involved in the adaptation to the environmental shift. In parallel, there is a reduction or elimination of polymorphism (black dotted lines) in the neighboring DNA of a mutation under recent and strong positive selection (selective sweep due to genetic hitchhiking).