Does this picture look like a face to you? It’s been generated somewhat randomly, along with tons of others at Mutating Pictures. You decide how much it looks like a face, and the more facey ones get to produce more offspring.

It’s not really natural selection, but it is a good example of how randomness plus feedback can cause certain features to recur. It even works when the feedback is very vague — you’re not saying exactly what it is that’s more face-like, but the more facey ones survive anyway.