Similarities between IE expansion and America colonization
Looking at the map of the Yamnaya admixture in Europe, I recall the situation in North America: In the area that today belongs to USA & Canada, very few original inhabitants DNA was left... but in Mexico; there is a substantial and evident proportion of amerindian admixture. I think this has to do with to causes, both of which could be applied to the expansion of the IE peoples:
a) in the North, the native population was more scarce, and thus, the indoeuropeans wiped off that people, or restricted dramatically their demographics. Further south, in the European SW, there was a more dense population that remained in place in a larger proportion, but that was nevertheless assimilated in many ways to the new comers. So they took the IE speech (with the exception of the basques. But amerindian languages survived in many places of latin-America aswell), but their genetic composition didn't change that much.
b) Interestingly, in Central and Northern Europe there is also a larger proportion of Yamna mtDNA, which suggests a large scale migration that involved entire families -men, women and children. In South America and Southern Europe; the new comers where mainly armored men; that took local wives. This explains the lower occurrence of Yamna mtDNA, and in the basque case, explains the persistence of the pre-IE language.