A small, livebearing fish commonly kept in aquaria, with current taxonomic classification:
Superorder: Acanthopterygii (spiny-rayed fishes)
Family: Poeciliidae (livebearing toothcarps, or livebearing topminnows)
Species: Poecilia reticulata (Peters 1859)
These fish were first collected in 1856 by the German ichthyologist Wilhelm C.H. Peters in Rio Guaire, Venezuela, but not described until 1859, when, for unknown reasons, only the females were described. The name given was Poecilia reticulata (Poecilia meaning that it resembled the family Poeciliidae, and reticulata describing the reticulated pattern of scales). Specimens collected in 1866 by the English botanist Robert John Lechmere Guppy were described as Giradinus guppyi, a name that stuck until a 1913 revision of the family Poecilidae changed it to Lebistes reticulata. A later revision in 1963 changed it back to Poecilia reticulata. At least a dozen synonyms for this species popped up between 1860 and 1920. I've noticed the name Lebistes reticulata in wide use in aquarium literature.
Guppies exhibit a high degree of sexual dimorphism. Males grow to about 2.5 cm and have significant, highly variable coloration in fins and body; females grow to 5 cm. and have much less coloration, particularly in the body. The variability has been reduced in domestic strains, commonly termed fancy guppies, due to inbreeding.
The native range is the West Indies and northern South America from western Venezuela to Guyana, although they've been introduced to warm-water sites in many parts of the world either accidentally or to control mosquitos. The U.S. Fish and Wildlife Service considers guppies a threat to cyprinids and killifishes in western states, particularly in desert springs.