 In the later period (1966–present), the advent of allozymes and molecular genetics permitted investigation of a less biased sample of genetic polymorphism. The modern characterization and analyses of allozymes and single nucleotide polymorphisms still retains the early emphasis on natural selection as the single most important evolutionary force shaping the hereditary material. Differently put, adaptive explanations for patterns of species divergence tend to be favored over non-adaptive explanations (see recent review of the causes of the rapid diversification among species of reproductive genes by Dapper and Wade [2020].)