Potato Phylogeny

The Curious Case of Potato Phylogeny: A lesson in convergent evolution

BY MAYA SEALANDER

Walk into the grocery store and head toward the produce section. You will find yourself passing displays of meticulously stacked fruits and perfectly organized vegetables. In the far back corner, you’ll eventually reach the potatoes. Sitting there in their tuberous glory, the tan russets, the appropriately named yukon golds, maybe a pile of red or purple-skinned spuds. And next to them are the sweet potatoes. Or are they yams? Aren’t they the same thing?

The humble potato can trace its roots (er, tubers) to the Andes Mountains of South America, where over 360 cultivated and wild varieties of potato flourish (1). Today’s potato plants, Solanum tuberosum, share a genus with tomato, eggplant, and over 1,500 other species (2). Variations within the species come from cultivars, plants selected over time for specific traits like tuber size, color, and shape in a process similar to animal breeding. The natural variation lends itself to the diverse physical characteristics of the potato varieties we see today. Despite not being closely related, potatoes, sweet potatoes, and yams are an example of convergent evolution, a phenomenon where organisms evolve similar traits despite their lack of relation. In this case, all three groups have developed starchy roots or tubers. Yet, comparing related species such as the tomato and potato show that only the potato grows large tubers. Convergent evolution appears everywhere in biology, although there is a lack of consensus surrounding its precise definition and cause (6).

So the next time you’re out shopping, think about the millions of years of evolution that separated sweet potatoes from potatoes, or how yams are phylogenetically closer to grasses than potatoes. Despite their genetic differences, potatoes, sweet potatoes, and yams ended up in the present day as delicious bundles of starch for plants to store energy and for us to eat. While it may not seem obvious at first glance, the produce section contains a rich history of convergent evolution.

Now that we’ve covered the potato, what about its sweeter cousin? The sweet potato, Ipomoea batatas, is a vine from a botanist’s view (3). The sweet potato and potato both belong to the order Solanales, and their respective families diverged around 88 million years ago, during the late Cretaceous period, long enough ago that dinosaurs were still roaming the earth (4). What is labeled in the grocery store as a yam is most likely a sweet potato, but true yams are not even relatives to sweet potatoes! The genus Dioscorea contains the species of yams most commonly grown today, and they are so distantly related to sweet potatoes that the smallest taxonomic group containing both is the angiosperms, or flowering plants, which encompasses almost 300,000 of the 374,000 known plant species (5).

yam sweet potato potato