On the possibility of constructive neutral evolution

The neutral theory often is presented as a theory of "noise" or silent chan ges at an isolated "molecular level," relevant to marking the steady pace o f divergence, but not to the origin of biological structure, function, or c omplexity. Nevertheless, precisely these issues can be addressed in neutral models, such as those elaborated here with regard to scrambled ciliate gen es, gRNA-mediated RNA editing, the transition from self-splicing to spliceo somal splicing, and the retention of duplicate genes. All of these are inst ances of a more general scheme of "constructive neutral evolution" that inv okes biased variation, epistatic interactions, and excess capacities to acc ount for a complex series of steps giving rise to novel structures or opera tions. The directional and constructive outcomes of these models are due no t to neutral allele fixations per se, but to these other factors. Neutral m odels of this type may :help-to clarify the poorly understood role of nonse lective factors in evolutionary innovation and directionality.