First, selection can be rapid.

The change in European skin pigmentation over five to ten thousand years represents very rapid evolutionary change on the timescale of human evolution.

This speed of change suggests that the selective pressure was strong — that the advantage of lighter skin in the specific context of Neolithic and Bronze Age European populations was substantial.

Second, population migration is more important than in situ evolution.

For much of the twentieth century, the default model for explaining population differences was adaptation — populations living in different environments evolved different characteristics over time.