America is sometimes described as a melting pot, and in many ways we are. Although first and second generation Americans tend to cluster with those of the same race or nationality, within a few generations that distinctiveness and separation begins to disappear. The process of amalgamation is accelerated by our increasingly mobile society.
The American south is the region I know best.
The south of the early 20th century was largely populated by two distinct ethnic groups. One group was composed of descendants of ancestors who had arrived from Britain and western Europe, mostly prior to the American Revolution. They were predominately Protestant. The other group consisted of descendants of African slaves. Mixing between the two groups was socially forbidden, although it did take place to some degree. This is evidenced by the fact that American blacks have an average 20% European genome.
My own family was perhaps typical of an old southern family of European descent. All but 2 of my 32 great-great-great grandparents arrived in America before the Revolution, and these 2 came shortly thereafter. Based on an analysis of the surnames of these 32 individuals, I am 50% English, 22% Scottish, 13% German, 6% Welsh, and 9% unknown. (Note that many of my English and Scot ancestors had Anglo-Saxon, Norman-French, or Scandinavian roots, but these racial accretions were overlaid upon a base of ancient Briton and Gaelic peoples.)
A Family Tree Autosomal DNA analysis performed in 2017 suggests that my family origins are 43% British, 37% West and Central European, 9% Scandinavian, and 4% Southeast European.
There have been significant changes in the ethnic composition of the American south since World War II. There was a considerable migration of whites from the north to the south and by blacks from south to the north. Also, there has been an influx of peoples from foreign lands. Prior to the late 20th century new immigrants had avoided the south like the plague. That is no longer so, and the racial admixture among southern whites is much greater than it was in the past.
I believe this is a good thing. I know that it has enriched my own family. I now have great grandchildren, nieces and nephews with parents from an increasing variety of lands and cultures.
Whatever our racial mix, we are all blessed and proud to be Americans. That is our new ethnicity.