Inferring Time-to-Speciation From Hybrid Zone Analysis Informs Assessments of Taxonomic Inflation

Measuring gene flow across hybrid zones can provide a direct evaluation of reproductive isolation (RI) between evolutionary divergent lineages. Geographic-explicit modelling of gene flow across hybrid zones, known as cline analysis, thus offers a gold standard for species delimitation under the biological species criterion. We here relate divergence time to post-zygotic RI as inferred from hybrid zone cline width from studies on amphibians, reptiles, birds, mammals and insects. The data confirm an overall significant negative correlation between cline width and divergence time (within-clade consistently significant only in amphibians). Out of 108 hybrid zones, hybridizing taxa were younger than 10 million years (Ma) in 104 cases, and younger than 7 Ma in 94 cases, implying that older lineage pairs are usually reproductively isolated. In amphibians, the correlation suggested that shallow and steep hybrid zones, characteristic of subspecies and species, are found with divergence times of < 2.6 Ma and > 7 Ma, respectively. In all taxa we revealed a trend of young lineages being increasingly described as new species over time by simulating past sets of taxonomically named species. Overall, the inferred proportion of species with post-Pliocene ages was highest in birds (27.3%) and mammals (21.8%), but lower in amphibians and squamates (< 10%). In amphibians we document a 56% increase from 1980 to 2010 in the number of newly described species that fall below the hybrid zone-derived divergence reference point of 2.6 Ma. While this trend so far impacts only a small proportion of the total amount of amphibian species, we commend that taxonomic hypotheses that delimit Pleistocene-aged lineages as species, or conversely, that consider lineages with Miocene ages or older as conspecific, should be carefully evaluated to avoid taxonomic inflation and deflation.