The detection of sympatric sibling species using genetic correlation analysis. I. Two loci, two gamodemes.

Four models are presented describing zygotic frequencies at two loci for one or two sympatric but genetically differentiated populations of "gamodemes." Linkage disequilibrium within gamodemes is allowed in two of the models. Maximum likelihood criteria are used to fit the models to the observed numbers of zygotes in a sample. A fitting-testing sequence for choosing a best model is described and the power of the test is analyzed. The statistical characteristics of the genetic parameter estimates were examined by simulation studies. In general, estimates were reliable when allele frequency differences between gamodemes were greater than 0.30 at both loci. This method may be used to study the population structure of samples with fewer heterozygotes than expected for Hardy-Weinberg populations, including the detection and genetic description of sibling species having overlapping ranges.--An example is given for Drosophila longicornis and D. propachuca, two sibling species within the mulleri complex of the repleta group which have been studied in detail using more conventional techniques. The reanalysis using the approach derived in this paper confirmed the reproductive isolation of these two species, and hinted at the possibility of further subdivision within D. propachuca.