Homologous chromosomes interact during meiosis by means of proteins involved in recombination and in the recognition and repair of mismatched base pairs. Recombination proteins bring homologous chromosomes or chromosomal regions together by facilitating the search for DNA homology and by catalyzing strand exchange between homologous molecules or regions. Mismatch recognition and repair proteins act as editors of recombination and appear to disrupt those DNA associations that contain mismatched base pairs. Thus, it may be that, as chromosomes diverge in their primary sequence and become increasingly polymorphic, recombinational interactions leading to chromosome pairing and recombination tend to be inhibited. Decreasing homologous interactions within and between chromosomes will clearly contribute to maintaining the integrity of individual chromosomes and may ultimately lead, as a result of sterile meioses, to the reproductive isolation of closely related species.