How do Mendel's ideas about the inheritance of traits relate to Darwin's theory of evolution by natural selection?

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.