Another developmental and evolutionary paradigm: the origin of the eyes
Eyes are obviously made to see, and they are found in most animal phyla. In some cases, they are very complex organs, performing their function close to perfection. In the early time of Darwinism, eyes were often taken as an argument to say that such complex structures could not proceed from a gradualistic evolutionary process. In other words, and in the same way that finding a watch implied a watchmaker, the observation of the perfect eye of a bird implied some intention in evolution, and has long been a creationist argument. Improvements in scientific knowledge have progressively changed the way we see and understand the evolution and development of the eye.
It was recently shown (Nilsson and Pelger 1994) by computer simulation, that a simple flat photoreceptor could evolve progressively into a complex spherical eye with a lens, by fixation of random mutations through natural selection.
The diversity of eyes which are found for example among molluscs (Strickberger 1990) is best explained by a gradualistic evolution and different levels of selective pressure. The similarity of complex eyes as found in an octopus and a vertebrate is simply due to convergent adaptive evolution. If eyes are recurrent adaptations occurring in diverse phyla, the question is: 'Does Developmental Biology provide some interesting insight about eye evolution?'
A positive answer was provided when it was shown in Drosophila that an over expression of a gene (the protein of eyeless) could induce the formation of ectopic eyes (Gehring 1998). The eyeless gene is homologous to Pax 6 in vertebrates, and mutants of the two genes result in eye abnormalities in fruit flies and vertebrates respectively. A possible but wrong conclusion would be that, despite their anatomical differences, eyes in insects and vertebrate are homologous. What is homologous in the two phyla is the basic process of inducing some cells into the photoreception pathways, with the production of the photosensitive protein, opsin. During the development of the eyes, the two genes are used again for morphological differentiation, including lens production in vertebrates, but these later functions are pure analogies.
How various genes, having a diversity of basic functions, may be recruited to perform a new task, is best illustrated by the lens proteins (crystalline) in vertebrates (Gerhart and Kirschner 1997).
The new function just consists in making a transparent solution within the lens, and many proteins can do that. Lens proteins are very diverse: some are constitutive, i.e., general proteins expressed in all species. But others are metabolic enzymes which have found a new site of expression, but no metabolic function: for example, lactate dehydrogenase in birds, alcohol dehydrogenase in camels, aldehyde dehydrogenase in macroscelids.
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