The single plant species, Brassica oleracea, serves as the common ancestor for an astonishing array of familiar vegetables, including cabbage, broccoli, kale, and cauliflower. The wide variations in appearance, texture, and taste found across these vegetables, known as cultivars, are all derived from one wild progenitor.
The Wild Ancestor: Appearance and Habitat
The original, uncultivated form of the plant, Brassica oleracea var. oleracea, is commonly known as wild cabbage. This plant is a tall biennial or short-lived perennial. In its first year, it develops a stout, loose rosette of large, thick, grayish-green leaves. These fleshy leaves help the plant store water and nutrients, enabling it to survive in challenging environments.
Wild cabbage is native to the coastal regions of Western Europe and the Mediterranean, growing on the limestone sea cliffs of Britain, France, Spain, and Italy. The plant exhibits a high tolerance for salt and lime, allowing it to thrive in the well-drained, nutrient-poor soils of the cliffs where competition is limited. In its second year, the plant shifts its energy to reproduction, sending up a woody flower spike that can reach up to 1.5 meters tall, topped with clusters of characteristic yellow, four-petaled flowers.
The Process of Artificial Selection
The transformation of this unassuming wild plant into the varied vegetables we eat today is a powerful example of artificial selection. This process is directed entirely by human preference and desire, unlike natural selection, which is driven by environmental pressures. Early agriculturalists in the Eastern Mediterranean region, possibly beginning as far back as 4,000 years ago, started propagating individual wild plants that displayed slightly more palatable or useful traits.
The mechanism involved consistently selecting and breeding plants that possessed a small, beneficial variation, such as slightly larger leaves or a thicker stem. Over many generations, this cumulative human choice exaggerated certain features of the plant, leading to the emergence of distinct morphotypes. Ancient Greek and Roman texts from the sixth century BC onward document the cultivation of early forms, suggesting the domestication process was well underway. The process was modular, meaning different groups focused on different parts of the plant, resulting in distinct developmental lineages.
The Diverse Cultivars: Selection Based on Plant Part
The wide range of cultivars within Brassica oleracea can be grouped based on which specific plant part was enhanced through selective breeding.
- The earliest cultivated forms, such as kale and collard greens, belong to the Acephala group (“non-heading”). These leafy vegetables were developed by selecting plants with large, tender, and abundant foliage.
- Cabbage (Capitata group) resulted from breeding plants where the terminal bud became tightly packed and heavily layered, creating a dense, round head.
- Brussels sprouts (Gemmifera group) were developed by selecting for greatly enlarged lateral, or axillary, buds that grow along the upright stem.
- Broccoli (Italica group) was selected for its immature, edible flower clusters and thickened stalks.
- Cauliflower (Botrytis group) represents an extreme selection for the flower-producing part, where the undeveloped flower stalk and meristem are highly condensed into the dense, white curd.
- Kohlrabi (Gongylodes group) was created by selecting for a swollen, globe-like stem base that develops above ground.