1. Department of Plant Sciences, University of Oxford, South Parks Road, Oxford OX1 3RB, UK
2. Department of Biology & Centre for Advanced Research in Environmental Genomics, University of Ottawa, Ottawa K1N 6N5, Canada
3. School of Biological Sciences, University of Auckland, Private Bag 92019, Auckland, New Zealand
4. Present address: London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, UK

Correspondence to: Rees Kassen^1,2 Correspondence and requests for materials should be addressed to R.K. (Email: rkassen@uottawa.ca).

Abstract

Taxonomic diversification commonly occurs through adaptive radiation, the rapid evolution of a single lineage into a range of genotypes or species each adapted to a different ecological niche^{1, 2}. Radiation size (measured as the number of new types) varies widely between phylogenetically distinct taxa^{2, 3, 4} and between replicate radiations within a single taxon where the ecological opportunities available seem to be identical^{5, 6}. Here we show how variation in energy input (productivity) and environmental disturbance combine to determine the extent of diversification in a single radiating lineage of Pseudomonas fluorescens adapting to laboratory conditions. Diversity peaked at intermediate rates of both productivity and disturbance and declined towards the extremes in a manner reminiscent of well-known ecological patterns^{7, 8, 9}. The mechanism responsible for the decrease in diversity arises from pleiotropic fitness costs associated with niche specialization^{10, 11}, the effects of which are modulated by gradients of productivity and disturbance. Our results indicate that ecological gradients may constrain the size of adaptive radiations, even in the presence of the strong diversifying selection associated with ecological opportunity, by decoupling evolutionary diversification from ecological coexistence.

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