I'm not sure what about the question. If you're asking about genetic variation within a specific species even in a more or less common area you're asking about the adaptive nature of species to specific areas. What we know is that there is genetic variations in all species (including humans) that will individually favour different circumstances. So as climate and other variables change, members whose genetic profile permit greater adaptiveness will thrive while those without that specific profile may not. That said, this becomes an extremely complex subject when rigorously examined, and tested. The answer here is only in the most general terms. Simply, built into all species, plant and animal, is a broad general genetic compliment and different members may better thrive (and thus reproduce) in different contexts.