Mixandburn, plants do evolve! Here\'s a very brief rundown, and here is a more detailed look at a few different plants.

It's interesting that you ask about apples. What we refer to as "natural selection" is a process involving environmental pressure--competition for limited resources, mates, etc. Human activity has also applied selective pressure to species of plants and animals, and the apple is a great example. For over two thousand years, people have harvested apples, and have affected the shape, flavor, and size of apples themselves by selecting which apple seeds to plant and cultivate. Today's apples--juicy, very sweet, and huge--are very different from the apples people grew a thousand years ago, which historical evidence indicates were much smaller and more tart.

Let's say you plant two apple trees--one on either side of a hill. The apples from those two trees will be a little different, because the immediate environments of the two trees are different. Now, let's say you pick the apple you like better, and then plant two seeds from that apple--again, one on either side of the hill. Again, you wait a few years and pick the "better" apple, and again you plant the seeds. After many generations, if you compare the apple you have with an original specimen, they may be quite different, because you influenced which apple trees reproduced.

If you look at the DNA from both a black man and an asian woman, they will both be similar. All humans are the same species, regardless of their external morphology--it's just that in the tens of thousands (probably more) of years that people have lived on earth, different people were exposed to different environmental pressures depending on where they lived. Hence, for example, Norweigans are quite pale (there is little direct sunlight in Norway for much of the year), and natives of the Andes have vascular and respiratory systems capable of dealing with life at very high altitudes.

Mud doesn't really undergo selective pressure, because it doesn't consume anything, doesn't reproduce, and isn't alive. Hence, mud doesn't really undergo selection or evolution.