a cultural analogue to Darwin's finches

Not only birds differentiate across the islands of an archipielago, headwear does, too!

Here and there in the doorways we saw women with fashionable Portuguese hoods on. This hood is of thick blue cloth, attached to a cloak of the same stuff, and is a marvel of ugliness. [...] The general style of the capote is the same in all the islands, and will remain so for the next ten thousand years, but each island shapes its capotes just enough differently from the others to enable an observer to tell at a glance what particular island a lady hails from.

—Mark Twain: The Innocents Abroad, chapter V.

The Voyage of the Beagle predates the one narrated by Twain in The Innocents Abroad by about thirty-three years.

See also: Darwin's finches and this paper about them.