MORPHOMETRIC DIFFERENTIATION IN NEW ZEALAND POPULATIONS OF THE HOUSE SPARROW (PASSER DOMESTICUS).

Abstract

During the nineteenth century settlers in various parts of the New World sought to familiarize their surroundings by importing common European birds. In so doing they unwittingly began replicate experiments in nature, which are now of special interest to evolutionists because both the time frame and antecedent stock are documented. Sufficient time has elapsed since the introductions were made to suggest that a careful monitoring of the introduced populations might provide insight into the evolutionary genetics and phenetics of colonizing species. By using extant European populations as representative of the ancestral stock it is possible to estimate the rate and amount of change that has occurred in the New World isolates. The house sparrow (Passer domesticus) has been a particularly successful colonizing species, presumably because it is a commensal of man. Flourishing populations are now established in North and South America, South Africa, Hawaii, Australia and New Zealand (Austin, 1962; Summers-Smith, 1963). In a series of elegant papers, R. F. Johnston and R. K. Selander have analyzed variation in continental populations of house sparrows in North and South America, and have calibrated these variations against samples from England and Europe (Johnston and Selander, 1964, 1971, 1973; Selander and Johnston, 1967; Johnston, 1969a, 1969b, 1972a, 1972b, 1972c, 1976a, 1976b). Several of their findings are of considerable theoretical and heuristic import to students of the evolutionary process, principal among which are the following: (1) in the evolutionarily minute time of about 100 years (100 generations), North American populations of house sparrows have developed significant interlocality differentiation in size; (2) interlocality differentiation evident in extant New World isolates matches that in the ancestral English and German populations, but is less than that expressed across all of Europe; (3) although size increases with increasing latitude in North America (at least to 45?N) and vice versa in Europe north of the Mediterranean, characters are similarly covariant in the two continents, as might be expected if differentiation has been molded by natural selection; (4) relative to the North American populations the South American ones are constrained phenetically, consistent with a population bottleneck from a small founding population; and (5) sexual dimorphism in size increases clinally with increasing latitude. Along with these interlocality phenomena it has been suggested from reanalysis of the classical Bumpus data (Bumpus, 1899) that there are viability components of fitness associated with size at the intralocality level (Johnston et al., 1972). Selection within localities for optimal size is most intense in the period of winter cold (Grant, 1972; Johnston and Selander, 1973; O'Donald, 1973; Rising, 1973; Lowther, 1977). Collectively, these studies provide some of the best empirical data yet assembled on microevolutionary changes in new isolates of a species. One natural experimental situation not covered by previous workers is that of temperate island populations. House sparrows were liberated in New Zealand at approximately the same time as they were in North and South America. In the period 1862-1871, 110 birds were imported from Europe, and most if not all were