Delineating human populations is one of the trickiest and most sensitive subjects in genetics. The reason is the fact that even scientific thinkers can be influenced by their cultural baggage. For centuries, Americans with the help of race scientists differentiated human populations into Negroids and Caucasoids. However, a closer look at this classification scheme will show that its origin and initial utility was to differentiate those groups who could be enslaved in perpetuity and those who could not.
However, at the same time, America's founders were challenged by European monarchists, who derided the profound irony of the so-called American revolutionaries' "all men are created equal" doctrine, when the U.S. was itself based on slave labor.
So what does that have to do with scientific classifications? Pressures within American society made it necessary to lump the Irish, English, Germans, French, and other European immigrants into a single race, when previously each nationality would have been classified as a race unto itself. But the psychological nature of these pressures created a marketplace among academics to rationalize slavery by providing theories showing that Africans were inferior to Europeans. Thus, slave traders were doing the former a presumed service by removing them from heathen-dom and placing them within a civilized setting.
By the mid=18th century, academics were manipulating skull sizes to show that the ex-slaves had smaller brains and were thus cognitively inferior to Caucasians. When that effort was eventually shown as a sham, the 20th century impulse to establish a racial hierarchy replaced craniology (the study of skulls) and craniometry (the measurement of skulls) with more refined methods of categorizing its citizens -- intelligence testing. It should come as no surprise that the cultural groups from which the testers emanated, always scored high on such tests, and marginalized minorities scored low.
By the 21st century, scientists realized that the highly abused racial categories might serve a function in differentiating disease susceptibilities in diverse ethnic groups. But we now understand that race (Negroids, Caucasoids, Mongoloids), is not scientific at the genomic level, even though biomarkers can be searched for in any person's genome to identify his or her so-called race.. Human beings can however be seen as "ecological niche populations". That is, their ancestors will over time have developed genetic variants or Single Nucleotide Polymorphisms (SNPs) that allowed them to survive in challenging environments. But these SNPs do not correspond to racial classifications. The SNPs that allow human populations to live in malarial areas bring together West Africans, with Southern Europeans and coastal East Indians. SNPs that make it possible for West Africans to survive the tropical heat of the sodium-deficient interior will not be carried by coastal Africans, who have the limitless availability of ocean salt for their sodium nutritional needs. Thus, ancestral populations that have adapted to the same ecological "niche" will have more in common with one another than other ways of delineating populations. This is because their genetic variants will align with the unique ecological conditions required to survive in whatever environments they inhabit.
This approach can also be classified as "ancestral genomics" because the SNPs that make unique environments livable for certain groups, may be maladaptive in other environments to which they have been translocated. A ripe field of research is the ancestral genomics of African-Americans of slave descent. They are admixed with Europeans but also appear to have differences in sodium-metabolism from coastal West Africans. Is the fact that the ancestors of these Black Americans remained healthy on 200 mg/sodium/day and their U.S. descendants consume 3400 mg/sodium/day, the reason why 75% of adult African-Americans suffer from salt-sensitive hypertension? These are the types of questions that explorations into ancestral genomics will over time answer.