Join Date: Oct 2010
Location: Victoria, Australia
Guppies can be very hardy (less so the expensive fancy strains). There are populations of feral guppies in Australia in places where the temperature gets very low. It is possible that a population of fish can slowly acclimate to less than ideal temperatures. Those who cannot tolerate the colder water die off leaving only the hardiest individuals behind to reproduce.
A lot of people do this with rainbows here, leaving a group in a pond over winter and when it gets to spring, only those fish who have survived the colder temperatures are around to breed. However, it doesn't mean that they aren't still tropical fish.