All about being old and shooting hairs: clade age and urticating hair explain the patterns of diversification in tarantulas

The extreme asymmetry of species richness distribution across the tree of life has always intrigued evolutionary biologists. Two competing explanations have been proposed to explain this pattern-the clade age hypothesis and diversification rate variation. While these two scenarios may not be mutually exclusive, to what extent time and diversification rates interact to explain species richness patterns remains understudied. Here, we investigate the relative influence of these two scenarios using tarantulas (Family: Theraphosidae) as a model. Tarantulas represent a speciose group of spiders found worldwide but exceptionally diverse in South America. These spiders show two distinct patterns of microhabitat use (ground-dwelling or arboreal) and defense strategies (presence or absence of urticating hairs). Using various trait-independent and dependent diversification models, we test the clade age hypothesis, the role of microhabitat, antipredator defense strategy, and geography in influencing diversification rates. Our results suggest that clade age is the primary predictor of species richness distribution across the tarantula subfamilies. However, the presence of urticating hair probably disrupted this pattern in some clades by increasing the net diversification rates, not by increasing the speciation rate but by reducing the extinction rate.