The reason they're so abundant in this part of the world is because they evolved as a plant on lime-rich chalk and soil with high salt content. The covering of soil over these chalk deposits is ideal because that's what they've evolved to live on.
So why doesn't it look like a typical cabbage? Because, from this original wild type, by a process of evolution driven by human selection over thousands of years, humans have produced a whole range of different plants for food, each filling a different function and each exaggerating a particular feature inherent in the wild form. This neatly illustrates how a wide range of morphologies can be obtained by a simple selection process. We did the same with the close relatives of the cabbage, B. rapa, B. nigra, B. napus, B. juncea and B. carinata
The reason we were able to do this is because, at some time in their evolution, B. oleracea, B. rapa and B. nigra hybridized to form three entirely new species, B. napus, B. juncea and B. carinata and this gave a wide range of genetic potential. This is of course, a complete refutation of three creationist assertions; that mutations are always harmful, that new information can't arise, and that evolution can't give rise to new species.
We know this because the three 'ancestral' species are diploid, in other words, they have just two of each chromosome, but the other three have multiples of two of these, termed allotetraploid. This is perhaps easier to understand if we look at the numbers:
|B. juncea||18 (B. rapa + B. Nigra)|
|B. napus||19 (B. rapa + B. oleracea)|
|B. carinata||17 (B. nigra + B. oleracea)|
What happens is that due to a mistake in producing gametes (pollen or ovules) instead of them getting one of each chromosome, they get two. Normally, a pollen from one species can't pollinate an ovule from another because they have a different number of chromosomes so they can't pair up in the resulting zygote. However, if a pollen with double the normal chromosome number pollinates an ovule with double the normal number, there is no problem with chromosomes paring because they are already paired. In fact, this form of tetraploidy is common in plants and even in some animals, especially amphibians and some lizards.
But, on the rare occasions in which it's happened in the Brassica genus, it's produced a new, stable hybrid species which can only reproduce with plants with the same chromosome number. Brassicas are self-fertile, in that they can pollinate themselves, so these new species could have arisen as single events. The rare occasion of a new species arising not by the normal slow process of accumulated small changes over time leading to divergence, but as a single speciation event.
So, humans now had a range of closely related species of Brassica, each of which had something edible such as stems, leaves, terminal and lateral buds and even clusters of flower buds and, over time, by selecting plants with different desirable features and saving the seeds from them to sow next season, we have produced a whole range of food plants from these six species which are derived originally from three parent species. Of course, almost all this took place many years ago by unknown people who knew nothing of the science of genetics but had noticed that if you planted seeds from particularly desirable plants, there was a good chance that you got desirable plants next season. There is no fundamental difference between this selective process and natural selection of course.
It's merely semantic to argue that somehow humans are not natural so therefore this is not natural selection. It is no less natural than leaf-cutter ants farming fungi and so producing a unique species or birds selecting berries to swallow whole and excreting the seeds somewhere else, so causing the right size berries to evolve.
Rich morphotypes of Brassica plants. (a) Morphotypes of B. rapa; top two lines from left to right: pak choi, heading B. rapa, turnip, oilseed, purple pak choi, caixin, mizuna, purple caitai and takucai; the third line shows additional morphotypes or varieties of the previous morphotypes. (b) Morphotypes of B. oleracea; top two lines from left to right: heading cabbage, Brussels sprouts, broccoli, cauliflower, purple cabbage, purple cauliflower, collard; the third line shows additional morphotypes or varieties. Some of the pictures were collected from the Internet.
Maybe a creationist could explain to me why the method of speciation I've outlined above is impossible, given the creationist mantra that new species can't arise by evolution, why these mutations weren't harmful but have resulted in a massive proliferation of the different Brassicae when it is an article of faith to creationists that mutations are always harmful, why the information controlling the different morphologies of the genus contained in their genomes appears to have changed when it is axiomatic to creationism that no new information can arise because it would 'contravene the Second Law of Thermodynamics' (sic) and why the wide range of different types of brassicae have arisen by selective breeding when creationists deny that selection can produce change in allele frequency (i.e evolution).
In short, maybe a creationist could explain why the cabbage family completely refute creationism yet creationists still insist that creationism can't be refuted.
Reference: Feng Cheng, Jian Wu & Xiaowu Wang; Genome triplication drove the diversification of Brassica plants.; Horticulture Research 1, Article number: 14024 (2014) doi:10.1038/hortres.2014.24
Thanks to Facebook user Jay Whitney for posting a link to the above reference.