Genetic drift and natural selection
We'll finish up anything we have left to finish on mutation and migration (including answering any questions) before we move on to investigating what happens when drift interacts with selection. You'll see some surprising results. Favorable mutations may be lost. Unfavorable mutations may be fixed. Balancing selection may increase the rate at which genetic diversity is lost. With a little luck, however, your intuition will be good enough by the time we finish that these surprising results seem understandable, if not obvious.