Ecological, morphological and genetic divergence of sympatric North Atlantic killer whale populations ANDREW D. FOOTE,*† JASON NEWTON,‡ STUART B. PIERTNEY,§ ESKE WILLERSLEV† and M. THOMAS P. GILBERT† *Institute of Biological and Environmental Sciences, School of Biological Sciences, University of Aberdeen, Lighthouse Field Station, George Street, Cromarty, IV11 8YJ, UK, †Natural History Museum of Denmark, University of Copenhagen, Universitetsparken 15, 2100 Copenhagen Ø, Denmark, ‡NERC Life Sciences Mass Spectrometry Facility, SUERC, East Kilbride, G75 0QF, UK, §Institute of Biological and Environmental Sciences, School of Biological Sciences, University of Aberdeen, Tillydrone Avenue, Aberdeen, AB24 2TZ, UK Abstract Ecological divergence has a central role in speciation and is therefore an important source of biodiversity. Studying the micro-evolutionary processes of ecological diversification at its early stages provides an opportunity for investigating the causative mechanisms and ecological conditions promoting divergence. Here we use morphological traits, nitrogen stable isotope ratios and tooth wear to characterize two disparate types of North Atlantic killer whale. We find a highly specialist type, which reaches up to 8.5 m in length and a generalist type which reaches up to 6.6 m in length. There is a single fixed genetic difference in the mtDNA control region between these types, indicating integrity of groupings and a shallow divergence. Phylogenetic analysis indicates this divergence is independent of similar ecological divergences in the Pacific and Antarctic. Niche-width in the generalist type is more strongly influenced by between-individual variation rather than within-individual variation in the composition of the diet. This first step to divergent specialization on different ecological resources provides a rare example of the ecological conditions at the early stages of adaptive radiation. Keywords: Atlantic, ecotype, killer whale, Orcinus orca, Phylogenetics Received 12 July 2009; revision received 22 September 2009; accepted 23 September 2009 Introduction Adaptive radiation is the process of diversification from a single ancestral form into a variety of ecological or geographic niches to produce new ecologically special- ized forms (Gavrilets & Losos 2009). Ecological diversi- fication leads to divergence in morphological and other phenotypic traits through phenotype plasticity and genetic divergence resulting in resource or trophic poly- morphisms, that may represent the incipient stages of speciation (Smith & Sku ´ lason 1996). Theoretical approaches have modelled mechanisms that allow this process to occur in allopatry or sympatry where the absence of geographic barriers to gene flow should erode differentiation (e.g. Dieckmann & Doebeli 1999; Doebeli & Dieckmann 2003). These theoretical models are being increasingly supported by empirical evidence from a range of studies, which identify ecological fac- tors as the driver of speciation (e.g. Funk et al. 2006). Phylogenetic tests combined with studies of the micro- evolutionary processes of ecological diversification at its early stages can provide a useful approach to investi- gating the influence of ecology on evolutionary diver- gence (e.g. Huber et al. 2007; Steinfartz et al. 2007; Wolf et al. 2008). Killer whales (Orcinus orca) are widely distributed throughout the world’s oceans (Forney & Wade 2007), however the worldwide genetic diversity of killer whales, based on mtDNA control region sequences, is low and consistent with a historical bottleneck followed by rapid expansion (Hoelzel et al. 2002). Despite this Correspondence: Andrew Foote, Fax: +441381600548; E-mail: a.d.foote@abdn.ac.uk Ó 2009 Blackwell Publishing Ltd Molecular Ecology (2009) 18, 5207–5217 doi: 10.1111/j.1365-294X.2009.04407.x