Subspecies of Eurasian Beaver
Historically, eight subspecies of Castor fiber were described, one for each of the eight 19th–20th century refugia where the species never became extinct. The basis of the differentiation was morphological, largely based on very small differences in cranial morphology, but has been recently refuted based on genetic studies. In 2005, Durka et al. showed that only two evolutionarily significant units (ESUs) exist based on mitochondrial DNA studies, a western phylogroup (C. f. gallicus, C. f. albicus and C. f. fiber) and an eastern phylogroup (C.f. ssp., C. f. tuvinicus, C. f. pohlei, C. f. birulai). In addition, Ducroz et al. found that even in the more genetically diverse eastern phylogroup, the degree of genetic divergence was below thresholds considered sufficient for subspecies differentiation.
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