Carlos Daniel Cadena, Felipe Zapata

Given the notion that species are population-level lineages and the availability of genomic data to identify separately evolving populations, researchers usually establish species limits based on gene flow or lack thereof. A strict focus on gene flow as the main –or only– criterion to delimit species involves two main complications in practice. First, approaches often used to apply this criterion to 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 selection in the origin and maintenance of species calls for an integration of genomics with phenomics in avian species delimitation.