Some of the most important quantitative variables that were used for data collection and analysis include mDNA, AFLP, AFLP sequences, morphology and plumage coloration (Brelsford et al. 2011). The amplified fragment length polymorphism analysis produced significant results indicating a clear cluster distinction between the parent species, D. coronata and D. nigrifrons (Brelsford et al. 2011). Additionally, the D. auduboni cluster was found to be between D. coronata and D. nigrifrons. Despite the strong genetic evidence supporting this strong model of homoploid hybrid speciation, reproductive isolation between D. auduboni and D. nigrifrons has yet to be demonstrated (Brelsford et al. 2011). Reproductive isolation, in any situation of possible hybrid species, is crucial and plays an important role in increasing overall genetic diversity. The lack of reproductive isolation is considered by Brelsford et al (2011), suggesting that a fusion event may be possible.
The importance of reproductive isolation is not lost with the yellow-rumped warbler complex. An excellent model of reproductive isolation has been made by studying the Italian sparrow, a hybrid species whose parents are the Spanish sparrow P. hispaniolensis and the house sparrow P. domesticus. The authenticity of the Italian sparrow was assured by Hermansen et al. (2011), as their findings suggested great divergence between the parental species and the hybrid, a mixed nuclear genome and no unique haplogroups found at various loci.