Domestication
Though the majority of domestic geese are descended from the Greylag Goose (A. anser), two breeds are direct descendants of the Swan Goose: the Chinese Goose and the African Goose. These breeds have been domesticated since at least the mid-18th century – perhaps even (in China) since around 1000 BC. They vary considerably from their wild parent in appearance, temperament, and ability to produce meat and eggs; the most conspicuous feature is the prominent bill knob.
Charles Darwin studied goose breeds as part of his work on the theory of evolution. He noted that the external differences between Chinese geese and breeds descended from the Greylag Goose belied a rather close relationship:
"The hybrids from the common and Chinese geese (A. cygnoides), species which are so different that they are generally ranked in distinct genera, have often bred in this country with either pure parent, and in one single instance they have bred inter se."
Read more about this topic: Swan Goose