A large part of the current population of South America are descendants of both native Americans and Europeans. In contrast, in north America the intermingling of native Americans and Europeans was significantly less common. What are the historic reasons for this difference?
One reason was that the "Anglos" brought their own women with them. For instance, there were women passengers on the Mayflower. And twelve years after the settlement at Jamestown, there was a boatload of women (in 1619), followed by many more.
The Spaniards also had more "multicultural" dealings, as noted in the comments above. The Spanish religious ideology was one of converting the "natives," which in practice meant absorbing them into Spanish society and intermarrying with them once they converted. English society did not have similar mechanisms for absorbing children of mixed parentage. In the rare situations where Anglos produced "half breeds" with Indians, the children almost always became "Indians" rather than Anglos.
"American" men DID produce children with African slaves. But they were consigned to the lowest levels of society (until modern times), and didn't "mix" with the rest of American society. "Segregation today, segregation tomorrow, segregation forever" was the North American ethos as late as the 1960s.
I think you could argue for a fundamental difference in the Spanish and English attitudes toward "colonization" of the Americas, based partly on different historical backgrounds and partly on where they started.
Once the Spanish got to the Americas proper (as opposed to the Caribbean islands), they encountered populous agricultural and urban societies, including sophisticated kingdoms and empires (especially the Aztecs and Incas). The model provided by Cortes and Pizarro thus became (crudely speaking): "Conquer the native kingdoms and rule over them (and marry their women). When you want more land, go on expeditions to conquer it." The strong Catholic missionary impulse of the time also meant a strong emphasis on converting the natives to Catholicism, which made it easier for Spaniards to intermarry with them. (Note that both Cortes and Pizarro married local princesses.) This was arguably a continuation of the Reconquista model: conquer the Moorish kingdoms and rule over the (non-Christian) locals as you try to convert them. It probably also was affected by the multinational nature of the Spanish/Hapsburg empire in the 16th Century: if Flemish and Italians and Germans could all be subjects of the Emperor, so could Indians in the Americas.
For the English colonists (as well as, e.g., the Dutch in New Amsterdam), the model was more "Buy or steal small amounts of land from the natives to set up your own independent settlements. Don't interact with the natives aside from trade, and when you want to expand, buy or steal more land and push the natives away (or kill them)." Thus, despite the interactions and occasional intermarriage that did take place (and the occasional efforts at missionary activity), English colonists tended to remain segregated in their own societies, building their own towns and gradually expanding at the expense of native populations.
Another possible factor: The lands the Spanish conquered included numerous long-established agricultural societies (with genuine cities), which generally meant they had higher populations. This probably ensured that the natives remained relatively numerous compared to the European conquerors/settlers despite the massive die-offs from Old World diseases. The lower population density in most of North America (fewer agricultural societies, and none of them with actual cities) probably meant that it was easier for English colonists to outnumber the natives over time.
Interracial marriage is not so uncommon. Probably the absence of same-race women played a role, but Portuguese were also well known to marry local wives in Africa, while British and French people usually didn't
The fact that Portuguese and Spaniards have a multirracial origin (arab, celtic, roman, goth) might also have some influence.
One theory is that King Phillip's war was the cause. The idea behind this theory is simple: When the pilgrims arrived at Plymouth Rock in 1620, the Native Americans aided them in establishing themselves, and helped them not to starve, by teaching the pilgrims, people who had lived as refugees in the urban Netherlands for a generation, to learn farming and survival skills. They even celebrated the first thanksgiving together. In New England, generally speaking, the contact between the Europeans and Natives Americans were more than cordial - there was intermarriage, and conversion to Christianity, and peaceful coexistence.
By about 1670, the mixing of the two populations, combined with declining Native American populations and quality of life, set the stage for "King Phillips War" Supporting Source and Wikipedia links. The common narrative goes like this: Metacom, a Native American nicknamed "King Phillip" by the settlers, started attacking and slaughtering the colonial population. The English colonialist responded in kind, slaughtering the Native American population. Both sides killed whichever "enemy" they came across, including women and children. Persons of mixed heritage were victims of both sides. 5% of the settler and 40% of the Native American population died. The general slaughter led to well defined racial identities that didn't exist before the war, and set the stage for future separate ethnic trends in North America.
There is, of course, an opposing theory presented in this book that the war was instead a civil war intended to increase British control of the region, which used "divide and conquer" strategies, split what was a unified community to increase colonial power. One results of the increase British control was marginalization of the Native population, and a stronger racial identity for the settlers. The British then used racial identity through the colonies to maintain power.
"Divide and Conquer"
Especially agree with the above comment on King Phillip's War...which was then followed by the French and Indian War. In the US South treatment of native populations was quite brutal..with both Northern and Southern "brutalities" converging as US Settlers started pouring into Ohio then heading West. Allan W Eckert wrote I think excellent albeit fictional accounts of the early era. The stories only grow tragic and more dramatic as "the United States" crosses the Mississippi into the Wild West.