The genomic revolution and species delimitation in birds (and other organisms): Why phenotypes should not be overlooked

Given the availability of genomic data to identify separately evolving groups of organisms, many researchers establish species limits based on assessments of the extent of gene flow among populations and often use analytical approaches to identify species in which gene flow is explicitly disallowed. Strictly considering lack of-or limited-gene flow as the main or only criterion to delimit species involves two main complications in practice. First, approaches often used to analyze genome-wide data cannot by themselves distinguish species limits from within-species population structure, particularly in allopatric organisms. Second, recognizing as species only those lineages one can identify using such approaches fails to embrace the role of other evolutionary forces (i.e. various forms of selection) in defining evolutionary lineages. Using examples from various groups of birds, we call for the importance of considering evolutionary forces additional to gene flow in species delimitation and explain why genomic approaches commonly used in taxonomic studies may be insufficient by themselves to properly uncover species limits. By considering the processes that structure genotypic and phenotypic variation during speciation, we argue that rigorous analyses of phenotypic variation remain crucial for species delimitation in the genomics era because phenotypes uniquely inform us about the role of selection maintaining the cohesion of evolutionary lineages. Evolutionary theory describing the roles of gene flow, genetic drift and natural and sexual selection in the origin and maintenance of species calls for an integration of genomics with phenomics in avian species delimitation.

Automated summary

Simply considering lack of-or limited-gene flow as the main criteria for species delimitation faces complications, as it fails to distinguish species limits from within-species population structure and overlooks the role of other evolutionary forces. Therefore, it is important to emphasize the significance of evolutionary forces beyond gene flow in species delimitation, and acknowledge that genomic approaches alone may not be sufficient to properly uncover species limits in avian species.