Heat tolerance could be a very useful indicator both at phenotypic and genetic level. Please take a look at http://www.kongressband.de/wcgalp2010/assets/pdf/0625.pdf or http://dx.doi.org/10.3168/jds.2013-6611
The opposite i assume might also apply in temperate conditions, tolerance to low temperatures could be a very useful indicator both at phenotypic and genetic level for adaptability.