Strains are started from seed. I assume pollination is somewhat controlled. Several Chinese trumpets are strains, like Pink Perfection and Golden Splendour. That's why the depth of the pink color in Pink Perfection varies from one plant to the next, they're all some shade of pink, but some are very pale and others may be deep, rich pink. If a particularly dark color shows up in a batch, it may be cloned and named.