All it would take is one infected planet. A trader comes to do business he brings back a hugger or 2 that sets it off again. The queen can stow away and populate another planet. In a make up world there is no proof except for what is written by the creator. Sorry.

If we consider evolution to mean a shift or direction in genetic frequency, Aliens have an unbelievably fast rate of change, perfectly suited for their lifestyle. Whenever a new or different species is infected, the resulting alien is different (predaliens, runners). I assume that the differences would be tailored specifically to help the aliens deal with the new species. This means that, while evolution in Terran circumstances takes a long time because the genotypes are being shifted through several generations, Alien evolution is, well, instantaneous. An interesting direction to go with this would be behavioral evolution. Hive behavior has it's advantages and disadvantages, and perhaps a queen *might* (ahem) try a different strategy. Nomadic? Solitary? Specialization instead of numbers? My theory? They are an incredibly advanced species in terms of evolution. They apparently have no problems with any environment without protection. Acid blood as a defense? That would take a long time (physiological changes are a bit different than morphologic changes). I say this because wouldn't it be better to have blood that gave off fumes that knocked humans (or whatever) out so capture and impregnating is easy? With acid for blood, potential hosts are wasted. Their eggs can apparently last indefinitely, yet react to motion. Just the fact that they can, by physical, behavioral, and physiological design, deal with other new and different species indicates that they have been around for a very long time.

True true but also they could be just like bacteria. How they change with the generation. Influenza changes when it comes into contact with a new less deadly antidote.