Author(s): Carlos Daniel Cadena (corresponding author) ; Robert E. Ricklefs ; Iván Jiménez ; Eldredge Bermingham 
Arising from: B. C. Emerson & N. Kolm Nature 434, 1015-1017 (2005); B. C. Emerson & N. Kolm reply
Emerson and Kolm  show that the proportion of species endemic to an island is positively related to its species richness and, assuming that endemism indexes speciation rate, they infer that greater species diversity accelerates diversification. Here we demonstrate that the same correlation between species richness and percentage endemism can arise even if within-island speciation is negligible, particularly when both endemism and species richness depend on attributes of islands (such as area) that influence the average age of resident populations. Island biogeography theory indicates that, where the average time to extinction is relatively long, diversity increases through colonization, irrespective of whether new species are formed ; at the same time, islands on which populations persist for longer accumulate more endemic species as local populations differentiate and populations on neighbouring islands become extinct [3, 4]. We therefore suggest that species richness and endemism are correlated fortuitously owing to their mutual dependence on the life spans of populations on islands, which is unrelated to speciation itself.
If the scenario we propose is correct, islands richer in species and with higher endemism would also have older populations. This prediction is supported by data for birds in the West Indies, a system in which the occurrence of within-island speciation is an extremely rare event: the proportion of endemic species increases linearly with species richness, and species-poor islands with few endemics have populations that are younger on average than those of species-rich islands with higher endemism (Fig. 1). We argue, therefore, that the correlation between species richness and endemism does not imply a causal relationship between these two variables, but rather...