Mustard and cabbage belong to the same genus Brassica, but they are two different species. What defines them as separate species?

Genus is the first name, and the species is the second name of a scientific name of any organism.

Genus ranks higher compared to species in the hierarchy of taxonomy.

Explanation:

Brassica balearica: Mallorca cabbage

Brassica carinata: Abyssinian mustard or Abyssinian cabbage, used to produce biodiesel

Brassica elongata: elongated mustard

Brassica fruticulosa: Mediterranean cabbage

Brassica hilarionis: St. Hilarion cabbage

Brassica juncea: Indian mustard, brown and leaf mustards, Sarepta mustard

Brassica napus: rapeseed, canola, rutabaga

Brassica narinosa: broadbeaked mustard

Brassica nigra: black mustard

Brassica oleracea: kale, cabbage, collard greens, broccoli, cauliflower, kai-lan, Brussels sprouts, kohlrabi

Brassica perviridis: tender green, mustard spinach

Brassica rapa (syn. B. campestris): Chinese cabbage, turnip, rapini, komatsuna

Brassica rupestris: brown mustard

Brassica spinescens:

Brassica tournefortii: Asian mustard

i think it is true in my reasonment ..