Creationists have often said that microevolution has been observed in nature, but that macroevolution has not. The correctness of this statement depends on the definition of those words. (Perhaps speciation counts.) But the following simple question expresses pretty much the same idea:
"Why have no new major groups of living things appeared in the fossil record for a long time?"
-- Alabama State Board of Education textbook insert, 1995
A famous biologist gave this answer:
In zoology, "major groups" would be called phyla - a phylum being a category such as mollusks, which includes snails and shellfish; echinoderms, which are starfish, sea urchins and so on; chordates, which are animals with spinal cords, including ourselves; arthropods which include insects and crustaceans. The question is, "Why have no major ones appeared in a long time?"
Well, major groups don't and shouldn't, according to the Darwinian Theory, just appear. They evolve gradually. Major phyla are different from each other, though ancestrally they were like brothers. They diverged and became separate species, then separate families, then separate orders. It takes time to do that.
Think of this analogy. Suppose you have a great oak tree with huge limbs at the base and smaller and smaller branches toward the outer layers where finally there are just lots and lots of little twigs. Obviously the little tiny twigs appeared most recently. The larger boughs appeared a long time ago and when they did appear, they were little twigs. What would you think if a gardener said, "Isn't it funny that no major boughs have appeared on this tree in recent years, only small twigs?"
-- Richard Dawkins, Oxford University