The role of mutational biases in adaptation: internal versus external influences on the direction of evolution

What determines the direction of evolution? One possibility is that evolution is primarily directed by changing ecological circumstances, so that organisms evolve via natural selection to whatever form is optimal under this extrinsically defined environment. A different possibility is that the developmental mechanisms within organisms are much more likely to vary in some ways rather than others, so that the course of evolution is primarily determined by the intrinsic features of organismal development and the types of variants that it tends to produce. However, rigorously adjudicating between these two possibilities has proved difficult. Here we propose to empirically evaluate the importance of intrinsic causes by considering the special case of adaptive changes to genes where each adaptive change has been caused by a single mutational event. We will catalog a large number of such adaptive changes and ask whether these adaptive changes are caused by types of mutations that occur at particularly high rates. Our preliminary results suggest that known adaptive changes are highly enriched for classes of mutations that are produced more often by development, indicating that intrinsic developmental mechanisms may indeed be capable of determining the direction of adaptive evolution.