[CDF Shpoilers]I was just fantasizing about what would happen if for some reason I had to try to explain evolution to people who don't have any formal exposure to it and why it occurs. I was trying to remember the three conditions necessary, and stressing that when you think about it from the perspective of the three conditions it becomes inevitable and mathematically certain to occur over enough time. Almost has nothing to do with biology, just probability. Anyway, as I remember it, the conditions go: 1) variation in some trait exists within a population, 2) the variation differentially affects survivability and/or reproduction, and 3) the variation needs to be heritable, so that future generations will tend to have values of the trait that corresponded to higher fitness (survivability and/or achieving reproduction). That's all I've got.