Genetic and Morphologic Divergence among Introduced Rat Populations (Rattus rattus) of the Galapagos Archipelago, Ecuador
Systematic Zoology, 24 (3):
296-310 (1975)Abstract
The roof rat (Rattus rattus Linnaeus) was initially introduced into the Galapagos Archipelago before Darwin's visit in 1835. The species is now known from seven of the 16 major islands and exhibits a wide range in both human and non-human associated habitat usages. Morphological distinctiveness of island populations was first noted by Heller in 1904, consisting of overall size and shape as well as pelage color differences. Analyses involving allozyme frequencies at 37 genetic loci, epigenetic cranial characters, and multivariate treatments of mensural characters confirm and extend these observations. The level of concordance between each analysis is extremely high; each delineates the same three groupings of islands based on overall similarity: (1) Isla Santiago-Bartolome; (2) Isla Floreana-Isabela-Pinzon-San Cristobal; and (3) Isla Santa Cruz-Baltra. An hypothesis of multiple origins best accounts for the similarity relationships between islands as each of the groupings fits a known separate period of human use activity. The initial introduction was most likely at Santiago in the late 1600's; the most recent on Baltra-Santa Cruz during World War II. Gene flow between the different island groups, past and present, is considered slight, but continual introduction of immigrants from outside source populations, particularly to Baltra, Santa Cruz, and San Cristobal, is highly probable. Genic variability as demonstrated by allozyme analysis is quite low (mean heterozygosity = 2.85 per cent) when compared to other rodent species. The pattern of variability, however, suggests that this low level does not result from a severe founder effect, but that it is a general characteristic of the species as a whole.