That "gradient" was caused by interbreeding, Arabs are darker due to importation of slaves and southern europeans are tanned due to medieval Muslim invasions.
Yes, it is like a Ring Species group, but does not rise to the level of speciation. Interbreeding is evolution, when populations change genetic characteristics.
The alleles for dark skin are more prevalent where filtering the sun is a survival advantage. You see, since there is a selection against darker skins in the North, eventually lighter skins prevail (as long as people are living outside). Today it might not work exactly the same way since we have Vit D supplements and many people never see sunlight in the modern world.
But there was a gradiation present prior to the relatively recent historical events you cite. For instance, you might site the example of the Eskimos in Alaska. But these are a recent immigration within the last 70,000 years over the land bridge that existed in the vicinity of the Bering Strait.
It took a lot longer than that for natural selection to turn into the gradient we see. But I would bet there is selection going on for lighter skin in native eskimo populations, especially if compared to native south american populations.
The gradient was caused by interbreeding. I will agree with that. However, it still is a case where allele frequencies have changed over time, and hence is an example of human evolution, regardless of cause.