Exactly what it says on the packaging.
Over the course of history, humans have continually and repeatedly manipulated the reproduction of organisms to manifest select traits of preference. This is known as artificial selection. Because at the time, farmers weren’t aware of genes and alleles, farmers selected based on phenotype, rather than genotype. Once they had chosen a trait they wanted to see prevalent in their livestock population, they isolated the organisms already expressing it to breed amongst themselves. By doing so for multiple generations, they ensured the organisms were “purebred,” or solely carrying the selected characteristics.
Since this selection isn’t natural, the target traits aren’t necessarily advantageous to survival, or useful at all, as can be seen with pigeon breeding, a popular hobby in England in Darwin’s time. In that case, they were selecting for size, shape of beak, color, and other such trivialities.
In contrast, plants like kale were selected for more defining traits, in this case the large leaves. Kale, like cabbage, Brussels sprouts, kohlrabi, broccoli, and cauliflower, was developed via artificial selection from wild mustard. Interestingly, though these are all drastically varying vegetables, and many of them are likely not mentally associated with each other, they’re all of the same species: Brassica oleracea.
This is because in artificial selection, the traits selective breeding is based on are all present within one species. This in itself seems obvious, since it’s a requirement of creating viable and reproducing offspring. However, in this case it means that even generations later, the results are manifestations of different alleles, not different genes, and as such their descendants are still of the same species. Furthermore, especially since farmers had no knowledge of alleles, but rather were choosing based on what they saw, they were selecting for many traits at a time, hence the drastic differences between resulting offspring.
