If you don't understand something this simple, ......
First off, it's not something solely described by the theory of evolution. It's just a normal fact described by creationists, IDers, evos and whoever else has any ideas out there. Things reproduce after their own kind but not exact duplicates. I doubt anyone has not known this since the beginning of recorded history. It's certainly not novel to Darwinists.
Secondly, just because you use the same word "evolution" to describe heritable change and the theory of evolution does not make them the same thing.
Thirdly, there is absolutely no reason to assume microevolution which we know decreases genetic diversity is macroevolution which requires an increase in genetic diversity and massive one at that is microevolution writ large.
In other words, despite evos like you insisting otherwise, merely saying something doesn't make it so.