The Leptodactylus fuscus species group consists of 25 currently recognized species; within this species group and distributed throughout the Amazon Basin, Atlantic Forests, Gran Chaco, and cerrados is the L. mystaceus species complex. This species complex consists of L. didymus, L. elenae, L. mystaceus, L. notoaktites, and L. spixi. Adult morphologies have been used to distinguish these species from each other except for L. didymus and L. mystaceus (Heyer, 1978; Heyer et al., 1996). Leptodactylus didymus and L. mystaceus are morphologically indistinguishable; the species are recognizable only by the characteristics of their advertisement calls: non-pulsed in L. didymus and pulsed in L. mystaceus (Heyer et al., 1996).

Traditionally, L. mystaceus and L. didymus have been considered "sibling species." The concept of "sibling species" was originally introduced by Mayr (1942: 151) to describe pairs or groups of morphologically identical or nearly identical species; however, in subsequent work Mayr (1976) interchangeably used the terms "sibling and cryptic species" to describe morphologically similar species. Mayr (1942: 151) considered sibling species to be important in understanding the full complexity of animal speciation. In order to differentiate these two terms, herein we take a narrow cladistic methodological approach (i.e., dichotomous speciation) by which we restrict the term "sibling" species to two taxa that share a most recent common ancestor; whereas, the term cryptic (derived from the Greek Kruptos, meaning 'hidden'; Allaby, 1991) species refers to "hidden" diversity and does not necessarily imply close phylogenetic relationship. Thus, the sibling species pair of L. didymus and L. mystaceus assumes two postulates: (1) the taxa shared a most recent common ancestor not shared with other species in the L. mystaceus species complex and (2) the two taxa could represent a recent speciation event (i.e., not enough time has passed to reach morphological differentiation, although this is not a requisite).

Herein, we analyze the genetic diversity among taxa in this species complex to determine if the sibling species L. didymus and L. mystaceus are sister taxa. If the assumptions about sibling species are correct, then we would expect that the two taxa involved would be genetically closer between themselves than with any other closely related species.

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Copyright © 2005 SEH. This paper first appeared in Herpetologia Petropolitana: Proceedings of the 12th Ordinary General Meeting of the Societas Europaea Herpetologica, 12-16 August 2003, Saint-Petersburg, Russia.

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