On age and species richness of higher taxa.

Many studies have tried to identify factors that explain differences in numbers of species between clades against the background assumption that older clades contain more species because they have had more time for diversity to accumulate. The finding in several recent studies that species richness of clades is decoupled from stem age has been interpreted as evidence for ecological limits to species richness. Here we demonstrate that the absence of a positive age-diversity relationship, or even a negative relationship, may also occur when taxa are defined based on time or some correlate of time such as genetic distance or perhaps morphological distinctness. Thus, inferring underlying processes from distributions of species across higher taxa requires caution concerning the way in which higher taxa are defined. When this definition is unclear, crown age is superior to stem age as a measure of clade age.