The previous replies to this I think where a bit to lofty, or a bit to snooty. I'll try and get it down to where you can understand it.
You know, evolution works kind of like doggie families. The Great Dane goes to visit the Cocker Spaniel one afternoon. And a little while later there's a whole batch of puppies that are bigger than the Cocker, and have longer hair than the Dane. But then the Dane visits another Dane, and you have a bunch of puppies that look just like the Dane! Isn't that special!
Now you have TWO bunches of doggies that don't look like each other. Great big ones with short hair. And medium sized ones with longer hair.
All you have to do is repeat the process for a few million generations, and you'll have two bunches of animals that are different species.[/babytalk]
Just as an asside, there's been lots of discussion on these threads about macro vs. micro evolution. I propose that we can see macro evolution, in the human community right now. Downs syndrome people do not have the same DNA count as normal people. Therefore (please don't anyone take offense here), they are technically not human. I'm not sure the results when Downs people have children, if they can. And it might be said that this genetic mutation is a "failure". But if instead it resulted in a stronger, or smarter, or some other mutation that was more successful, I believe that in a few short number of generations you would see an entirely different "humanoid" species.
This is a very good example. Thanks for posting it. There are literally hundreds of human mutations
which infrequently come to light. Most of them are evolutionary dead ends.