For today’s answer, we’re turning to Harold McGee’s On Food and Cooking. According to McGee:
The sweet potato is the true storage root of Ipomoea batatas, a member of the morning glory family. It is native to northern South America, and may have reached Polynesia in prehistoric times. Columbus brought the sweet potato to Europe, and by the end of the 15th century it was established in China and the Philippines. China now produces and consumes far more sweet potatoes than the Americas, enough to make it the second most important vegetable worldwide.
There are many different varieties, ranging from dry and starchy varieties common in tropical regions, some pale and others red or purple with anthocyanins, to the moist, sweet version, dark orange with beta-carotene, that is popular in the United States and was confusingly named a “yam” in 1930s marketing campaigns.
Did you hear that? What we think of as a “yam” here in the United States is actually a sweet potato. Back to McGee:
True yams are starchy tubers of tropical plants that are related to the grasses and lilies, a dozen or so cultivated species of Dioscorea from Africa, South America, and the Pacific with varying sizes, textures, colors, and flavors. They are seldom seen in mainstream American markets, where “yam” means a sugary orange sweet potato. True yams can grow to 100 lb/50 kg and more, and in the Pacific islands have been honored with their own little houses. They appear to have been cultivated as early as 8000 BCE in Asia.
SO, in short: sweet potatoes (Ipomoea batatas) and true yams (species of the Dioscorea family) are completely different plants. Both species are old; both are tubers; both come in various textures and colors. And to add to the confusion, if you’re in a grocery store in the United States, what is labeled a “yam” is most likely a sweet potato. You may have never actually eaten a true yam, which you are more likely to encounter in international or specialty markets and in African and South American cuisines.