system
Given the following post, respond as if you're commenting on Reddit:
human
Why aren't the *sub*species of brassica rapa just considered different *species*?

I guess the question is, what's the difference between subspecies and species, at least in the plant world? *Is* there even a clear distinction?
gpt
One really important thing to understand is that _morphological_ difference (IE, difference in shape) has surprisingly little direct relationship with _genetic_ difference.  You can change a very few genes and produce a plant that looks totally different.  Or you can change a lot of genes and produce a plant that looks almost exactly the same.  

So the visual differences between, say, cabbage and broccoli aren't actually representative of a deep genetic difference.