Don't mean to jump in, but I am online and I have done a fair bit of research so I will hopefully be able to provide some answers. And the first is that you are correct in that the "Variatus" platy is a distinct species, Xiphophorus variatus, the "Variable" platy.
Xiphophorus hellerii, the green swordtail, is the type species of this genus that includes 28 distinct species native to areas of Belize, Guatemala, Honduras and (primarily) Mexico. Most aquarium fish (meaning swordtails and platys here) have been developed from three species, X. hellerii (green swordtail), X. maculatus (southern platyfish) and X. variatus (variable platyfish). The genus name is derived from the Greek xipho (sword) and phorus (bearer), an obvious reference to the male swordtail; the species epithet honours an aquarist, C. Heller. Five species described as X. brevis, X. guntheri, X. jalapae, X. rachovii and X. strigatus have subsequently been determined to be conspecifics [= the same species] and these names are now synonyms [= invalid as distinct species].
”Type species” of a genus refers to the species that has all the physiological characteristics which separate it from all species in other genera. Today, the type species is usually the first species bearing the characteristics to be described by science. All species subsequently included within the genus will share all these characteristics.
The original wild swordtail fish is the common green form with minimal red markings, and almost never seen unless imported specifically. Due to cross-breeding with platy (which are in the same genus) from the species X. maculatus and X. variatus, swordtails are now available in a variety of colour forms and finnage including hifin, wagtail, lyretail and tuxedo, with fish that are completely red or mixed red and black in addition to the original green with red.
The subject species is the common swordtail seen regularly in the hobby. There are several other distinct species that are rarely available; Xiphophorus montezumae is probably the one that might be encountered over the remainder. It is highly unlikely that any of the other distinct species will be encountered in the hobby.
Last edited by Byron; 10-17-2013 at 06:56 PM.
Byron Hosking, BMus, MA
Vancouver, BC, Canada
Something we all need to remember: The fish you've acquired was quite happy not being owned by you, minding its own business. If you’re going to take it under your wing then you’re responsible for it. Every aspect of its life is under your control, from water quality and temperature to swimming space. [Nathan Hill in PFK]