For a less US-centric perspective, regional differentials in development persist even for countries with 'homogenous' populations. Italy is a canonical example, with significant differences between the rich North and the poor South. By US ethnic standards, Northern and Southern Italians are basically indistinguishable clones, though Italians themselves may beg to differ. See also Germany, Belgium, Poland or Romania.

https://commons.wikimedia.org/wiki/File:Map_of_Italian_regio...

https://en.wikipedia.org/wiki/List_of_German_states_by_GRP_p...

https://en.wikipedia.org/wiki/List_of_Belgian_provinces_by_G...

https://en.wikipedia.org/wiki/Poland_A_and_B

https://commons.wikimedia.org/wiki/File:Romania_-_Nominal_GD...