Darwin's early accounts of the distinctive species on the Galapagos Islands were central to the foundations of evolutionary theory. Since that time, data from numerous species have supported Darwin's hypotheses that native species of the Galapagos arose from their earlier inhabitants, remaining unique from other populations because of their geographical isolation. As an ultimate test of the effects of geographical isolation on residents of these islands, Frank Hailer and researchers at the Smithsonian Institution and the University of Missouri–St Louis sought an improbable candidate for isolation effects, the magnificent frigatebird (Fregata magnificens). This seabird is widely distributed and capable of easily travelling hundreds of kilometres, giving it great potential for mixing genes with other populations. As predicted by the capacity of frigatebirds to wander, Hailer and his colleagues document that this species does exhibit extensive gene flow amongst most populations; however, on the Galapagos Islands, even this highly adaptable vagrant is distinct from its mainland counterparts.
The authors obtained biological samples from fresh and museum specimens from eastern Pacific and Atlantic populations spanning the distribution range of this avian vagabond. Then they used mitochondrial DNA, microsatellite markers and nuclear introns to assess genealogical relationships. They revealed extensive gene flow across most of this species' range, including across the Isthmus of Panama, which is a major barrier to gene migration that has not been bridged by any other tropical seabird. Other researchers have also shown that magnificent frigatebirds banded in the Galapagos have been recovered over 1000 km away in Central America, demonstrating that island and continental populations may interact. However, the genetic data from this study show that the Galapagos population is unique. Measurements taken on magnificent frigatebirds in museums also revealed that the Galapagos birds are significantly larger than those from the mainland. Despite the birds' nomadic lifestyle and potential for mixing, these genetic and morphological results indicate long-term isolation on the Galapagos, probably for several hundred thousand years. So just what explains the uniqueness of magnificent frigatebirds on the Galapagos?
The authors speculate that the magnificent frigatebird may exhibit a tendency to breed at their birth location or a previous breeding site (philopatry), perhaps because of a movement barrier unique to the Galapagos or to a behavioural mechanism related to their intricate courtship routine. Indeed, Galapagos seabirds have been found to forage primarily in the nutrient-rich waters west of the archipelago while seabirds from the mainland of South America forage mainly on the continental shelf. These distinct foraging preferences could impart geographical isolation of the two populations. As the territory of the magnificent frigatebird on the Galapagos overlaps with that of the great frigatebird (F. minor), these birds may be especially picky in choosing their mates in order to avoid pairings with the other species. This type of finicky mate selection could also result in the rejection of mainland birds as potential mates, leading to reproductive isolation.
Future studies are necessary to determine the mechanism behind the evolutionary distinctiveness of the Galapagos magnificent frigatebird. In any case, the work by Hailer and his colleagues is an elegant modern day recapitulation of Darwin's early hypotheses on the unique species of the Galapagos Islands. These findings may also dictate a reconsideration of this bird's IUCN status (currently listed as Least Concern), given that this population consists of only about 1000 pairs.
