Generation of evolutionary novelty by functional shift

That biological features may change their function during evolution has lon g been recognized. Particularly, the acquisition of new functions by molecu les involved in developmental pathways is suspected to cause important morp hologic novelties. However, the current terminology describing functional c hanges during evolution (co-option or recruitment) fails to recognize impor tant biologic distinctions between diverse evolutionary routes involving fu nctional shifts. The main goal of our work is to stress the importance of a n apparently trivial distinction: Whether or not the element that adopts a new function (anything from a morphologic structure to a protein domain) is a single or a duplicated element. We propose that natural selection must a ct in a radically different way, depending on the historic succession of cc -option and duplication events; that is, co-option may provide the selectiv e pressure for a subsequent gene duplication or could be a stabilizing fact or that helps maintain redundancy after gene duplication, We review the evi dence available on functional changes, focusing whenever possible on develo pmental molecules, and we propose a conceptual framework for the study of f unctional shifts during evolution with a level of resolution appropriate to the power of our current methodologies. (C) 1999 John Wiley & Sons, Inc.