Organisms use light directly as an energy source or as a source of information about their surroundings. The ability to detect and characterize objects at a distance is important in the way of life of many or even most animals. A variety of attributes of light are used, including direction, brightness, image, distance, and motion of objects. The evolution of vision in turn affected the evolution of the organisms being detected by their light images. This occurs in many ways, not least being the evolution of mating rituals and display, protective camouflage and deceptive coloration (e.g., caudal eye spots that disguise a tail as a head to trick predators, and Batesian mimicry in which tasty species mimic bitter ones), light-based attractants in flowers and fruit, motion behavior to escape predators.Visual cues are also used to set traps, by disguising a predator. Fireflies have species-specific flash signals to attract each other (and their predators who mimic them to lure a lusty but careless fly to its death). These phenomena variously use color alone and/or coloration pattern.
As important as it has been, however, seeing is not a specific problem to be solved by organisms, and no environment demands any particular type of vision. Indeed, the presence of various primitive photoreception mechanisms shows that organisms can use light information without having a brain (e.g., primitive organellar vision in unicellular organisms; jellyfish). Vision probably also illustrates the role organismal selection may play in nature. Organisms use what they have, and this may sort them out as well as classical darwinian selection does, but without fitness differences.
The homologies indicated by Pax6 were dramatic findings. But this was only among the first of many similar findings of deeply conserved genetic mechanisms involving regulatory, enzymatic, and structural genes in many different structures and organ systems. The eye story, along with a few others like the Hox axial patterning system, led the way in these discoveries, but findings like this are rather general. Indeed, we have now come to expect to find such conservation. This unifies the animal world and may suggest that life is younger than we thought, relative to the evolution of genetic mechanisms—there has not been enough time for the sharing of these genetic mechanisms to be erased, replaced by selection, or subjected to phenogenetic drift, as sequestration among species almost guarantees will eventually occur if the Earth remains hospitable to life for a long enough time. However, the sharing that has persisted so far has perhaps tempted overstatements of homology because the conservation has been far from complete or simple, as eyes and vision illustrate.
Photoreception is evolutionarily related in interesting ways to olfaction and has diverged from a common ancestral mechanism for cells to sense external conditions. They use related members of the 7TMR family (and Pax6 is involved in olfactory development). This homology can be overstated because there are so many genes in this family and they serve such diverse purposes. However, like olfactory receptors, photoreceptors are G protein-linked receptors that activate cyclic nucleotide-gated channels using cGMP to affect cAMP concentration (it is synthesized by guanylyl cyclase and degraded by cGMP phosphodiesterase). A major step was the evolution of the different binding properties of these related genes—one for chemical shape and the other for light energy.
As is true of hearing and olfaction, seeing is not just a molecular trick played by the brain with opsin firing. The eyes and head can be moved relative to the environment, and the brain must integrate that movement with the neural information it receives from the eyes or other sensory neurons. Although this book is about genetic mechanisms and how they have been used to receive diverse information from the environment—especially unique or unpredictable information—we should not forget that morphology is also a major aspect of organisms. Morphology (and its coordinated use) is responsible for the mobility of external ears, eyes, nostrils, heads, and bodies, and this is all an integral part of how organisms detect and respond to their environment.
Most animals can respond to the world in ways that are not prespecified. Some merely need to know where a light source is. Others need a spatially arranged map of some aspects of the external environment and have evolved complex means to integrate spatially arrayed input, as we will see in Chapter 16. There is a correspondence between the properties of optics and the structures found in the eyes in nature, strongly suggesting that adaptation by natural selection has been at work.
But in nature, there are many ways to see, and no organism has them all. Animal brains have evolved to be able to receive organized light signals and resolve them in one way or another. There isn't one species that might not do better with additional visual ability, but all make do with what they have, and that has been good enough.
In Greek mythology, Juno jealously suspected that her husband Jupiter had a mistress who took the form of a heifer to disguise her identity (Juno was right). She hired the herdsman Argus to keep an eye—actually, to keep his hundred eyes—on the heifer to prevent further mischief. Argus never slept with more than two of his eyes closed and so was ever-vigilant. This evolutionary experiment ended suddenly, however, when Jupiter commissioned Mercury to get rid of Argus. Mercury lulled Argus to sleep with story-telling and then killed him in a stroke. Juno's revenge included bedecking the peacock's tail with Argus' eyes, which are now seen but no longer see. Being covered with 100 eyes always on the alert might have been selected for, but it wasn't (except, perhaps, in the scallop, whose hundred or so eyes are arrayed, beadlike, along its mantle).
Aristotle and his peers through Classical times had many ideas about what species could see. Aristotle, for example, thought that moles were blind. But in a huge compendium to debunk long-held mistaken ideas published in 1646, Thomas Browne (Browne 1646) reported that he observed mole embryos to develop eyes and that in fact moles could see. That a species could not see as well as others do, he notes, is a value judgment of which we need beware: "if the ^gle were judge, wee might be blinde our selves."
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