The original distribution of the family was the Southeastern United States to north of Río de la Plata, Argentina, and Africa, including Madagascar.

Due to release of aquarium specimens and the widespread use of species of the genera Poecilia and Gambusia for mosquito control, though, poeciliids can today be found in all tropical and subtropical areas of the world.

In addition, Poecilia and Gambusia specimens have been identified in hot springs pools as far north as Banff, Alberta.

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. 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.

The conflict hypothesis suggests the placenta is a nonadaptive byproduct of genetic "tug-o-war" between the mother and the offspring for resources.