The genetic variation within the deer population can result in them developing differently. It can also result in them developing different adaptations, for example super senses, that are used to help them survive.
Deer that have gone through genetic variation and have learned to adapt to their environment are more likely to reproduce and survive. While others, aren't because of how weak they have become.
The population of Deer will only increase if the population has learned to adapt. In learning to adapt develop learn-able senses or have gone through genetic chnages that have allowed them to survive.
More white tailed deer have reproduced. As the deer reproduce their offspring they are passing down their genes that have already gone through these genetic changes along with other adaptions they have learned.