ALLOZYME FREQUENCIES IN A LINEAR SERIES OF SONG DIALECT POPULATIONS

Evaluating the biological consequences of avian song dialects is important in at least two ways. First, where appropriate experimental evidence is available, we know that dialect markers in song are learned (Marler, 1970; Marler and Mundinger, 1971). Dialect systems thus offer an opportunity to determine if and how culturally transmitted vocal signals, a fundamental feature of many songbird societies, influence patterns of gene flow. Second, despite much fine descriptive work, few attempts have been made to connect the field of behavioral research on song dialects to that of evolutionary genetics. The central issue is whether or not population specific vocal signals (dialects) cause mating to take place primarily among members of the same dialect population. This question does not specify any particular behavioral mechanism that might be hypothesized nor should it suggest that gene migration is an all-or-none phenomenon. Gene flow between adjacent dialects may vary in amount or direction depending upon such possible influences as dialect discrimination affecting mate selection, recency of contact of the populations (Remington, 1968), availability of unoccupied breeding habitat, or major environmental disturbances. If song dialects reduce gene migration among dialect populations, however, then it is reasonable to predict that neighboring dialect populations may exhibit genetic differences (Templeton, 1980a). The divergence may occur because of founder effect, genetic drift, or because of selective differences in occupied habitats as when, for example, dialects occur along an environmental gradient (Endler, 1977). differential coadaptation of genomes may also be involved. Possible outcomes of the interaction of gene flow, environmental gradients, magnitude of selection pressures, and population size have only recently begun to yield to theoretical analysis (Endler, 1977; Slatkin, 1978; Lande, 1980) and we hope that our studies may be of some use in this regard. Our field data, while not constituting a test of this developing theory, may serve to underscore the significance of gene flow in population differentiation. The four song dialect populations of Whitecrowned Sparrows treated here extend over a total of only 35 km distance in an environment of apparent homogeneity in climate and vegetation. In view of the potentially high vagility of these birds, the possible consequences of discontinuities in gene flow caused by social structure may be more easily discovered. In this report we provide allozyme frequency data from four dialect populations occurring in a linear sequence along the central California coast. From these data we describe the nature and extent of genetic differentiation within and among the four song dialect populations to test the hypothesis of genetic differentiation of dialects.