Variability in Island Populations of the House Finch

--Coefficients of variation and generalized variances are compared for nine morphological characters from five mainland and four island populations of the House Finch (Carpodacus mexicanus). The purpose is to test the idea that variability is reduced in isolated populations and that there is a "population variation parameter" that determines the level of variation in most characters. Variability is greater in bill characters for Guadalupe and San Benito Islands than for mainland and other island populations. There are no consistent differences among samples in variability of wing, tail, or hind-limb characters, except for a tendency toward increased tarsus-length variability in a southern Baja California population and reduced variability in this character in San Clemente Island and Guadalupe Island populations. In bill characters, increased variability is found in the most divergent populations. The results do not support he generalization that geographic (= genetic) isolation per se causes much of a change in variability in island populations of birds, nor is there support for the idea of a pervasive quality of the gene pool that determines the level of variability in most characters. Received 29 June 1981, accepted 30 September 1982. ISLAND populations are isolated to varying degrees and, as a result, are often subject to reduced gene flow. They are also often subject to strong directional selection, especially just after colonization of a novel environment. Grant (1979) has proposed that phenotypic variation is reduced in well-isolated populations as a consequence of both diminished gene flow and strong differentiation through directional selection and random genetic drift. Soul• (1972) also identified isolation, as well as the intensity of stabilizing selection, as factors in the reduction of variability in island populations. In addition, Soul• has stated that there is strong evidence for a "population variation parameter," implying that some pervasive quality of the gene pool determines the level of variability in most characters. I tested the ideas of an association between variability and isolation in island populations and of the similarity in variability among characters by using morphological data from island and mainland populations of the House Finch (Carpodacus mexicanus). "Variability" is used here to mean within-population variation as measured by the coefficient of variation or variance of log-transformed data for single characters. For multiple characters, a generalized variance is used. The House Finch is widespread in North America and occurs on the California Islands (Fig. 1). Samples from many of the islands are large enough to provide reliable measures of variability. Divergence has occurred in some characters on many of the islands (Power 1979, 1980).