Effective population size in a songbird: some possible implications

Abstract

Song dialects in White-crowned Sparrows (Zonotrichia ieucophrys nuttalli) may act as partial barriers to the free exchange of genes. Within a dialect population, a genetic neighbourhood comprises about 100 individuals. Other factors combine to reduce the effective number to less than 50. By Wright’s methods, the genetic divergence among subpopulations (Fst) expected in a population exhibiting this size effective number with neutral alleles is about an order of magnitude larger than the observed Fst calculated from electrophoretically detectable alleles. Natural selection may therefore be maintaining greater homogeneity within a dialect than expected by assuming that alleles are neutral. From the torus model of Maruyama, however, the expected Fst values are close to those observed, suggesting no major departures from panmixia within dialect populations. The degree of genetic isolation exhibited between dialect populations may provide an explanation for the more rapid rate of speciation in passerine birds.