Not exactly. They're both species of live-bearing carps, but they're not the same species. Because they're so fertile and similar, though, the two species have been cross-bred to give colour to the swordtail species. The original swordtail was the green swordtail, and it was crossed with a red platy to create red swordtails (That's from "Aquarium Fish from Around the World", by Klaus Paysan). From there, all the different colour varieties grew.
