Response to comments on Species diversity can drive speciation

Abstract

This issue of Ecography features three responses to our paper Species diversity can drive (Emerson and Kolm 2005a), each presenting arguments of a different nature as to why our original analyses and interpretations may be flawed. We welcome the opportunity to respond in turn to each of these, and to clarify some points of confusion that may have emerged from our earlier paper. The distinction between speciation rate and diversification rate was perhaps not made clear in our earlier paper (Emerson and Kolm 2005a). In several instances we referred interchangeably to both speciation rate and diversification rate. However, in a subsequent paper we did explicitly refer to speciation rate, not diversification rate (see Fig. 1, Emerson and Kolm 2005b). Rate of species origin can be considered a measure of speciation rate, while rate of species accumulation can be considered a measure of diversification rate (Futuyma 2005, p. 400). Our model makes no explicit prediction regarding diversification rate, only about speciation rate. Whittaker et al. (2007) criticise our approach because the index we used is not a rate of species production. They argue that the proportion of single endemics (our index of speciation rate) does not measure rate of species production, and is inconclusive as to the mechanisms leading to the relationships between species richness and the proportion of single endemics. To be clear, the index we used is not a rate of speciation. It is the proportion of single endemics that we use as an index of relative speciation rate on an the higher the proportion the greater the probability of a species on an undergoing anagenetic or cladogenetic change. An analogous approach has been taken by MacArthur and Wilson (1967, p. 173) where they related percentage endemism to turnover rate (see below). Whittaker et al. (2007) are prepared to accept that the proportion of single endemics reveals something of the opportunities for speciation, reflecting an increasing opportunity for speciation with age, followed by eventual loss of endemic species on older isl s through the progression rule. Exactly how extinction features in their model is unclear as it is not depicted in their Fig. 1, but outside of abiotic cata trophes they expect it to range between extremely low and very low. To this end they put forward a conceptual model they call the island immaturity sp ciation pulse model (IISP) that appears to predict a specific relationship between age and speciation rate. Implicit within their model is that speciation is the product of opportunity, and opportunity presents itself in he form of vacant niche space and competitive release, important primers for adaptive radiation. There is no doubt that these are particularly important variables contributing to speciation rate in young islands. However, it is interesting to note that in a meta-analysis of arthropod phylogenetic data from the central and western Canary Islands, Emerson and Oromi (2005) found little evidence for adaptive radiation on young islands. They did however find increasing evidence for evolutionary patterns consistent with adaptive radiation on increasingly older islands, a result that sits more comfortably with mechanisms of competitive interactions rather than competitive release, phenomena that are increasingly being recognised as drivers of evolutionary change (e.g. Grant and Grant 2006, Nosil and Crespi 2006, Jordal et al. 2006, Meyer and Kassen 2007). Some of the criticism from Whittaker et al. (2007) is directed toward apparent contradictions between the writings of MacArthur and Wilson (1963, 1967) and ourselves. In their original model MacArthur and