Poeciliidae is a family of fresh-water fish which are live-bearing aquarium fish (they give birth to live young). They belong to the order Cyprinodontiformes, tooth-carps, and include well-known aquarium fish such as the guppy, molly, platy, and swordtail. The original distribution of the family was the southeastern United States to north of Rio de la Plata, Argentina, and central and southern Africa, including Madagascar. However, due to release of aquarium specimens and the widespread use of species of the genera Poecilia and Gambusia for mosquito control, poeciliids can today be found in all tropical and subtropical areas of the world.
Although the whole family Poeciliidae is known as "live bearers", there are egg-scattering species with external fertilization in addition to the true live-bearing viviparous and ovoviviparous species, which have internal fertilization and the eggs hatch before being laid, so the female gives birth to live young. All African species are egg-layers, and (with the exception of the members of the genus Tomeurus) all American species are live-bearers. This distribution suggests Poeciliidae antedate the split between Africa and South America 100 million years ago, and live-bearing subsequently evolved in South America. Poeciliids colonized North America through the Antilles while they were connected 44 million years ago. Poeciliids then moved to Central America by the Aves land bridge. When South America reconnected to Central America 3 million years ago, there was some dispersal southward, but South American species did not move into Central America.
Among the live-bearing species, differences in the mode and degree of support the female gives the developing larvae occur. Many members of the family Poeciliidae are considered to be lecithotrophic (the mother provisions the oocyte with all the resources it needs prior to fertilization, so the egg is independent of the mother), but others are matrotrophic (literally means "mother feeding": the mother provides the majority of resources to the developing offspring after fertilization).
Members of the genus Poeciliopsis, for example, show variable reproductive life history adaptations. Poeciliopsis monacha, P. lucida, and P. prolifica form part of the same clade within the genus Poeciliopsis. However, their modes of maternal provisioning vary greatly. P. monacha can be considered to be lecithotrophic because it does not really provide any resources for its offspring after fertilization - the pregnant female is basically a swimming egg sac. P. lucida shows an intermediate level of matrotrophy, meaning that to a certain extent the offspring's metabolism can actually affect the mother's metabolism, allowing for increased nutrient exchange. P. prolifica is considered to be highly matrotrophic, and almost all of the nutrients and materials needed for fetal development are supplied to the oocyte after it has been fertilized. This level of matrotrophy allows Poeciliopsis to carry several broods at different stages of development, a phenomenon known as superfetation. Because the space for developing embryos is limited, viviparity reduces brood size. Superfetation can compensate for this loss by keeping embryos at various stages and sizes during development.