Eyespots

History recently aired a series called “Evolve” which focused upon various aspects of evolution: wings, sex, guts, etc. But the most interesting episode was probably the one on eyes. It’s pretty clear they started out with eyes because of all the hub-bub made by creationists who find this organ to be too complex to have evolved by blind chance (what about the brain?). Natural selection is not chance (though it is blind – half credit), but such a misconception is one of the reasons eyes needed to be a starting point.

What History didn’t do, however, was get down to the cellular level of eye evolution. They may have touched upon photoreceptive cells, and that’s technically the cellular level, I suppose, but it overlooks an important aspect of evolution: everything which evolved today began its evolution (eventually) at the cellular level. So it is to my delight that I came across a Wikipedia article on eyespots.

Eyespot apparatus of euglena

Eyespots are a lot like they sound they are. They’re photoreceptive areas found in some plants cells (algae) and even in single celled organisms such as the euglena featured above. These are simply areas which cause a reaction to light – it becomes too bright, you may be too close to the surface. Swim away.

The advantage should be quite clear. The ability to detect light – not shape, size, dimension, or detail – gives an organism a lot of information about its environment. Specifically, History did address the eyespots of certain squids. These squids, only having nervous systems (no brains), would simply go into a sort of lull when coming near a certain wave length of light. As it not-so-coincidentally happens, that wave length corresponds with the wave length common near the food source of these squids.

Here’s the interesting kicker for which I think everyone should perk up: the origin of these eyespots works upon signal transduction, which is initiated by enzymes. This is what happens in every cell everywhere. Enzymes catalyze various things within cells. Lactase, for example, catalyzes lactose into galactose and glucose monomers – you drink milk, proteins (lactase) in your body will break it down into its constituents. It’s basic biology.

So how do new enzymes arise that can cause the formation of eyespots? This is a matter of a mutation within the DNA of a cell. Some “letter” of DNA is changed through some sort of error in copying. It happens all the time. You have 50-100 mutations in you right now (most, if not all, are probably neutral). DNA replication isn’t perfect. So a simply mutation can quite easily code for a new enzyme, which can cause the formation of an eyespot – the beginning of the eye. A slow, cumulative building through, perhaps, further mutation combined with the non-random action of natural selection can (and has separately over 40 times) evolve a complex eye worthy of fighting on the evolutionary stage of life.

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2 Responses

  1. […] Eyes in Evolution Posted on November 25, 2008 by Michael I’ve already blogged about eyes and evolution, so I won’t go on about further research. But I will post an interesting article from […]

  2. […] useful. (I’m ignoring historical contingency for the sake of brevity.) For instance, an eyespot won’t enable any creature to see danger or prey from miles away, but it is useful for […]

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