How and why did the eye evolve from natural selection?

Answer 1

There are two questions here; but if we look at the first one (how the eye evolved), the second one will also be answered. The simplest "eye" is simply light sensitivity. For example, some simple sea organisms can only distinguish between light and dark; if it's dark they shrink - some cells on their bodies are light sensitive and it triggers the response. It's obviously basic but better than no sensitivity to light at all. The next stage is a light sensitivity with some indication of motion; again some sea organisms have just that - some extra processing indicating which cells "see" the dark area first. Again, this is better than simply having light or dark sensitivity. One can see from this, that its possible to imagine lots of "next steps" - speed as well as direction, different directions, two eyes to be able to estimate direction, layers of transparent cells to form lenses in front of the light sensitive areas and so on. What's surprising is that examples can be found of most of the "intermediate" stages. A key point is that there's no "final" target that evolution is aiming for; the next stage is reached because the offspring that have more of the next stage present will tend to survive to reproduce and eventually a new species will arise that all have the next stage. Because modern eyes are essentially the result of millions of minor improvements of earlier models we've ended up with some odd results; in our own eye the retina is essentially back to front; the nerves which lead off to the visual context are in front of the light sensitive cones and rods - also if we wanted to design an eye, we wouldn't have a blind spot where the optic nerve joins the eyeball. The "why did the eye evolve" part of the question is that each step provides an advantage over not doing it.

Answer 2

The first eyes were very simple: a region of light sensitive cells called the eyespot. They could not do most of the functions our eyes do. All they did was distinguish between light and dark by detecting changes in the intensity of light. They could not detect color, shapes, nor the direction the light was coming from.

Over time this region folded inwards to form a depression called the eyecup. This allowed for limited directional sensitivity. The primitive eye folded deeper inward, forming a pinhole that light could pass through, forming a more focused and distinguishable image. To build further on this, a lens (made of transparent cells) developed over the pinhole, allowing light to be focused for an even sharper and clearer image. Two or more eyes develop to distinguish depth. A retina forms, made of rods and cones that form a very sharp image. The eye became an eyeball and muscles surrounded it. A muscle also develops around the iris to control the amount of incoming light.

The great thing about the evolution of the eye is we can see intermediate forms of the eye in all the animals we see today. Microscopic bacteria have the eyespots, for example. As we move up through the animal kingdom we observe eyes becoming more complex. Planaria have the eyecups, the nautilus has the pinhole camera eye, and insects have eyes with lenses. The eye in each type of animal is modified differently to suit its needs. Some can live just fine with limited vision, whole others need sharp color vision in order to survive.

Eyes of course developed through natural selection. Whenever there was a modification in the eye that proved beneficial (such as the ones listed above) it would be passed on to the next generation. Thus, eyes became progressively more complex.