It's my understanding that biologists generally
have abandoned the idea of "race." I have.
A couple of notes on what you wrote:
"the loss of skin pigmentation in Caucasians"
Btw, a tremendous number of Caucasians are dark.
In fact, the Caucasus Mountains are not very far north. I am sure I am part Caucasian, and some of my siblings get great tans, while others don't.
The event you cited took place thousands of years ago...maybe even thousands of generations. The numerical increase of the trait may have occurred as follows:
In ancient times, folks with less pigmentation apparently gained an advantage in chilly northern climes during ice ages. Many of our ancestors did not meet their vitamin D need through eating the organs of ocean mammals; they were able to manufacture D in sunlight. Perhaps you have heard this before... that babies wrapped in furs all day in winter could develop rickets as they grew up if they had so much pigmentation that it inhibited Vitamin D production during the few minutes of sunglight on their face. Their parents didn't know about the need for sunlight.
Thelighter skin let in more sunlight for vitamin D production. This gave
lighter skinned babies an advantage...one of many examples of how a trait increased in frequency.
What you wrote is important to the general question. Thanks and congratulations.