THE FOUNDING OF A NEW POPULATION OF DARWINS FINCHES

We report the natural colonization of the small Galapagos island Daphn e Major by the large ground finch (Geospiza magnirostris). Immigrants of this species were present in every year of a 22-yr study, 1973-1994 . Typically they arrived after a breeding season and left at the begin ning of the next one. Geospiza magnirostris bred on the island for the first time in the exceptionally wet El Nino year of 1982-1983, and br ed in all subsequent years except drought years. In agreement with the oretical expectations the frequency of inbreeding was unusually high. Pronounced fluctuating asymmetry in tarsus length, together with sligh tly reduced breeding success of inbreeding pairs, suggests a low level of inbreeding depression. Despite this, the population increased from 5 breeding individuals in 1983 to 20 breeding individuals in 1992, an d probably more than twice that number in 1993, largely through recrui tment of locally born birds. The study illustrates the joint role of c hance and determinism in colonization. The original colonizers were ap parently a nonrandom sample with respect to morphological traits, and they and their offspring differed significantly in bill size from the immigrants that did not stay to breed. Because the traits appear to be heritable, the colonists were a genetically nonrandom sample. Genetic drift may have occurred, as only 6 of 13 founders produced recruits a nd small nonrandom tendencies in the colonists were amplified in the n ext two generations. An analogous process affected a culturally inheri ted trait, song type. This changed radically in frequency, apparently for reasons unconnected with properties of the song type itself; males singing one song type had better fledging and recruitment success tha n those singing another. The addition of a fourth species to a communi ty of three is nor expected on simple biogeographical grounds. It owes more to repeated immigration and the unusual but unidentified conditi ons favoring colonization than to a change in food supply on Daphne. C olonization and subsequent immigration may be the model pattern of fou nder events, applicable to continental and many insular situations.