Postglacial evolution in lateral plate morphs in Norwegian freshwater populations of the threespine stickleback (Gasterosteus aculeatus)

Abstract

Norwegian freshwater stickleback populations were founded after the last glacial period, and the progressive uplift of the land has produced an age range (1000 – 13 000 years) of the stickleback habitats. Most of the freshwater populations of today have probably been formed by isolation of marine sticklebacks in the process of land uplift. The freshwater threespine stickleback is known for its great morphological variability. Three distinct morphs ("low," "partial," and "complete") are recognized on the basis of variation in the lateral row of plates. Among the Norwegian populations, all three morphs were found, but the low morph was by far the most common and occurred mostly in monomorphic populations. The presence of the complete and partial morphs was mostly restricted to young lakes near the sea. It is likely that the plate polymorphism in this region is a transitionary evolutionary stage from a founding population dominated by complete to a monomorphic low population. The hypothesis of a polytypic origin of the low morph is discussed, and an alternative hypothesis is proposed. Within each plate morph, the number of plates also varied, and populations with exceptionally low plate numbers were mostly confined to three different areas. Within these areas, populations with plateless specimens also occurred. These plateless specimens tended to inhabit old lakes. The low plate number and plateless populations were found in parts of Norway that were deglaciated early. The advanced plate reduction can therefore be a result of a longer period of isolation and freshwater evolution. Other populations may be on their way towards extreme plate reduction, but have not yet reached the level of platelessness.