Originally Posted by Northern
The question is, how does the creature know which train to develop? Does the organism receive some kind of signaling that it needs to go in a certain evolutionary direction, or genetic mutations are completely random and it's just that those that accidentally turned out more successful get to live on?
It's the last one. There's a lot of subtlety to it though. I would recommend reading something like Richard Dawkins' "Blind Watchmaker" (I feel that in recommending Dawkins, I don't want to come across as some kind of zealot, I'm not, but his earlier books are a bit more toned down and very well written). You can probably get in on Amazon for the price of a cup of coffee.
The main point is that there's no pre-planned
direction to evolution: mutations happen randomly during reproduction, changes beneficial to the organism reproducing survive in the population and ones that are detrimental to reproduction do not. That's basically it.
a particular change might be beneficial to an organisms chances of reproducing can be more difficult to explain, such as the commonly cited "peacock's tail". Another caveat is that, given an example of an animal that displays a particular feature, you cannot usually work backwards through all the small steps that evolution took to get to the feature (without doing looking at fossils, or close living species, to find examples that match in every way except for the feature in question). These two things are the reason that people can get away with saying things like "That feature is useless, it doesn't help the survival of this animal at all. Therefore I have disproved evolution". Or, "This feature is too complex to have arisen without planning.
Hope that helps. (remember to buy the book, or a similar one, it will help a lot!)