Linking Darwinian Mechanisms with the Evolution of Form

摘要

As early as the 1960's, prior to elucidation of genome structure, eukaryotic gene regulation, or the mechanisms controlling development, R. Britten and E. Davidson had compiled enough evidence indicating to them that the evolution of the multi-cellular body plan occurred through changes in the circuitry regulating development rather than through the adaptive substitution of point changes in proteins. But twenty years earlier, C.H. Waddington had already proposed his theory of canalization, a mechanism by which robust developmental pathways could suppress phenotypic (and genetic) vari-ability, and give rise to an abundant classes of "hidden" variation upon which Darwinian selection can act when called upon. More recently, evolutionary studies of cis-regulatory modules – the func-tional unit of gene expression control – have revealed surprisingly rapid structural evolution of their functional elements, even when there is strong evolutionary constraint on gene expression. Adaptive morphological evolution has also been traced to evolutionary changes in CRM's, on the one hand reinforcing the Britten and Davidson belief that changes in gene regulation are paramount to evolu-tionary change, but also elevating Darwinian mechanisms involving the accumulation of mutational changes of small effect on the other. Here I attempt to sort through both the dialectical and empirical evidence supporting these opposing views. I also describe our current work on variation and evolu-tion in pattern formation in Drosophila, a system I believe will allow resolution of the molecular ba-sis for Darwinian selection acting on phenotypic variation filtered through developmental processes and pathways.