Convergence of Darwin’s finches through hybridization on a remote Galápagos Island

Studies of Darwin’s finches in the Galápagos archipelago have thrown light on the classical model of speciation in which divergence of related populations in separate locations is followed by the establishment of sympatry. In this context one situation is difficult to understand. On the island of Darwin, the most northerly and least accessible island of the archipelago, large finches have been variously described as Geospiza propinqua (formerly G. conirostris), G. magnirostris, or united as a unique and exceptionally variable species G. darwini. To resolve their status and investigate their evolution, we analyzed 24 specimens collected in the years 1897 to 1906 and compared them with 89 specimens of the two species on Genovesa, the only other island where G. propinqua occurs. The specimens from Darwin fall into two groups on a bivariate plot of beak length against width that correspond to G. propinqua and G. magnirostris from Genovesa. Compared with their Genovesa counterparts, Darwin G. magnirostris are much smaller in beak size whereas G. propinqua are larger in all dimensions except beak length. Their morphological convergence on Darwin places them below the minimum difference for sustained coexistence. This is suggestive evidence of introgressive hybridization. It appears to be an example of a reversal of speciation under harsh ecological conditions.