Single-gene models

An alternative view of the evolution of functionally novel proteins includes gene-recruitment [12] or gene sharing [13]. In this model, a period of bifunctionality of a single gene product precedes the duplication event upon which one copy maintains the ancestral function while the other adopts the new function. The various aspects of bifunctionality can hereby range from conservation of structural properties as demonstrated for crystallins [14] to what O Brian and Herschlag [15] call promiscuous activity in ancestral structures. 

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