Abstract
Two genera of terrestrial slugs (Arion and Geomalacus) display a striking disproportion in species richness in the Iberian Peninsula. While there are 17 Iberian endemic species in Arion, morphological criteria only recognize four species within Geomalacus. Sequence data were used to test whether these differences could result from: (1) cryptic diversity within Geomalacus; (2) an earlier origin for Arion (older clades are expected to accumulate more species); (3) distinct patterns of diversification rates (higher initial speciation rates in Arion), and (4) some combination of the above factors (e.g., an older clade with higher speciation rates). Species delimitation tests based on mitochondrial and nuclear data revealed eight cryptic lineages within Geomalacus that lessened the asymmetry; nevertheless, the disparity required further investigation. No meaningful differences in crown group ages of each recovered clade were found. Regardless the different premises of the two equally plausible diversification models (similar initial speciation rates vs. higher initial speciation rates in Geomalacus), both coincide on diversity-dependent diversification for the two groups but weaker rate declines in Arion best explains the observed asymmetry in species richness. Also, the broader environmental tolerance combined with a faster dispersal and wider distribution may have represented an evolutionary advantage for Arion.
Highlights
Patterns of species richness are often unbalanced throughout the tree of life[1]
We explored the following hypotheses to explain the differences in endemic species richness between two slug genera, Geomalacus and Arion: (1) cryptic diversity within Geomalacus; (2) differences in the crown group age of each genus; (3) distinct patterns of diversification rates, and (4) some combination of the above factors
Within-group estimates of average divergence within Geomalacus based on cytochrome oxidase subunit I (COI) data ranged between 0% (G. oliveirae - Peña de Francia) and 6.8% (G. maculosus) Net divergence between groups ranged from 11.4% to 20.6% (Table 1)
Summary
Patterns of species richness are often unbalanced throughout the tree of life[1] These disproportions may reflect differences in clade ages given the expectation that the number of species will increase through time, and older clades are expected to display more diversity, as long as extinction rates remain constant. We explored the following hypotheses to explain the differences in endemic species richness between two slug genera, Geomalacus and Arion: (1) cryptic diversity within Geomalacus; (2) differences in the crown group age of each genus; (3) distinct patterns of diversification rates (e.g. higher initial speciation rates in Arion), and (4) some combination of the above factors (e.g. an older clade with higher initial speciation rates). We analysed diversification rates over time for each genus, testing which model of diversification fitted best the data
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