MuSingMIDI for Mac

From Geneffects:

The MuSing^TM software suite includes beat generating programs designed to produce striking, purely original rhythms. MuSing uses the powers of evolution to grow families of beats. Everything you create is entirely original - there are no prefabricated beats or patterns! For the hobbyist just learning to make beats or for the experienced musician looking to take a new artistic direction and break out of the mold - MuSing offers unsurpassed creative potential.

Here's how it works: Once you open MuSing, it creates five beats. These first generation beats may not be the exact sound you are looking for, but one of those beats may be closer than the others. You select the beat you like best then hit the "evolve" button. MuSing gets rid of the other four and then replaces them with mutations of the beat you selected. The mutations could change drum patterns, which drums go with a pattern, velocities or any number of other parameters. You may find that you like one of these new beats more. If so, you select the one you like and keep repeating this process until you have evolved the perfect beat!