The Modern Synthesis describes the fusion (merger) of Mendelian genetics with Darwinian evolution that resulted in a unified theory of evolution. It is sometimes referred to as the Neo-Darwinian theory. The Modern Synthesis was developed by a number of now-legendary evolutionary biologists in the 1930s and 1940s.

The Modern Synthesis introduced several changes in how evolution and evolutionary processes were conceived. It proposed a new definition of evolution as "changes in allele frequencies within populations, " thus emphasizing the genetic basis of evolution. (Alleles are alternate forms of the same gene, characterized by differences in DNA sequence that result in the construction of proteins that differ in amino acid composition.) Four forces of evolution were identified as contributing to changes in allele frequencies. These are random genetic drift, gene flow, mutation pressure, and natural selection. Of these, natural selection—by which the best-adapted organisms have the highest...