Complex sentence: Zanzibar had a population of around 230,000 Africans--some of whom claimed Persian ancestry and were known locally as Shirazis--and also contained significant minorities in the 50,000 Arabs and 20,000 South Asians who were prominent in business and trade.
Split into simpler sentences: Zanzibar had a population of around 230,000 natives, some of whom claimed Persian ancestry and were known locally as Shirazis. It also contained significant minorities in the 50,000 Arabs and 20,000 South Asians who were prominent in business and trade.