Identification of hybrids based on morphological characteristics can be extremely difficult. Hybrids may express a mosaic of parental traits or display extreme phenotypes compared to parental forms (a phenomenon known as transgressive segregation). In addition, extensive backcrossing can result in a continuous gradient of phenotypes across a hybrid zone.
Jennifer Walsh (University of New Hampshire) and colleagues took a closer look at the hybrid zone between Saltmarsh Sparrow (Ammodramus caudacutus) and Nelson’s Sparrow (A. nelsoni). Based on genetic markers, they were able to identify eight recent hybrids, 44 backcrosses to Nelson’s Sparrow and 98 backcrosses to Saltmarsh Sparrow. However, using the morphological variation across the hybrid zone they could only distinguish between Nelson’s and Saltmarsh groups; backcrosses were indistinguishable from the pure parental species. These results indicate that hybrid identification without genetic data will likely lead to a substantial overestimation of the proportion of genetically pure individuals in a population. Hybrid zone research should be a combination of genetic and morphological approaches.
More information on hybridization in the genus Ammodramus and the family Emberizidae can be found here.
Walsh, J., Shriver, W. G., Olsen, B. J., O’Brien, K. M. & Kovach, A. I. (2015). Relationship of phenotypic variation and genetic admixture in the Saltmarsh-Nelson’s sparrow hybrid zone. The Auk 132, 704-716.