N.Drew,
first you should know that my original link to berkely wasnt aimed at you. You had a specific question about humans, my link was more about the basic understanding of evolution.
Obviously the simplest answer to why species branch out is because time and environment allows it. Life has existed for millions of years, and over that period, the look and structure of animals has changed drastically because the environment has changed drastically.
Picture if you will, a steady gradient from Yellow to Red. Now in this spectrum of colour there are hundreds of different hues, but there is clearly a steady progression from yellows, to oranges, to reds. If you were to zoom in on the gradient and compare one hue to the one next to it, you would not see any difference. Work your way up the gradient with this method and you will see the same thing. Each hue looks exactly the same as the one preceding it - but you would not think that the gradient was a solid block of yellow, or a solid block of red.
My point being, gradual changes over time can create a very different and unexpected result from the original starting point. Just like the gradual process of a child becoming an adult.
Obviously, since beetle and fruit fly evolution is relatively young, they have evolved, but still remain fruit flies. Over a longer period of time I would expect this to branch out even further.
And watch my video link on page 1 if you haven't already.