Data from: 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
postzygotic 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 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 such
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 interpreted to avoid taxonomic
inflation and deflation.