Biology was dogged for nearly thirty years by this problem. There was a rift between 'evolutionists' and 'geneticists', because the former claimed that mendelian variation (like yellow/green peas, blood types, etc.) was irrelevant for natural selection, and the latter argued that one would have to adapt evolutionary reasoning to mendel's laws (in the best case, but usually the 'geneticists' were by then fixists, that is, they denied evolution).
Ronald Fisher, Sewall Wright and John Haldane developed between 1918 and 1932 the mathematical basis for understanding evolution based on the mendelian laws of inheritance. Population genetics became the basis for what was called the Modern Synthesis and settled the above described dispute.
Any of their work is a bit hard to read, but instead of diverting to "popular" simplifications that do more harm than good, I would suggest reading the introductory chapters of the classical books they (and Motoo Kimura, another great developer of evolutionary theory) have written.