As population genetics advanced, the reconciliation of Mendelism and Darwinism occurred, leading to an understanding of the genetic basis of _ and _.

Explanation

The correct answer is variation and natural selection. The reconciliation of Mendelism and Darwinism in the context of population genetics led to a deeper understanding of how genetic variation underpins natural selection. Mendel's work explained how traits are inherited, while Darwin's theory of natural selection described how these traits could lead to evolutionary change. The incorrect options either misinterpret the key elements that were reconciled or propose alternatives that do not primarily relate to genetic principles as discussed in population genetics.