What good is half an eye?
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By David Annis
Article ID: 1236
One frequent objection that creationists use about evolution is to question how a complex structure could evolve – what good is half an eye? What possible evolutionary path could there be that would lead to such a complex structure?
So, how would an eye evolve? Start with simple organisms that don’t have an eye. In my fish tank I have a Tridacna maxima which is a saltwater clam. It has no eyes, but if I move my hand between the light and the water in a way that makes a shadow pass over the clam, it will close its shell. The clam has no eyes but is able to avoid predators by being able to sense light and dark. It is easy to see how this example of something that isn’t even half an eye results in a competitive advantage. Many organisms, not just clams, have the ability to sense light and dark.
So, how do we get from being able to sense light all the way to a human eye? The next step is eyespots (these are light-sensitive areas that may or may not be attached to a nervous system). Flatworms are a great example of eyespots. According to Wikipedia:
“Flatworms do have a bilateral nervous system; they are the simplest animals to have one. Two cordlike nerves branch repeatedly in an array resembling a ladder. The head end of some species even has a collection of ganglia acting as a rudimentary brain to integrate signals from sensory organs such as eyespots.”
Once we have eyespots, it is pretty easy to see how forming a dimple below the eyespot would help the animal. Since light from the side would hit only one side of the depression, the deeper the dimple the better the animal would be able to sense direction. Some flatworms and the nautilus have deepened these depressions enough to create a design like a pinhole camera, allowing the eye to focus without a lens.
Getting a lens over the eye to protect it and better focus the light is an easy step and one from which nature gives us many examples.
Some creationists will object to the idea that the thousands of varying eye designs represent steps in an evolutionary change. From photosensitivity, to eyespots, to dimpled eyespots, to lensless eyes, to the eyes of an eagle, they argue that the small advantage conferred by any single step wouldn’t be enough to make sure that the change persisted in the population. They ask for an example of a small, inefficient design evolving into a more refined form in a laboratory setting. Since most laboratories are far smaller than the entire planet, and have been operating for a far shorter time than Earth itself, it is difficult to recreate examples of most evolutionary events.
Nevertheless, there is a tantalizing example of what is probably an inefficient mechanism evolving and then being refined into an efficient adaptation in a carefully controlled laboratory setting. It occurred in a lab at Michigan State University, which has been growing 12 populations of E. coli since 1988. I’ve discussed this experiment in detail in a previous article titled Macro-evolution observed in the laboratory. This is our example of macro-evolution under carefully controlled laboratory conditions.
In this experiment, we saw the ability to use citrate under oxic conditions evolve in E. coli. It took about 30,000 generations and only happened in one of the 12 populations studied. Just like the eye, the wing, or any other complex trait, it evolved first as a very inefficient mechanism and then got better. In fact, in this experiment the Cit- population almost out-competed the inefficient Cit+ population before the Cit+ population became more efficient. From the paper:
“No Cit+ cells were found in the samples taken at 30,000, 30,500, or 31,000 generations. Cit+ cells constituted 0.5% of the population at generation 31,500, then 15% and 19% in the next two samples, but only 1.1% at generation 33,000. It appears that the first Cit+ variant emerged between 31,000 and 31,500 generations, although we cannot exclude an earlier origin. The precipitous decline in the frequency of Cit+ cells just before the massive population expansion suggests clonal interference, whereby the Cit- subpopulation produced a beneficial mutant that out- competed the emerging Cit+ subpopulation until the latter evolved some other beneficial mutation that finally ensured its persistence. The hypothesis of clonal interference implies that the early Cit+ cells were very poor at using citrate [emphasis added], such that a mutation that improved competition for glucose could have provided a greater advantage than did marginal exploitation of the unused citrate.”
So, as you can see, half an eye, an eye that sees half as well, half a wing, or even an inefficient way to use citrate can all confer reproductive advantage. This lays the groundwork for the evolution of a more complex mechanism.
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