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Reduce this… Part 1

May 18, 2010

Irreducible Complexity is the idea that a biological component or system can be so complex that were you to remove one part or “reduce” it the component would cease to function. Common examples of this are the eye and the bacteria flagellum, among others.

Intelligent Design proponents and Creationists like to tout this assertion as an argument for their side. “How can something irreducibly complex ever evolve without a designer”, they postulate? Right off the bat, that should be a red flag to anyone. It’s basically saying “I can’t think of a way for this to evolve, therefore, it didn’t”. It’s an argument from ignorance, at its root. Just because we can’t think of it, doesn’t mean the explanation doesn’t exist. It just means we or, more specifically, the ID or Creationist proponents haven’t thought of it yet.

When you break the argument down it has two basic assertions:

1)      Things which are “irreducibly complex” could not have evolved because intermediate forms are useless.

2)      That taking parts out of something which is “irreducibly complex” renders it completely useless.

I’ll tackle these two arguments by using the same two examples I gave above – the eye and the bacteria flagellum. In this post, I’ll only cover the eye. I’ll cover the bacteria flagellum in a later post.

The Eye

For the eye, I will show how it’s possible that intermediate forms can exist and have an evolutionary and naturally selective advantage and how natural variation within a population will lead to the eventual evolution of an eye.

Step 1

Remember, we’re working forwards, not starting with a fully formed eye and working backwards. So, first, let’s start with a photosensitive skin cell. Most animals have these but they are too few and spread out to be of any real use. I read somewhere that humans do as well but I can’t substantiate that claim at the moment. They aren’t of any use to us now because we have an evolved eye which is much more efficient. But, you can imagine that, were we blind, we might become more sensitive to these cells as time and generations passed.

Step 2

Imagine a whole population of individuals with no eyes and these photosensitive skin cells around their body. It would be within natural variation that some of these individuals would have more cells than others and, thus, be more sensitive to light. If you’re more sensitive to the light then you’re more likely to know if it’s night and day. You could track prey more easily if and detect if you’re under some kind of shelter. It’s clear these individuals would have a slightly better chance of survival. Over several generations, most of the population would have a higher number of these photosensitive cells.

Step 3

Now take this population of the same species and you can imagine, due to natural variation within a population, some of these individuals would have patches of skin where more of these cells are than others. We have similar patterns within our species, don’t we? Some individuals have patches of skin which contain more melanin than other patches. It’s not hard to imagine that some individuals within the population of this unnamed species. So, now, these individuals would be able to get their light sensitivity from one or two specific places on their bodies rather than all over. They could also be more sensitive to the light and be able to point whatever body part happens to have these patches towards something to figure out if it’s emitting light or not. It’s clear these individuals would have more advantages than others at detecting light and, therefore, would survive longer and, thus, reproduce more. It’s easy to imagine that, over several generations, this population might be filled with individuals who have similar photosensitive patches.

Step 4

It would be within natural variation in this species that some parts of their skin may not be perfectly smooth. Take a close look at your skin and you’ll notice there are peaks and valleys all over. It’s not perfect. Some of these imperfections are acquired but many are inherited. Now, what if one of these valleys happens to fall on one of these patches of photosensitive skin? Take a look at the figure below to see a cross section of what this would look like.


Yeah, it’s overly simplistic, I get that. But notice that, were light to be shown at an angle with this arrangement, some of the cells would be illuminated and some would not. We now have directional capabilities for these skin patches because of the way light would be angled! Granted not very good directional ability but, before, we had none at all. Clearly, these individuals have a selective advantage and would survive longer to pass down their genes for this.

Step 5

Moving on, imagine if a thin layer of transparent skin cells are put over the patch. It isn’t really too hard to imagine since our own epidermis is transparent. Now this patch of skin cells is protected and now we can use our primitive eye longer without it getting damaged. Again, these individuals would survive better than others and these genes would be passed down.

Step 6

From here, some of the transparent skin cells will be thicker than others. Now, if the variation of thickness is relatively evenly spread, that’s not going to do anything. But, you can imagine that some individuals will, by sheer statistics, have a thicker layer near the center than on the outside – again within normal variation of a species. Now what will happen? Well, what happens to light as it passes through a lens of a similar shape? I’m sure you’ve seen a magnifying glass before – it focuses the light. Now, on top of having much better directionality, we have a clearer picture of what we’re looking at.

I’ll stop here since this is essentially all we need for an eye. Look back and see where we started. We started with an entirely blind species and went from no eyes to at least something that serves the function of an eye. Current archeological evidence suggests that the eye itself actually evolved in parallel several times so “our” eye is not the quintessential eye. Indeed, it should be noted that this is not our eye. Actually, this is precisely the evolution of the eyes of a specific family of marine animals – mollusks. Jelly fish would be the first step – just a random dispersal of photosensitive cells. Octopi and squid happen to have the most sophisticated eyes, even better than our own, of the group. All throughout their phylogenetic tree you can see each of these stages of an eye. And, keep in mind; we did all of this using only the natural variation within a single species. There’s even one deep ocean species that has a version of an eye inside a transparent skull.

Are there other “examples” of irreducible complexity? Sure. But they all fall into the same pattern that I just laid out. The whole idea that the eye has to be fully formed in its current state to be useful ignores the actual evolutionary pattern of the eye. In short, this form of irreducible complexity arises from an ignorance of the process of natural selection and natural variation within a species.

Next up – The Bacteria Flagellum.

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