Over at Gene Expression, Razib Khan has up an interesting post that compares and contrasts the genetics of South Africa’s Afrikaner population with New England whites:
Afrikaner ancestry is overwhelmingly Northern European. But as you see in the PCA above they are notably African and Asian shifted when compared to their potential ancestral populations (I used Dutch and German individuals above). For me this is the part [of the study] that is important, if not surprising:
The individual with the most non-European admixture had 24.9% non-European admixture, and only a single Afrikaner individual (out of 77) had no evidence of non-European admixture…Amongst the 77 Afrikaners investigated, 6.5% had above 10% non-European admixture, 27.3% between 5 and 10%, 59.7% between 1 and 5% and 6.5% below 1%.
So about 87% of Afrikaners in their sample had between 1 to 10 percent non-European ancestry. As suggested by genealogical evidence, genetics indicates this is a relatively recent admixture, occurring during the 17th and 18th-century. The early decades of the Cape Colony. It’s a mix of diverse Asian and African components. In some ways, it seems that the non-European ancestry in modern Afrikaners is just the same phenomenon which gave rise to the Cape Coloured population, which is a mix of European, Asian (Indian and Austronesian) and African (Bantu and Khoisan).
But, this result is more interesting in light of how it contrasts with another case. Also in the 17th-century, there emerged another European settler society on the edge of a vast ocean rooted in a deeply Calvinist faith. By this, I mean the colonies of New England. Though New England has been reshaped by later migrations, between 1640 and 1790 30,000 English settlers expanded and grew into a region with 750,000 Americans. In the early 19th-century, New England spilled out over much of the northern swath of the United States of America, in part due to the fact that the fertility of New Englanders was quite high (the early Mormons were fundamentally a New England-derived subculture).
And yet unlike the Afrikaners or the whites of Latin America, the scions of New England have no non-European ancestry. One might argue here that this is due to the lack of opportunity, as the number of slaves in New England was always very low, and there were no native peoples. King Philip’s War falsifies the latter contention. There were numerous native people. At least initially. But the New Englanders were very efficient and effective at marginalizing and exterminating the native peoples of the region. To a far greater extent than occurred in the South.
Razib suggests that the almost nonexistent level of admixture between the English settlers in New England and their counterparts in Africa can be attributed to two things:
- The New Englanders came over from England as families and congregations, with a favorable male to female ratio
- The New Englanders had a confident culture ready to assert itself as itself from the beginning (or as Razib puts it, “by the latter portion of the 18th-century New England was unique because it was beginning to see itself as not just a complement of the metropole, but a potential rival”).
While Razib takes an angle informed by population genetics, the general observation that the English colonists in general and the New England settlers in particular had much lower intermarriage rates with indigenous peoples than colonists from other places is not new. One of the most interesting hypotheses for why this is so put forward in Alfred Crosby’s Ecological Imperialism: The Biological Expansion of Europe, 900-1900. If you have never read this book before I strongly recommend it—Crosby was a talented prose writer and a thoughtful historian. I blame the lack of attention the book has received outside of historical circles on its wordy and political sounding title. This is a shame. Most of the good ideas you find in Jared Diamond’s Guns, Germs, and Steel actually come from Crosby, though Crosby is both the better historian and the finer writer.
The question Crosby sets out to answer in Ecological Imperialism is this: why did European colonists almost completely displace native cultures and populations in Australia, New Zealand, North America, and Argentina, create mestizo populations and cultures in Central America, the Caribbean, the Andes, and coastal Brazil, but fail to even make a dent in most of Africa, interior Brazil, or Asia? For Crosby, the answer to all of these questions can be found by looking at the weeds.
A wild fact: the majority of the wild flora in the San Joaquin Valley did not exist in California two centuries ago. For most of temperate Canada and the United States, Australia, New Zealand, and Argentina the ratio of invasive to indigenous flora is similar, ranging from 30-60% of the flora in the places most of the population lives. The spread of disease from the Old World to the New is now quite famous: less famous is the spread of European weeds (dandelions, Kentucky bluegrass, nettles, white clover, etc.), pests (rats, roaches, houseflys, etc), and other disruptive species (earthworms, pigs, peach trees, and so forth) that transformed the landscape into something decidedly more European. Most interesting of all for the original question: the displacement of entire local biomes by European biota happened only in those places where the colonists displaced the local people.
Crosby suggests the first caused the second. The Europeans’ biological footprint in Africa—outside of the Mediterranean climate of the Cape—is near non-existent. African diseases killed European crops, animals, and colonists. European weeds, which had an advantage in North American biomes whose plants had not faced grazing herds since the ice age, were outclassed by African grasses that evolved next to an even larger contingent of grazers than found in Europe. In Mexico and the Andes the environment was not hostile to the Europeans entirely—but the climate was different enough from Europe, and the local biota resilient enough in face of European imports, that Spanish colonists could not transplant their entire mode of agriculture to Mexico whole-sale (as they did in Argentina). The colonists in these areas needed indigenous crops and styles of farming to survive. They depended on the indigenous farmers for those crops. Eventually, they would intermarry with them. 
Of all the “New Europe” zones discussed, the New England colonists had the least number of hiccups in setting up an Old World society somewhere in the New. The New Englanders famously faced far lower rates of disease than settlers in other parts of the Americas, and within a generation of the founding were living generally healthier lives than the English at home. They were also one of the most resistant to adopting local sources of food. Explains David Hackett Fisher:
“The Puritans of Massachusetts created one of the more austere food ways in the Western world, For three centuries, New England families gave thanks to their Calvinist God for cold baked beans and stale brown bread, while lobsters abounded in the waters of Massachusetts Bay and succulent game birds orbited slowly overhead… the coastal waters of New England teemed with mussels, oysters, lobsters, and clams. The rivers were choked with salmon and shad. Wild fowl flourished in abundance. Native delicacies such as glasswort sprouted along the seashore and fiddleheads carpeted the woodlands.
The Puritans showed little interest in these delights except when driven by hunger to consume them. Shellfish was regarded with grave suspicion. Shad roe, a gourmet’s delight, was used as a fertilizer. In the first year John Winthrop complained when he was compelled to eat oysters and wild duck instead of the staples of old England. “My dear wife,” he wrote, “we are here in paradise though we have not beef and mutton.“
That was true in 1630, when Winthrop wrote those words. It was not true in thirty years later, when the New Englanders had successfully transplanted the entirety of their agricultural system to their new home. Their success here is my favored explanation for why their marriages with non-colonists were so few.
 Razib Khan, “Afrikaner Genetics Show How Unique New England Culture Is,” Gene Expression (27 February 2019).
 This question is posed most clearly in Alfred Crosby, Ecological Imperialism: The Biological Expansion of Europe, 900-1900 (Cambridge: Cambridge University Press, 1986), 145-148; for his answers see 148-195; 269-304. Another famous book that I have not read is also relevant here: William Cronon, Changes in the Land Indians Colonists and the Ecology of New England (New York: Hill and Wang, 1985).
 Bernard Bailyn, The Barbarous Years: The Peopling of British North America: The Conflict of Civilizations, 1600-1675 (New York: Random House, 2012), 425-226.
 David Hackett Fisher, Albion’s Seed: Four British Folkways in America (Oxford: Oxford University Press, 1989), 135-137.