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Jacek M. Szymura, Nicholas H. Barton, The Genetic Structure of the Hybrid Zone between the Fire-Bellied Toads Bombina bombina and B. variegata: Comparisons between Transects and between Loci, Evolution, Vol. 45, No. 2 (Mar., 1991), pp. 237-261
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Szymura et al. (1991) studied this question.
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