Chorotypes—statistically significant groups of coincident distribution areas—constitute biogeographic units that
are fuzzy by nature. This quality has been referred to in the literature but has not been analyzed in depth or methodologically developed. The present work redefines chorotypes as fuzzy sets from a pragmatic perspective and basically focuses
on the methodological and interpretative implications of this approach. The amphibian fauna in the Iberian Peninsula was
used as an example to explore the fuzzy nature of chorotypes. The method on which this article is based is a widely used
technique to define chorotypes. This method involves the fuzziness that is inherent to the identification between degree of
similarity and degree of membership and includes a probabilistic analysis of the classification for the objective delimitation
of chorotypes. The main innovation of this paper is a procedure to analyze chorotypes as fuzzy biogeographic units. A
set of fuzzy parameters to deal with the biogeographic interpretation of fuzzy chorotypes is also described. A computer
program has been developed and is freely available. History may be related to the degree of fuzziness of chorotypes. In our
example, with amphibian distributions in Iberia, less fuzzy chorotypes could have a historical explanation, and the internal fuzziness of chorotypes increases with their distance to hypothetical Pleistocene refugia.
