[1][2] Poeciliopsis monacha is endemic to northwestern Mexico where it is present in the upper reaches of streams and arroyos on the western side of the Sierra Madre Occidental mountain range.

[3] In addition to the stress factors induced by trying to survive in summer in decreasing and warming waters, some pools have very low oxygen levels, exacerbated at night by the lack of the photosynthetic oxygen produced by plants, and often lower at the pool base where the fish feed, or under the mats of algae that tend to accumulate.

Under these circumstances, the fish may resort to "surface skimming", a behaviour that allows them to obtain enough oxygen at the interface of water and air.

[4] In fact this all-female breeding line can be maintained indefinitely in the laboratory by repeatedly back-crossing the offspring to males of P. lucida and P.

[4] In the headwaters of the Fuerte River in northwestern Mexico, it is found that only the bisexual form of P. monacha is present in the highest waters, and the proportion of unisexual individuals increases progressively downstream from here.

These findings indicate that clonal reproduction in a vertebrate lineage is not necessarily transient, but can achieve a substantial evolutionary age.