Let me give you a non-biological example of what I mean - the humble zip (or zipper or zip fastener or whatever it happens to be called in your part of the world!). For a zip to work, you have to have:
- A series of teeth that are shaped in a way that allows them to interlock with each other,
- A way of attaching the teeth to each piece of cloth that ensures that they are all nicely and evenly spaced with just the right gaps between them, and
- A device that you slide up and down that is shaped in the correct way to lock or unlock the teeth.
What's more, if you want to make improvements to an existing zip, you need to change all three of those factors in parallel. If you changed the shape of the teeth, but didn't change their spacing and the shape of the slider to match then it would just get stuck.
This means that making and improving something like a zip is really difficult to do by random processes because the changes have to coordinate with each other very closely. If a piece of molten metal happened to randomly cool into a rudimentary slider, for example, it would be (at best) totally useless unless you happened to already have some adequately shaped teeth that had randomly managed to become arranged in a nice regular pattern along the edges of the two sides of the cloth. However, those teeth would have been long since rejected because, without the slider, they would be very unlikely to be a more efficient solution than just using buttons (have you ever tried to do up a zip without the slider bit?).
This type of thing occurs all the time when you are designing or building things. You are forever designing two or more things to work together in various ways. Even if you are changing something that exists already (as we were discussing last time), you regularly have to change multiple parts of the system in a coordinated way rather than just making one isolated change.
The same applies in nature. Take your blood circulation system, for example. For this to be of any use to you, you need to have blood, a heart, and a series of tubes for the blood to flow through (among other things). Without any one of those, the other two are not likely to be that useful.
Or take sexual reproduction. There would be no point in developing some kind of "male" living thing unless a "female" developed at the same time. And if you did get them both to come into existence at the same time they would be at a reproductive disadvantage - they would have to find each other before they could reproduce, while all the other non-mutants around them could go on reproducing individually without any such hurdle. The advantages of sexual reproduction only become apparent after a number of generations by which time the whole idea would have been selected out. What's more, the reason sexual reproduction is advantageous is that it helps to minimise the spread of mutations. Exactly the opposite of what you want for evolution.
Now, maybe we could come up with theories of how these things kinds of things could come about by gradual, independent changes - for example a set of gradual, independent changes that would allow a button or lace system to slowly change into a zip (with each step being better and more efficient that the preceding one). And similarly for the circulatory and reproduction systems. Not to mention the myriad of other mutually dependent systems that exist in nature.
So I won't say that it is impossible for these things to happen (impossible is way too contentious a word!). But it sure makes it a lot more difficult and less likely. There is a very narrow path that all these changes have to follow. Just think of all those wasted mutations that would have been just the ticket, but were selected out because some other dependent change hadn't come along yet!
Note: This is part of a series of posts about why I believe in God. See my post "You Believe that Stuff???" for more info and links to the other related posts.
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