Evolution is where some characteristic becomes part of the genetic makeup of virtually all members of a species. The characteristic appears spontaneously in an individual, through some random mutation and gets passed on to its offspring. If the characteristic provides some advantage to the individual, that individual is more likely to survive and can pass on the characteristic to more offspring than other individuals. This is the key. The characteristic has to result in more offspring being produced than by other individual members of the species. Eventually, over a very long period of time, the members of the species who have the characteristic begin to outnumber those who don't, simply because they are reproducing at a greater rate. Finally, all members of the species will have the characteristic and the evolutionary change will be complete.
There are variations, of course. If two groups of the same species are isolated from each other, then characteristics that evolve in one group don't necessarily evolve in the other group. Over a long period of time, the two groups can diverge enough so that they, in effect, form two different species.
Humans, of course, are subject to the same evolutionary process as any other species. That is why we aren't still apes (at least some of us, anyway). However, we have managed to arrest the process.
Let's say I was born with some characteristic that was new to the species, one which gave me an advantage. For example, say I could tolerate large amounts of Ultra-Violet radiation, or say I could thrive on 1/4 the calories that a normal human needs, or say I could produce my own Vitamin C. Any of these things would mean that I would be healthier and theoretically could produce more offspring in my lifetime than any other human.
But what happens? Would I actually produce any more offspring? No. I would still have my 2.3 children, just like everyone else. So individuals with this fantastic new characteristic would never outnumber those without it and it would never become part of the genetic makeup of the entire species.
So, why is this? Basically, it is because we are monogamous and because our medical advances mean that I don't have to have a large family in order to ensure the survival of a few of them. If humans reproduced to their capacity, then the fantastic new characteristic would have a chance of becoming part of the genetic makeup of the species, since I would out-reproduce any other individual. But that is not the case, so the fantastic new characteristic would be limited to my limited number of offspring.
Yes, there are some places in the world where evolution IS continuing - basically in primitive cultures where medicine isn't as advanced and where the number of offspring is greater. I have been talking here about our "advanced" civilization.
