As others have noted, the Endler livebearer,
Poecilia (Acanthophacelus) wingei, closely resembles the common guppy,
Poecilia reticulata, and some authors have considered both to be the same species. The two species do hybridize and produce fertile offspring, as was mentioned by qwe123. However, Schories et al. (2009) determined that
P. wingei is now unequivocally defined by the molecular phylogeny as a valid species. A third distinct guppy species,
Poecilia (Acanthophacelus) obscura, was described from Trinidad by Schories et al (2009); you can read the paper here if interested:
http://www.mapress.com/zootaxa/2009/f/zt02266p050.pdf
The three guppy species have been placed in the sub-genus
Acanthophacelus, that is considered to be generically different from all other taxa in the Poeciliinae subfamily, forming a cryptic species complex.
The common name "Endler's Guppy" honours John Endler who "discovered" the species in 1975, although it was actually first discovered in 1937 by Franklin Bond. The species is feared to be extinct in the habitat lake in which it was discovered due to pollution from garbage. A second possible site was mentioned by Dr. Endler but he was unable to find it. Schories et al. (2009) document this species occurring in the Cumana region.
It was not until 2005 that the species was scientifically described as a distinct species by F.N. Poeser, M. Kempkes & I.J.H. Isbrucker. The genus name
Poecilia comes from the Greek
poikilos meaning "with different colours" and the species epithet honours the ichthyologist Dr. Ojvind Winge. Poeser et al. (2005) proposed re-establishing the genus name
Acanthophacelus, originally erected by Eigenmann in 1907 and subsequently deemed a synonym for
Poecilia, as a sub-genus of
Poecilia. As mentioned above, Schories et al. (2009) have followed this provisional sub-classification for the (now) three guppy species on the basis of generic differences from the other species in the subfamily.
