Some traits are simpler than others-blood type for instance isn't affected too much by environment, and has a fairly classic recessive, dominant and co-dominant setup. Other traits, like height, is affected by diet. If you look at historic clothing and skeletons, it is obvious that people were shorter even 100 years ago on average. It is unlikely that we have developed genes for height suddenly, a more reasonable explanation is that our ancestors had the genes that would have allowed a taller stature if they had the necessary nutrients.
The evolutionary pressure that led to this is simple-a species has an advantage if they can respond to changing conditions, instead of requiring a narrow range of conditions. An example: if humans adapt to a poor diet by shifting nutrients to life functions instead of being automatically used for building height, then more people survive in poor conditions, since a higher stature may be an advantage but is not necessary to life.
Man isn't the only species where multiple factors play a role-I don't know of a simple species other than some single cell organisms.
