1. ¹CEFE-CNRS, UMR 5175, Montpellier cedex 5, France
2. ²Ornithological Station, Museum and Institute of Zoology, Polish Academy of Sciences, Gdask, Poland
3. ³Origine, Structure et Evolution de la biodiversité, UMR 5202, Département Systématique et Evolution, Muséum National d'Histoire Naturelle, Paris, France
4. ⁴Department of Ornithology and Mammalogy, California Academy of Sciences, San Francisco, CA, USA

Correspondence: Dr L Gay, Centre for Ecology and Conservation, University of Exeter, Cornwall Campus, Tremough, Penryn TR10 9EZ, UK. E-mail: L.Gay@exeter.ac.uk

Received 4 February 2008; Revised 28 July 2008; Accepted 15 August 2008; Published online 24 September 2008.

Abstract

We investigated the role of selection in generating and maintaining species distinctness in spite of ongoing gene flow, using two zones of secondary contact between large gull species in Europe (Larus argentatus and Larus cachinnans) and North America (Larus glaucescens and Larus occidentalis). We used the pattern of neutral genetic differentiation at nine microsatellite loci (F_ST) as an indicator of expected changes under neutral processes and compared it with phenotypic differentiation (P_ST) for a large number of traits (size, plumage melanism and coloration of bare parts). Even assuming very low heritability, interspecific divergence between L. glaucescens and L. occidentalis in plumage melanism and orbital ring colour clearly exceeded neutral differentiation. Similarly, melanism of the central primaries was highly divergent between L. argentatus and L. cachinnans. Such divergence is unlikely to have arisen randomly and is therefore attributed to spatially varying selection. Variation in plumage melanism in both transects agrees with Gloger's rule, which suggests that latitude (and associated sun and humidity gradients) could be the selective pressure shaping differentiation in plumage melanism. We suggest that strong species differentiation in orbital ring colour results from sexual selection. We conclude that these large gull species, along with other recently diverged species that hybridize after coming into secondary contact, may differ only in restricted regions of the genome that are undergoing strong disruptive selection because of their phenotypic effects.