As an iguana owner, back in the heady days just after Jurassic Park hit, and iguanas were big - or, more precisely, they were little - we had lots of conversations on how to keep them healthy and enable them to be big in the future. Iguanas eat tree leaves in the wild, particularly protein-rich leaves from trees related to peas. There's not much like that available in grocery stores in the US.
One thing we rapidly learned was that the majority of 'green leafy vegetables' in stores were actually the same plant. Brassica oleracea is the plant, and it's high in oxalates (that impair calcium absorption, a no-no in iguanas and some humans) and in goitrogens, compounds that interfere with the functioning of the thyroid.
How did one plant come to dominate the produce section - and why can't we spot it immediately as a plant that shouldn't make up all of our (or our iguana's) diets?
Because Brassica oleracea is a master of disguise. It's a science-fiction-style shapeshifter that lurks unseen in the produce section.
You may know it by such names as cabbage, kale, collard, spring greens, kohlrabi, Brussels sprouts, broccoli, broccolini, and cauliflower.
Mustard greens are of the closely related species Brassica juncea. Napa cabbage (won bok), and Chinese cabbages (bok choi, pak choi, pei tsai), rapini and turnips are Brassica rapa. Brassica napus includes the oilseeds (rape, canola) and Russian Kale. Rutabaga, mustard seeds, horseradish, mizuna, radish, cress, watercress, wasabi and daikon are all also closely related.
How does one weedy wild mustard plant turn into kohlrabi (with its swollen stem), a headed cabbage, a leafy cabbage and a giant flower like a cauliflower? Plant breeding over thousands of years. (Not to be confused with genetic modification, despite the latter being described in comment sections of articles on GMOs as mere plant breeding recently; don't be fooled by a Big Ag spin.)
There are a couple of recent reviews of Brassica oleracea and its rise to kitchen supremacy. One was in The botanist in the kitchen and the other in Vox, here. Both are quick reads.
Some of the compounds that the cruciferous vegetables produce in an effort to stop mammals eating them are actually useful to us. They slow down cell division, which gives the plants some anti-cancer properties.
They're an interesting Old World institution.