The differences between kale and broccoli are visually striking. One presents as a mass of tightly bundled, immature flowers and stems, while the other is a collection of broad, often curly, deep-green leaves. Despite their vastly different appearances, these two popular vegetables share an intimate connection. They both trace their lineage back to the same wild plant, a weedy species native to the coastal Mediterranean region.
Same Species, Different Cultivars
Kale and broccoli are the same species, Brassica oleracea. This scientific classification means they share the same fundamental genetic blueprint and can still interbreed. Within the single species Brassica oleracea, they are categorized as different “cultivars.” A cultivar is a plant that has been specifically bred and maintained by humans for desirable characteristics.
Both vegetables originated from the wild cabbage, which is a biennial plant that stores food in its thick, fleshy leaves during its first year of growth. The human practice of selecting for different physical traits over time led to the striking divergence in appearance. Kale is classified under the Acephala group, which means “without a head,” while broccoli belongs to the Italica group, reflecting its specialized structure.
How Selective Breeding Created the Differences
The process that transformed the wild cabbage into the many vegetables we see today is known as selective breeding, or artificial selection. This method involves farmers purposefully choosing plants with a desired trait and breeding them together, a practice iterated over thousands of years. Early cultivators started by choosing seeds from plants that had the most desirable leaves.
For kale, the selective pressure focused entirely on the vegetative parts of the plant, specifically the leaves. Farmers repeatedly selected plants with larger, more abundant, and more palatable foliage, resulting in modern kale that prioritizes leaf production. Broccoli was created by selecting plants that produced immature flower heads and thick stems. These were plants whose flowers were prevented from fully opening, resulting in the dense, edible cluster of undeveloped flower buds.
The Broader Brassica Family
The Brassica oleracea species includes many other common vegetables that share the same botanical identity. Each of these vegetables represents a different part of the original wild plant that was exaggerated through targeted selection. This single species includes:
- Cabbage
- Cauliflower
- Brussels sprouts
- Kohlrabi
Cauliflower is the result of selection for a highly compressed, undifferentiated flower meristem, which forms its dense, white head. Brussels sprouts were developed by selecting for enlarged lateral buds, which grow into the small, tight heads along the plant’s main stalk. Cabbage was created by selecting for a large, tight terminal bud, where the leaves are packed together on a very short stem.