Light & Eyes

It's interesting and useful to make a distinction between linear- and non-linear responses to natural selection.

There can be a direct correlation between some environmental factor and the resulting adaptation. For example the coat-thickness of some animal will likely be a directly response to environmental temperature: when it's cold there will be selection for a thick coat and when the temperature rises there will be selection for a thinner coat. The result is a simple, linear, predictable correlation between the adaptation (coat-thickness) and environment (temperature).

There are other instances in which the relation between environment and adaptation is less linear and predictable.

Consider two time-frames:
1 There is light. There is a population of animals that don't have eyes and can't see.
2 There still is light (perhaps the same amount as in the first time-frame). Now the animals have eyes; complex constellations of functional parts, making functional wholes, that can that can perform the act of seeing.

What I'm trying to get at is that, although the evolution of seeing/eyes has of course a lot to do with the fact that there is light, the elementary fact that there is light is not a full explanation of the origin the complex system that is an eye.

More generally: a complex adaptation can evolve in response to a simple environmental factor, but that does not mean that all parts of that complex adaptation are determined by that factor.

The second, non-linear response is a, let's say, don't get me wrong, more creative response to environmental selection.
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