While mandarins are natural, in the sense that they grow on trees planted in soil, the popular varieties sold in the supermarket are the product of decades of human intervention. In other words: they are heavily designed.
There is, I think, a fascinating tension in our understanding of food, and this quote gets at the heart of it. In part, it has to do with our use of the term ‘natural’ as an approximation for a whole bunch of intentions. When we call a food natural we probably also mean healthful and wholesome and pure, and on some level believe it could not hurt us. But almost nothing we eat is actually natural, in the sense that it exists in the same form in nature. All food, almost by definition, has co-evolved with humans for thousands of years, being chosen out of a wild population for some favored quality and selectively propagated through the generations. What the series of posts at Design Decoded points out is that now we have added marketability to the list of qualities under selection for a growing number of produce items, which is, I guess, the natural progression of things.