Opening the "Black Box": The Genetic and Biochemical Basis of Eye Evolution
By Andrew J. Petto and Louise S. Mead (both authors at Ecology, Evolution, and Marine Biology, University of California,
Santa Barbara, CA)
(Evolution: Education and Outreach, Oct. 2008)
Eyes provide a rich narrative for understanding evolution, having attracted the attention of preeminent scientists and communicators alike. Until recently, this narrative has focused primarily on the evolution of eye structure and far less on biochemistry or genetics. Although eye biochemistry was once likened to an unknown "black box;" the flood of discoveries in biochemistry is now allowing an increasingly detailed understanding of the processes involved in vision. As a result, evolutionary comparative ("tree-thinking") analyses that use these data currently allow a new and still unfolding narrative, both richer in detail and more comprehensive in scope. Rather than toppling evolutionary theory by finding irreducibly complex molecular machines, eye evolution provides detailed accounts of how natural processes tinker with existing genetic components, duplicating and recombining them, to yield complex, intricate, and highly functional eyes. Understanding the new biochemical narrative is critical for researchers and teachers alike, in order to answer antievolutionist claims, and to provide an up-to-date account of the state of knowledge on the subject of eye evolution.
The evolutionary history of eyes is one of the most intriguing and often-told stories in biology. It is a topic researched and discussed by members of the pantheon of evolutionists, including Darwin (1859), Salvini-Plawen and Mayr (1977), Gould (1994), and Dawkins (1996). The commonly told story is familiar and has not changed much since Darwin first proposed it: A light-sensitive nerve gradually changes over evolutionary time, adding complexity across the generations. To Darwin's sketch, Salvini-Plawen andMayr (1977) added detail, and Nilsson and Pelger (1994) tested the temporal component. The ancient canon of gradual evolution is certainly valuable for demonstrating that some assumptions are met for the hypothesis that natural selection generates complexity, serving as a colorful and graphical example (Dawkins 1996). However, this account - if taken too far - can easily be criticized, as demonstrated, for example, by proponents of Intelligent Design (ID; Behe 1996) who point out that the Darwinian canon ignores complexity at the molecular level. The structure of this article is first to briefly present the "gradual morphological" account of the evolution of eye form and to describe how this account relates to understanding evolution by natural selection. Section II will point out two primary limitations of this "gradual morphological" model, including one highlighted by ID proponents. Third, the paper will present molecular and biochemical details describing how phototransduction (the cascade of signaling events that generate a nervous impulse in response to light) and complex lenses evolved. These two case studies provide specific details about how eyes evolved. The processes elucidated, such as duplication and co-option (the use of existing components during evolution in new contexts) generalize to any complex trait.