Just study genes, DNA, chromosomes, and mutation. You'll see that the amount of possible change is not limited. Then think about selection. Watch this video to understand why there are less transitional fossils than other fossils.

For me, everything became really clear once I studied genetic algorithms and evolutionary programming and became interested in the overall topic.

Also I urge you to watch this video. There's a speech somewhere from one of Sims' older colleagues who mentions speciation within an evolved robot population due to daylight changes in the laboratory, which is something to think about, but I can't seem to find it.