They arose from a variety of sources. Assuming they all were fathered by slave owners is erroneous. That’s not to say that that did not occur, but it’s far from predominant, let alone being due to rape as is commonly accused. There were randy younger male relatives of the owner, there were overseers. The largest mulatto communities in my awareness arose from basically outlaw camps in the wilderness on colonial borders or state lines pretty far back, comprised of runaway white indentured servants, runaway black slaves and some natives who for whatever reason joined them.
Off topic. This is a Hurricane Irma thread.
I’ll have to jump back in my research, but one mulatto woman from Monticello was given a parcel right in the middle of her presumed father’s (and 1/2 brothers land.)