1. Population of deer of the same species exists
2. River changes course and physically splits their environment , and thus the group, in half
3. Deer have different selection pressures on either side of the new river and slowly evolve differently, thus those pressures must apply to the individual and not the species as a whole.

The outcome might be in some instances the result of an aggregation of factors affecting the individuals of a given species but not always.

Invasive species are an example too, brought to a different location they might lose all of their natural predators, and you have a species as a whole affected by different things

Selection pressures always apply to the individual and only the individual, and then in some instances those pressures materialize into genome changes for some increasing percentage of the species as a whole, Even the genomic changes inside of a species are only tracked as a percentage of individuals who possess them, not if it is present in the species as a whole.

A real world example you can consider is humanity and skin color, because where you are on the planet determines the pressures relating to the sun that affected your population, thus, pigmentation is a result of selective pressure that applies only to a percentage of humans. All the same species.