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Biochemical methods can detect variation at individual genetic loci, making possible the direct assessment of natural hybridization and introgression between fish populations. Protein electro‐phoresis has been used to confirm and extend knowledge of many situations where species hybrids have been detected by morphological analyses. New cases of natural hybridization, including some at the subspecies level, have also been identified. Biochemical studies have provided the first conclusive evidence of natural post F₁ hybrids and of introgression between fish taxa. The strongest cases for introgression have used a combined analysis of nuclear protein genes and taxaspecific maternally inherited mitochondrial DNA variation. Information on the significance of introgression as a source of gene flow between taxa, particularly below the species level where sympatric subspecies and sibling species are involved …