Why brown people are different shades of brown....

Why are brown people so many shades of brown? If you were raised in a South Asian family I'm sure that you've had to deal with the "color" issue somehow. This isn't a cultural blog, so I'm not going to go there, but I do think that the salience of complexion in South Asian culture makes this new paper, A genome-wide association study of skin pigmentation in a South Asian population (PDF), of more than passing interest. If you plotted a frequency distribution of skin reflectances of South Asians within 2 standard deviations from the median you would see a range from brunette white from nearly black skin. Even eliminating the confounding factor of environment (upper class South Asians often are at great pains to prevent the exposure of their skin to sunlight so that it can stay as light as possible, an option that rural peasants don't generally have), the question is why such a diverse range?. First, the Indian subcontinent is a subcontinent. From the northwest to the southeast you have a diversity of peoples which is second only to Africa (this is often born out by studies of neutral markers). Over the past few thousand years there have been many newcomers to the subcontinent, as evidenced by the languages (Indo-Europeans from the northwest) or lifestyles (rice agriculture from the northeast). It is therefore a common explanation that the variation in complexion in South Asia is a function of ancestry. The lighter skinned populations tend to cluster in the northwest and amongst the upper castes, and these are the groups with the greatest levels of exogenous genetic input (note that there is a great overlap here!). That being said, neutral markers seem to imply that about 90% of the ancestry of South Asians is indigenous at least to the threshold of 10,000 years before the present. In plainer language 9 out of 10 ancestors of any given South Asian was probably a resident of the Indian subcontinent 10,000 years ago. The balance of the ancestry is of course not trivial. It seems likely that the 1 out of 10 ancestors that were exogenous made significant contributions to South Asian culture, from the Indo-Aryan (and likely Dravidian) languages to the Islamic religion which 1 out of 3 South Asians are members of. The story of lactase persistence also suggests that the genetic contribution of that 1 out of 10 was also very significant in driving recent adaptations. But in any case, the studies of skin pigmentation allows us to go beyond folk biology and explore the genetic architecture of this trait. Here is the major section of the abstract: