All animals with eyes or eyespots that have been studied so far share a gene in common. when mutated, the gene pax-6 causes lack of eyes in fruit flies, tiny eyes in mice, and missing irises (and other eye parts) in humans. the sequence of pax-6 in humans and mice is identical. there are so few sequence differences with fruit fly pax-6 that the human/mouse version can cause eye formation in eyeless fruit flies, even though vertebrates and invertebrates last shared a common ancestor more than five hundred million years ago.
fruit-fly eyes are of the compound type, which is structurally very different from the camera-type eyes of mammals. even the camera-type eyes of molluscs, such as octopi, are structurally quite different from those of mammals. yet, fruit flies, octopi, and mammals possess very similar versions of pax-6. the fact that the same gene produce very different types of eyes is most likely due to
(1) the few differences in nucleotide sequence among the pax-6 genes of these organisms
(2) differences in the control of pax-6 expression among these organisms
(3) the independent evolution of this gene at many different times during animal evolution
(4) variations in the number of pax-6 genes among these organisms