On the causes of evolutionary transition:transversion bias

Arlin Stoltzfus; Ryan W. Norris

The conservative transitions hypothesis proposes that transitions collectively are more conservative than transversions. How well do mutant fitness studies distinguish conservative replacements from radical ones? We construct two versions of this distinction, EX B and U B (the "B" indicates a binary distinction, as opposed to a continuous measure), simply by designating higher-exchangeability replacements as "conservative", and the remainder as "radical". Table 2 shows how well studies of mutant fitness distinguish conservative from radical replacements, and how well they distinguish transitions from transversions. The measure of effect-size used here is the chance that a mutant designated as "conservative" is more fit than a randomly chosen "radical" mutant. This statistic is not affected by the relative sizes of the 2 classes; its range is from 0 to 1, with a null expectation of 0.5; higher values indicate that nominally "conservative" changes are indeed conservative. We call this measure AUC because it has the same meaning as the area under a ROC (receiver-operating characteristic) curve for a binary classifier.