Freshwater and marine threespine stickleback, Gasterosteus aculeatus, differ remarkably in armour plate number and body shape, although differences in other morphological characters are also common. Most freshwater populations have apparently evolved after isolation of marine sticklebacks in freshwater. After colonisation of freshwater habitats, they show rapid morphological changes and associated genetic isolation within as few as eight generations. I transferred fish from marine tide pools to two isolated freshwater ponds, differing in habitat characteristics, at the beginning of the breeding season, when females had ripe ovaries and males had breeding coloration. The first generation fish that I sampled from the ponds had significantly fewer armour plates than their marine ancestors and differed in shape. I also found some significant differences between fish sampled from the larger pond and those from a smaller, adjacent pond. This extremely rapid morphological divergence suggests that either the marine sticklebacks were highly phenotypically plastic or that there was very strong natural selection acting on the first generation within freshwater habitats.
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I’m grateful to Jón Gardarsson and his family in Nedri Ás for asking me to release stickleback in his ponds, and allowing me to conduct this experiment. D. Noakes and V. B. Pálsson assisted me in the field. Thanks to D. Noakes, S. Skúlason, G.Á. Ólafsdóttir, M. Robillard and R. Sturlaugsdóttur for comments on earlier drafts of this manuscript. This study was supported by Hólar University College.