The species? Brassica oleracea. Its other varieties include cabbage, broccoli, savoy, kohlrabi and Brussels sprouts, to name just a few commercially relevant examples. How did one species of plant come to be so diverse? Selective human breeding and exceptional genetic diversity.
Above: A garden plot with several varieties of the B. oleracea at the Jardin botanique de Montréal | Photo Credit: Montréalais | CC BY-SA 3.0.
Over at Vox, Jason Stromberg has a nice little primer on cultivars (i.e., varieties of plant selectively bred for specific traits), including background on how some of the more popular B. oleracea cultivars came to be:
In Ancient Greece and Rome, people began growing the plant in their gardens. To maximize the amount of food they got from it, they preferentially planted seeds from plants that grew more leaves, and after many generations, this sort of artificial selection produced a leafy version of wild mustard that looked more like modern-day kale or collard greens.
Later (sometime after the year 1600), farmers selected for variants of the plant that produced enlarged leaf buds in particular. After many generations, this led to plants with huge heads of tightly rolled leaves — plants that we would call cabbage.
One of my favorite plants happens to be a cultivar of B. oleracea. Popular among students first learning about fractals, Romanesco broccoli sprouts buds in a gorgeous logarithmic spiral, making it one of the more striking (and genetically interesting) patterns in nature: