Decoupling of latitudinal gradients in species and genus geographic range size: a signature of clade range expansion

Aim. Clade range size is a function of species range sizes but also depends on the geographic deployment of species; clade range expansion should therefore depend partly on a clade’s tendency to produce new species. Under high speciation pressure, species-rich clades can migrate outside its present distribution, and thus overcome niche conservatism. A simple probabilistic mechanism for an out-of-tropics dynamic (OTT) for clade range expansion can thus operate in the absence of species-level differences in probability of range expansion, with speciation into novel climates more likely to occur in species-rich clades. In this scenario, (a) species and clade range sizes are decoupled and (b) clade range expansion is a function of per-clade species diversification. To test these predictions that can discriminate between a weaker and a stronger climatic niche conservatism, we contrast the latitudinal and thermal range sizes of marine bivalves at the species and clade (genus) level.
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