That is, small populations experience large changes due to drift in a short amount of time. In contrast, large populations experience relatively fewer changes over the same period of time. Natural selection is NOT evolution. NS is differential survival and reproduction. It is a process that leads to evolution.
Evolution is defined as change in composition of a population, or descent with modification.
Based on the graph, you know that there has been selection for shorter snout length. That is, shorter snouted individuals have higher fitness, longer snouts have lower fitness. Your graph should be a straight line with a negative slope. If selection is favoring shorter snouts and NOT narrower jaws, then it is likely that that jaws and snouts are correlated traits. The change in jaw width is merely a consequence of the process.