Figure 5.

The Innovation-Amplification-Divergence (IAD) model of evolution by gene duplication explains the evolutionary history of the CrCO, PpCOL, and AtCOL genes. (A) The CrCO gene in Chlamydomonas is a hub gene whose main functions are light response and carbon/nitrogen metabolism. Additionally, CrCO presents links to secondary functions such as response to chemical stimulus and developmental processes that can be considered innovations developed by this gene. (B) During the evolution of multicellular land-based plants such as Physcomitrella and Arabidopsis these secondary functions became selectively valuable which facilitated the fixation of duplications of CrCO. Initially, duplicated genes are identical and conserved exactly the same links as the ancestral gene. (C) In order to fully cover the demands exerted by selection of a more complex regulation over developmental processes, an amplification consisting of multiple gene duplications took place, giving rise to the 10 gene family of PpCOLs and the 17 gene family of AtCOLs. (D) After duplication and amplification the same selectively forces facilitate the accumulation of mutations over the duplicated genes that make them diverge, specialize and acquire new functions as it can be observed in the PpCOL and AtCOL genes. This is reflected in the network by the removal of links and the establishment of new ones. Note that in spite of divergence the genes tend to keep links to the main ancestral functions.