Eyeballing Evolution and Vision
Eyes are beautifully complex structures, and their evolution was a source of some mystery to Charles Darwin. The idea that the eye could not have arisen from the process of natural selection is a common misconception even today and is rooted in the idea of irreproducible complexity. Irreproducible complexity states that complex structures could not have arisen as a result of a gradual evolutionary process because humans can't imagine how intermediate forms would be advantageous.
Intermediate stages of development
Darwin never suggested that natural selection couldn't produce the eye, of course; he just admitted that he didn't know exactly how the process unfolded. Fast-forward to today, when scientists know that many of the intermediate stages exist in other animals. From this fact, they can imagine the series of small steps that would lead from the simplest light-sensitive cell to a more complex eye:
- Step 1: Start with the simplest light-sensitive cells. A patch of these cells can determine only the presence or absence of light.
- Step 2: The cells are set slightly into the body in a little pit or cup. The light-sensing apparatus then becomes capable of determining the direction from which the light is coming.
- Step 3: The edges of the pit grow together so that light enters the pit through a very small opening. This arrangement is the principle behind a pinhole camera. Even though the camera has no lens, restricting light to traveling through a small hole results in a crisper image.
- Step 4: A lens is added to the opening of the light-sensitive cells. You don't have to imagine the lens evolving all at once as a lens; you can easily imagine that a layer of translucent cells over the opening of the pinhole had a protective function. When that layer was in place, any changes that resulted in a crisper image would be selectively advantageous.
Imagining such intermediate steps goes a long way toward understanding how a series of small changes can lead to complex structures like the eye.
A common pattern that's repeated across a large number of animals in different locations is the evolution of cave blindness — the evolution of sightlessness in lineages that have come to inhabit caves. Cave blindness is an excellent example of convergent evolution, in which the same trait evolves independently in different organisms.
The ancestors of most cave-dwelling organisms came from non-cave environments that had light. In fact, you can go to any big cave with its own ecosystem, and you'll find blind cave animals whose closest relatives (in the tree of life) can see.
You can easily see why the selective pressures on organisms existing in darkness would be different from those existing in light: Perhaps the energy required to produce those structures was needed for other functions — it's wasted making eyes in the dark. Cave critters often have a good sense of smell, extra-sensitive tactile feelers and antenna, or other stuff that's good to have in the dark, so perhaps the energy normally spent developing vision was spent further heightening the other senses.