As a supplement to my series on simple evidences for evolution, I thought I would also write occasional articles addressing common misunderstandings regarding evolution. In this one, we'll address that common stumbling block, "But how could something so complex have come about by chance? The probability is just too small."
To address this, we need to understand the role that randomness plays in evolution. Imagine you are standing in a room with a white and black checkerboard tile floor. Each tile is one foot (30 centimeters for our international friends) on a side. In your hands you hold a deck of cards. You hurl the cards into the room, scattering them all over the floor. By simple chance, we should expect that roughly half the cards land on black squares and half on white squares.
However, in this particular room, the black squares are actually not tile at all; they are "liquid hot magma," as my friend Dr. Evil would say. Any card that touches a black tile is incinerated, while the ones that land on white squares are safe.
Now imagine that an observer enters the room at this point. She might be given to wonder how it is that all the cards in the room are on white squares. Surely this could never have happened via a random process! What are the odds that all the cards in the room would just happen to be on white squares? Surely some intelligent being placed the cards there! Right?
As we have seen, our friend would be incorrect in her conclusion. No one intelligently placed the cards on the white squares. They were scattered by random forces. It just so happened that the environment killed off the cards that landed on the squares that were not conducive to card-existence. The only cards that were left were the ones that, by chance, happened to land on safe squares.
Now let's move our analogy to the natural world. The hand that hurls the cards is reproduction. Every day, entire new batches of cats, dogs, yeast, tulips, wheat, and so on are being thrown out into the world, millions of little creatures that are different in various ways from each other and from their parents. Random forces create the diversity.
The floor in our analogy is the local environment. All of creatures land in a different spot and try to make it in the world, but some of them will be swiftly wiped out if their design is not appropriate for the environment. It's cold and harsh, but that's the way it happens, just like with the cards. The only remaining creatures will be the ones that the environment "selected" as being fit.
Notice that I say that it is the local environment that does the selecting. This is because the environment changes dramatically from place to place. What might be fit in one spot is not fit in another. Consider the lion in Africa versus the lynx in North America. Neither would survive for long in the other's environment. By wiping out all but the most successful versions of each batch of offspring, the environment causes a species to gradually become more and more suited to it, because the ones that happen to be more suited to it are successful. One can easily imagine a feline progenitor living in a cold, snowy environment becoming more and more honed toward lynx-like features over generations because the offspring with the longer hair, the bigger feet, the tufts of hair in their ears, and so on, will be more adept at survival in that harsh environment.
So what we have are two competing forces at play. Random chance in reproduction throws out a million different candidate versions of a species. The cold, hard realities of the local environment "choose" which of those versions are worthwhile. It's a lot like selective breeding, except in this case the breeder (the local environment) brings the axe down hard on any that he doesn't like.
Therefore, random chance does play a very important role in evolution. However, it is meaningless without an environment to do the selecting. Chance only provides the raw material for the environment to select from. To say that evolution does not make sense because of the chance involved would be to vastly misunderstand what evolution is. Indeed, evolution can only occur because of random events.
SNG
Appendix: Links to articles so far in my "evidences for evolution" series:
Retroviral sequences: http://www.jehovahs-witness.com/6/86797/1.ashx
Cytochrome c: http://www.jehovahs-witness.com/6/87238/1.ashx