Half an eye IS better than none

Alright, Ginger, here you go. I'm going to do this off the top of my head because my references are at home (or on line, but if I start calling up this stuff I'll never take a break to get any work done).

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Evolution works in small steps. Lots of creatures are born with small genetic variations that did not exist in their parents (replicating our 3,000,000,000 base pairs of DNA is bound to make a mistake somewhere eventually). Sometimes that means you're born with Muscular Dystrophy; usually it means nothing at all: now and then it means you're a little bit better at something.

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One of the creationist arguments that evolution cannot be right is that this kind of small incrementation cannot possibly result in complex structures that don't work until all the parts are there. Having just some of the parts would be useless and not advantageous, so you couldn't get them all together by chance. An oft-taken example is the eye.

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But it simply isn't true that the modern human eye must be complete and perfectly functional for it to be advantageous. Certainly, when competing one human against another, being myopic is not as good as having clear vision. And being color-blind is not as good as being able to see the usual spectrum of colors. And having macular degeneration is disadvantageous too. But that's comparing partial vision to good vision, when you should be comparing partial vision to even less vision or none at all.

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Early in the course of animal life on earth, imagine that no creature can see. Then one creature happens to have a patch of cells that are somewhat sensitive to light. This may indeed be advantageous because it gives the creature an edge in knowing where to find shelter, or which direction to go to find food plants that grow in the light. Then one of this creature's descendents had a light-sensitive patch that was bigger, and so could start to detect the movement of something dark against the sky. This creature might avoid predators better than its peers and thus survive to reproduce more. Then in one of its descendants the light-sensitive patch developed in a sort of indentation, and this concentrated the light in one area, resulting in better resolution. And so on. The indentation deepens to a globe, an eye. The light-sensitive cells become sensitive to different wavelengths, giving color vision. The globe is capped with a lens of very stable, clear, flexible proteins (the crystallins, a family of enzymes used for completely different purposes, just happen to fulfil these critera, and now make our lenses) that helps focus light into sharp images. Every step of building an eye is more useful to some animals than the eye without that step. Examples of different eyes, down to simple light-sensitive patches, abound in the animal kingdom (and that's where I need my references to cite them for you).

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The very definition of what's a good eye varies with different animals. Cats' eyes are well developed for gathering all the light they can in the semi-dark, but that night-vision sacrifices daytime detail. Predators tend to have front-facing eyes with excellent acuity in a narrow zone in front where their prey is, with a concurrent sacrifice in peripheral vision. Prey animals, in contrast, have their eyes on the sides of their heads, making for excellent peripheral vision to spot that stalking lion but poor depth perception in front. Deep-living animals, if not completely blind, are often content with light-sensitive patches signalling 'up'.

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So in the course of evolution, changes that head toward something more useful than the previous version tend to let their animals live more successfully. What is better depends on what you need - whether you're prey or predator, living in bright light or shadows. Changes that don't help, or that hinder, either go unnoticed, or disappear from the gene pool, or are known to us as birth defects or genetic illnesses, or contribute to the diversity of the species. Just think of the color of your iris: living in most places, this color doesn't impact your physical fitness but it well may impact your sexual fitness by making you more desirable to the opposite sex (notably if your eyes are blue); in other places, like an equatorial desert where the sun is harsh, having a dark iris may protect your eye from too much light and thus have a real impact on your ability to function outdoors. Usually eye color is placed in the basket of normal variation without impact, but is it really...

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Anyway, the point is that the complex and magnificent thing that is an eye can indeed be developed through evolution, and there is plenty of evidence that it did so. Every part of the eye is explained, from its existence, position, color, eyelashes, tear ducts, even the awkward inside-out design (with the light-sensing retina behind other structures, and a central blind spot where the optic nerve enters - if one were to design an eye from scratch, surely this organisation would be avoided! but it's a leftover from earlier versions). A person denying evolution may then say, 'well, that's just a bad example, let me take another one.' But that one too will be argued and we'll just go on and on and on.

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I'll give you more if you ask for it, but it's time to prepare today's oncogenetics class. (cancer - now there's evolution gone wild!)

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