A large fraction, sometimes the largest fraction, of a eukaryotic genome consists of repeated DNA sequences. Copy numbers range from several thousand to millions per diploid genome. All classes of repetitive DNA sequences examined to date exhibit apparently general, but little studied, patterns of "concerted evolution." Historically, concerted evolution has been defined as the nonindependent evolution of repetitive DNA sequences, resulting in a sequence similarity of repeating units that is greater within than among species. This intraspecific homogenization of repetitive sequence arrays is said to take place via the poorly understood mechanisms of "molecular drive." The evolutionary population dynamics of molecular drive remains largely unstudied in natural populations, and thus the potential significance of these evolutionary dynamics for population differentiation is unknown. This review attempts to demonstrate the potential importance of the mechanisms responsible for concerted evolution in the differentiation of populations. It contends that any natural grouping that is characterized by reproductive isolation and limited gene flow is capable of exhibiting concerted evolution of repetitive DNA arrays. Such effects are known to occur in protein and RNA-coding repetitive sequences, as well as in so-called "junk DNA," and thus have important implications for the differentiation and discrimination of natural populations.