Genetic and Morphologic Divergence among Introduced Rat Populations (Rattus RATTUS) of the Galápagos Archipelago, Ecuador

Patton, J. L., S. Y. Yang, and P. Myers (Museum of Vertebrate Zoology, University of California, Berkeley) 1975. Genetic and morphologic divergence among introduced rat populations (Rattus rattus) of the Galdpagos Archipelago, Ecuador. Syst. Zool. 24:296310.-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 Crist6bal, 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. [Rattus, GalApagos, allozymes, morphometrics. ] Lewontin (1965) has summarized the features of an ideal colonizing species as including ". . . effective dispersal, high somatic plasticity, and high interspecific competitive ability." Of the handful of non-domestic mammals which can be considered exceptional colonizers, the roof, or black, rat (Rattus rattus Linnaeus) clearly possesses these characteristics. The species, along with two other human commensals (house mouse, Mus musculus, and Norway rat, Rattus norvegicus), has been transported nearly the world over during the past several centuries. It is thus one of the most successful mammalian weed species, being equally adept at establishing feral as well as human associated populations. While a large body of literature has been 'Contr. 190, Charles Darwin Foundation. 2 Present address: Museum of Zoology, University of Michigan, Ann Arbor. amassed regarding epizootic, ecological, distributional, or systematic attributes of roof rats (e.g., Storer, 1962; Tomich, 1968; Tamarin and Malecha, 1971, 1972; Eche, 1955; Schwarz and Schwarz, 1967; and Brooks, 1966), few'studies have been concerned with the underlying genetic bases for their colonizing success. The present report views the latter from the context of patterns of genetic and morphologic variability exhibited by populations introduced into the Galapagos Islands, Ecuador. The general questions asked relate to distinguishing between genic based variability and phenotypic plasticity as fundamental to colonizing success in the species. In doing this, an attempt is made to discern the relative roles of time of immigration, interisland exchange, and current extra-island influx as determinants in the documented patterns of variability.