Charles Darwin’s On the Origin of Species is a delightfully sophisticated account of evolution. But the core ideas are not that difficult to understand. Variations in traits in individuals arise by chance, due to what we now think of as mutations in genes. Some of those trait variations are functionally better adapted to part of the environment than others. Individuals so advantaged will tend to survive and leave more offspring. Accordingly, the advantage, and the frequency of the genes causing it, will increase from generation to generation. Conversely, genes causing less advantageous or harmful variations will decrease in frequency. That is natural selection.