Darwin 200: An eye for the eye

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A celebration of one of evolution's crowning glories.

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"To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree. Yet reason tells me, that if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real." – Charles Darwin, _On the Origin of Species_

Charles Darwin was well aware that the eye — so obviously, so brilliantly 'designed' — might represent an impediment to the acceptance of natural selection. In fact, it has come to be seen as one of evolution's crowning glories. Across the animal kingdom, eyes have evolved in different ways, to different purposes, in exuberant diversity. Yet they are all sculpted by the lawfulness of light — and the imperatives of survival.

This panoply of eyes sees the world in different ways, some concerned with colour, others with movement, others with acuity. Yet time and again, unrelated eyes hit upon common solutions to the problem of how to safely gather and focus information from a sunlit world. The eyes of the cuttlefish grow from invaginations of the skin; those of the human grow in part out of the front of the brain; each uses completely different receptors to pick up the light they focus. Yet cuttlefish and people see the world in the same way, through eyes whose similarities outweigh their deep differences.

Eyes are largely built from building blocks designed for other things. The lenses of vertebrates use proteins that bacteria developed to deal with stress; the flexible guanine mirrors that make a cat's eye glow in the dark provide gas-proofing for the swim bladders of fish. And the components are put together in a bewildering number of ways. The brittlestar has an entire carapace pitted with optically tuned calcite crystals. The scallop eye uses a curved mirror to focus incoming light; the pram bug Phronima elongates each element of its eyes with a natural optical fibre. The dragonfly's compound eyes — each an array of up to 28,500 individual visual organs — may look ungainly, but they are extremely resistant to motion blur as it hunts on the wing.

In dim conditions, an eye must gather what light it can, regardless of image clarity. By using box mirrors to focus light on a common point, the eyes of shrimp and lobsters enjoy more than 250 times the light-catching power of the human eye.

The compound eye is a dead end in design terms; no matter how big you make it, it produces poor images. But it is well adapted to the zigzagging life of a hoverfly. The abrupt changes in the fly's direction of flight, which occur several times a second, help the eyes produce a fuller picture of the world.

Human eyes are rare in their concern for producing images; most of the eyes in the animal kingdom are tuned to movement and colour. The band running across the eye of the mantis shrimp contains receptors tuned to 16 different wavelengths (humans typically have three), giving colour vision of extraordinary subtlety and complexity. Nor are two eyes a universal norm. The fish Anableps anableps effectively has four eyes in two sockets; each eye has one half for seeing above water and one half for seeing below. Another fish, Bathylychnops exilis, has two pairs, one set to look up, which is how it sees its prey, and one to look down — but for what, nobody knows. The better your predators can see, the more need there is for disguise. For some animals, looking inedible is not enough: the unnervingly detailed mock eye on a moth wing is meant to convince predators that something big is looking right back at them.

The principal high-resolution eyes of true spiders are as big and as powerful as the eyes of small rodents. Subsidiary eyes are used to spot movement in the periphery, and sometimes, by harnessing the polarization of sunlight, to navigate.

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Ings, S. Darwin 200: An eye for the eye. Nature 456, 304–309 (2008) doi:10.1038/456304a

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