A few weeks ago I posted my fixations which put the spotlight on some elementary population genetics formalisms. I thought I'd put my elementary Illustrator skills to use and throw up the canonical diagram use to elucidate basic population genetics in many classrooms. Here you see two demes, two breeding populations, and the arrows show the factors which increase and decrease genetic diversity. While mutation increases genetic variation, genetic drift tends to remove variants. Similarly, while balancing selection, whether it be frequency dependent or heterozygote advantage, tends to perpetuate variation, positive or negative selection either constrains or sweeps across populations to maintain or generate homogeneity. Add to this the added parameter of deme-to-deme gene flow, and you have another parameter can speed up evolutionary processes, or, if you accept the Shifting Balance model, dampen it.