Dr. Houde notes that when female guppies drop fry, they also release a pheromone (useful in nature) that alerts the male guppies to the need to court the female and with her cooperation he can mate with her. It must be done within about 48 hours of the dropping of the fry for the mating to effectively impregnate her.

A pretty good case, on the basis of the shape of the gonopodium of guppies and Endler's being exactly the same and DNA samples being the same can be made for that. Endler's have been described as Poecilia wingii. That may not hold up though. I might want to see them as different species and will certainly keep strains apart (as would most responsible aquarists) but what I desire and what goes on in the real world are often different. ;)

Mollies are close enough to guppies that one wouldn't be surprised to find that their females also release a pheromone and also need to mate within so many hours of birthing young. The rest of their behavior just passes the time.