Research article Unusual number of pectoral fin rays in an Icelandic population of threespine stickleback (Gasterosteus aculeatus) recently isolated in freshwater BJARNI K. KRISTJA ´ NSSON 1,2, *, SKU ´ LI SKU ´ LASON 1 and DAVID L. G. NOAKES 2 1 Ho ´lar University College, 551, Skagafjo ¨rdur, Iceland; 2 University of Guelph, Guelph, Ontario, N1G 2W1 Canada (*author for correspondence, tel.: +354-455-6386; fax: +354-455-6301; e-mail: bjakk@holar.is) Received 9 March 2004; accepted 27 August 2004 Co-ordinating editor: Kaitala Abstract. Threespine stickleback most often have 10 pectoral fin rays, and it seems to be a highly canalized trait. We observed an unusually high frequency of stickleback with 11 pectoral fin rays in a population recently isolated from the marine environment in a freshwater lagoon in Iceland. These new morphologies may be beneficial for the fish in the new habitat but may disappear because of strong canalizing selection for the typical 10 pectoral fin rays. Key words: divergence, isolation, rapid evolution Introduction The threespine stickleback, Gasterosteus aculeatus, inhabits marine and fresh- water habitats throughout its holarctic distribution (Wootton, 1984). It is believed to have originated in the ocean, but populations have repeatedly been isolated in freshwater (Bell and Foster, 1994). These populations adapt quickly to diverse freshwater environments and thus display a wide array of phenotypes often forming sympatric or parapatric morphs and species (Klepaker, 1993; Bell and Foster, 1994; Ziuganov, 1995; McKinnon and Rundle, 2002). These phe- notypes usually differ from the marine ancestors, which appear to be more can- alized in their morphology (Bell and Foster, 1994). West-Eberhard (2003) defined canalization as ‘the evolved ability to maintain a single phenotypic norm in the face of deviation-inducing genetic and environmental effects during ontogeny’. Icelandic marine sticklebacks show some morphological variation, but much less than those in fresh water (Kristja´nsson, unpublished; Kristja´nsson et al., Evolutionary Ecology 18: 379–384, 2004. Ó 2004 Kluwer Academic Publishers. Printed in the Netherlands.