Poeciliopsis is a genus of live-bearing fish in the family Poeciliidae, native to freshwater and brackish habitats from the southwestern United States down through Mexico and Central America to Panama. Species are small to medium livebearers, from about 1–5 in in the atlas's range, and the genus is a classic model system in evolutionary biology: several species reproduce through unusual all-female hybrid lineages, and the group has been used for decades to study how live-bearing fish evolve different levels of maternal nutrient provisioning to their developing young. The atlas covers 25 species.
The porthole livebearer (P. gracilis) is the genus's main aquarium-trade export, a hardy, prolific fish that has also become established outside its native range wherever it has been released. Several relatives carry real conservation weight instead: the Gila topminnow (P. occidentalis) and Sonora topminnow (P. sonoriensis) are range-restricted fish of the Arizona-Sonora borderlands sustained today mainly through refuge populations and captive-breeding work, and the atlas flags nine Poeciliopsis species for CARES conservation-priority breeding, one of the higher counts of any genus in the livebearer family, reflecting how many of its members are small-range desert and highland endemics rather than the widespread species typical of the family.