The Lunatic said:
So, it's off topic, but I'm going to come down hard on this because it's frankly dangerous to indulge scientific racism even a little..
"Caucasian" was a term from the common "three races" system of scientific racism, popular in the 19th century. Like all forms of scientific racism, it was bullshit, and although people sometimes use the word as shorthand for white people, there is no such thing, scientifically speaking, as a "Caucasian".
Indo-European is a language group which we theorize derives from a common source language called proto-Indo-European (or PIE). Because of the distribution of Indo-European languages, it is generally believed that over about 3000 years, Indo-European speaking people migrated outwards from somewhere in the near east (there are various theories as to where, but the dominant one depicted by your map has PIE originate on the Pontic steppes) to both Europe and Central Asia.
This is also backed up by archaogenetic evidence, which suggests that originally, Indo-European speakers did have a distinct ethnic identity, and that some of their genes were assimilated into populations where they migrated. However, beyond a few genetic markers which we use to track their migrations, we know almost nothing about them.
However, this is where reality begins to separate from scientific racism and myth. There's no evidence whatsoever that Proto-Indo-Europeans were the ancestors of white people or were responsible for creating some kind of "caucasian" race. For comparison, Neanderthals went extinct tens of thousands of years before the Indo-European migrations, and we can still identify distinct trends based on the relative presence of their DNA in European and Middle Eastern populations versus those elsewhere in the world. PIE migrations definately influenced the genetics of people they migrated into, but they became one of many, many factors influencing the genetics of people in that region.
The reason why North Indians and South Indians sometimes look a little different is much less dramatic than an invasion of magic white people. There isn't a magical barrier preventing people crossing the Indus. Much of North India is very rich and fertile, so naturally people have always migrated from Central Asia into India. It is possible and indeed likely that some of these migrations were invading armies, there have after all been several invasions of India by Altaic conquerers during our recorded history. But there is no evidence of a single "Aryan invasion" of India.
"Aryan", incidentally, is the etymological root of the word "Iran". It literally just refers to the region and its people, not to any mythical race.