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 500 million years ago.
The appearance of Pax-6 in all animals with eyes can be explained in multiple ways. Based on the information above, which explanation is most likely?
A) Pax-6 in all of these animals is not homologous; it arose independently in many different animal phyla due to intense selective pressure favoring vision.B) The Pax-6 gene is really not "one" gene. It is many different genes that, over evolutionary time and due to convergence, have come to have a similar nucleotide sequence and function.
C) The Pax-6 gene was an innovation of an ancestral animal of the early Cambrian period. Animals with eyes or eyespots are descendants of this ancestor.
D) The perfectly designed Pax-6 gene appeared instantaneously in all animals created to have eyes or eyespots.
Answer: C
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 helps produce very different types of eyes is most likely due to A) the few differences in nucleotide sequence among the Pax-6 genes of these organisms.
B) variations in the number of Pax-6 genes among these organisms.C) the independent evolution of this gene at many different times during animal evolution.
D) differences in the control of Pax-6 expression among these organisms.
Answer: D
Pax-6 usually causes the production of a type of light-receptor pigment. In vertebrate eyes, though, a different gene (the rh gene family) is responsible for the light-receptor pigments of the retina. The rh gene, like Pax-6, is ancient. In the marine ragworm, for example, the rh gene causes production of c-opsin, which helps regulate the worm's biological clock. Which of these most likely accounts for vertebrate vision?
A) The Pax-6 gene mutated to become the rh gene among early mammals.B) During vertebrate evolution, the rh gene for biological clock opsin was co-opted as a gene for visual receptor pigments.
C) In animals more ancient than ragworms, the rh gene(s) coded for visual receptor pigments; in lineages more recent than ragworms, rh has flip-flopped several times between producing biological clock opsins and visual receptor pigments.
D) Pax-6 was lost from the mammalian genome, and replaced by the rh gene much later.
Answer: B
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