Race crossing 1-2

due to trade. War. Napoleon Columbus. Marriage alliances. Industrialisation and urbanisation education jobs transport.

Great resistance and banned marriages Jim crow law which banned interracial marriage.

Cultural taboo on it and even in India caste marriages create honour killing.

But race does not exist and sometimes interbreeding is favourable. 

Andamanese tribals with mainland indians are mixed now.

USA race crossing with blacks, chinese, whites and native Americans. Barack Obama is an example. 

In india intercaste marriages are still 5% so, race crossing is still low. 

Historically, David Reich study on Indian's genetics shows how indians are a mix of various populations from central asia, Dravidian and old natives.