Poecillia (fam. Poeciliini) is a genus of fish most known for their uncommon trait of bearing live, fully developed young instead of laying eggs. This genus has been the subject of much debate, as many fish now belonging to it were originally classified in other genuses. Such is the way with taxonomy.
Currently, this genus contains such members as the guppy (Poecilia reticulata), the molly and sailfin molly (Poecilia latipinna, Poecilia velifera), and Endler's livebearer (Poecilia sp. "Endler's").
Many members of this genus are popular with aquarium hobbyists due to their hardy and prolific nature. This genus is also the subject of intensive selective breeding and therefore, many unnatural varieties exist in the hobby.
Note, due to the confusion in the original classification of these fish and the fact that many of them can interbreed with other species, many scientific names may exist for the same species. Also, many classified species are not such at all, but rather hybrids of two distinct others.