J
Jason_Meyers
Guest
Actually, recent evidence suggests that the eye did not evolve multiple independent times, but rather that the eye is a rather good example of divergent evolution. There are two major lines of evidence that have supported this change in the ideas.
First, the genes that specify tissue to become an eye are very highly conserved (meaning that they are found in most animals with few changes to the sequence). In particular, there is a gene called Pax6/eyeless that helps trigger the initial formation of the eye during development, that is similar enough that the gene in frogs can replace the gene in flies and function perfectly fine (and vice versa). This suggests that the developmental pathway to make an eye was established before the split between vertebrates and invertebrates, and then since the split vertebrates and invertebrates use those genes to make very different structures, but that share a common heritage.
The second interesting line of evidence comes from the observation that invertebrates have a very different structure to their photoreceptors from vertebrates and use a photoreceptive pigment that is different from all of the vertebrate photopigments. (Incidentally, this was some of the evidence originally cited in favor of independent evolution of the eyes in vertebrates and inverts) Recently, however, it has been found that the output cells of the vertebrate retina (called retinal ganglion cells) are related to the invertebrate photoreceptors in structure and gene expression. Of particular interest, although they were not thought to be photosensitive, these cells have photoreceptive pigments that are much more closely related to invertebrate photopigments than any of the vertebrate photopigments. It turns out that this photopigment in vertebrates is likely playing an important role in sensing light and dark for setting night/day body rhythms.
Thus one of the leading hypotheses now is that both vertebrate and invertebrate eyes are derived from simple eyes present in a common ancestor and have diverged since then. In particular, it is though that this common ancestor likely had both vertebrate-like and invertebrate-like photoreceptors, and in the invertebrate lineage the vertebrate-like photoreceptors were lost while in the vertebrate lineage the invert-photoreceptors became the retinal ganglion cells. Notably, a marine worm has now been described that has both invertebrate and vertebrate type photoreceptors.
Thus, while the eye used to be cited as an excellent case for convergent evolution, it is now likely that it is an even better case for divergent evolution.
Sorry if that was too technical, but since it reflects both current progress in evolution and shows further evolutionary linkage between all of God’s creatures including humans, I thought it worth bringing it forth.
JRM
P.S. Just for the record, I am both an active biologist and an active Catholic.
I’d like to gently ease into a few of your comments if you don’t mind. You’re comments reflect that of a proponant of Intelligent Design hypothesis.
