Tribal society vs civilized society
Hunter-gathers are living in small tribes. They survive in group. When come to competitions with other tribe, it is about group survival and selection. So in barbaric (hunter-gather) society, tribe elimination or genocide is often rule than exception. The way they compete is very similar to that of wild animals.
However, in civilized society, human subjects are equivalent to domesticated animals. So called civilized societies were often ruled by king or emperors in early days. The rule of game in civilized society is productivity and taxation. Highly productive individual domesticated human or animal is selected to survive in such condition. Non-compliant non-productive barbaric human or wild animal would be rid of. As ruler or breed selector, it does not matter what ethnicity or breed of subjects are. What is really matter is productivity. Thus, in civilized society, ruler and subjects can have different ethnicity or be from different origin. England have been invaded numerous times. But original Celtic blood survived under different rulers as long as they survived brutal taxation imposed by increasing demanding successive rulers. New ruler either by inheritance or conquering often want more wealth than previous ruler. Higher taxation will be imposed on subjects. Only highly productive subjects can survive. Only obedient subject (law-abiding) can survive just as domesticated animal. The net result of thousands year of social Darwin evolution is highly productive civilized (domesticated) obedient citizens.
Ruler's greed for wealth to higher tax to higher productivity of subject creates evolutionary cycle leading to high material wealth in civilized society. Civilized productive subjects are descendants of brutal exploitation.
When English established its empire, it was about wealth and taxation. If local subjects are taxable, they can survive like Hong Kong. If they were nontaxable barbarians, they would be replaced by domesticated species, English settlers in north America, Chinese in Singapore, ects. To ruler, it does not matter what subjects are. Only matter is tax. Same thing happened to domesticated animals against wild animals. Domesticated wheat or rice are replacing wild plants around world because they are useful to civilized society. Corn or potato in new world survive onslaught of old world invasion because they are productive to civilized society.
Group behavior based tribalism is true for people with short history of civilization. Group behavior is different story for civilized people, which is more of loyalty to the master who might have different ethnicity.
In civilized society, a royal king is more interested in his subjects loyalty and productivity than their ethnicity. Thus, WASP less ethnocentric behavior had its histological and evolutionary background.
Tribalism is relic of barbarians. In today, tribalism express itself in form of racism