Darwin, speaking on pigeons, mused that all pigeons descended from a single, mated-pair.
No, he didn't.
Great as the differences are between the breeds of pigeons, I am fully convinced that the common opinion of naturalists is correct, namely, that all have descended from the rock-pigeon (Columba livia), including under this term several geographical races or sub-species, which differ from each other in the most trifling respects.The confusion may have arisen from this passage:[...]
from these several reasons, taken together, I can feel no doubt that all our domestic breeds have descended from the Columba livia with its geographical sub-species.
[...]
Fourthly, pigeons have been watched, and tended with the utmost care, and loved by many people. They have been domesticated for thousands of years in several quarters of the world; the earliest known record of pigeons is in the fifth Ægyptian dynasty, about 3000 B.C., as was pointed out to me by Professor Lepsius; but Mr. Birch informs me that pigeons are given in a bill of fare in the previous dynasty.
-- Charles Darwin, "On the Origin of Species", 1859
I have discussed the probable origin of domestic pigeons at some, yet quite insufficient, length; because when I first kept pigeons and watched the several kinds, knowing well how true they bred, I felt fully as much difficulty in believing that they could ever have descended from a common parent, as any naturalist could in coming to a similar conclusion in regard to the many species of finches, or other large groups of birds, in nature.However, in the context of the rest of the discussion surrounding this passage, it's entirely clear that he's talking about a common parent *species*, not a common parent individual, since the chapter concerns itself with whether different breeds of domesticated animals (e.g., all domesticated pigeon breeds) were derived from different wild species, or the same wild species.