Summary: Phenotypic variation and sexual dimorphism in
anadromous threespine stickleback: implications for
postglacial adaptive radiation
WINDSOR E. AGUIRRE*, KAITLYN E. ELLIS, MARY KUSENDA and
MICHAEL A. BELL
Department of Ecology and Evolution, Stony Brook University, Stony Brook, NY 11794, USA
Received 7 December 2007; accepted for publication 25 January 2008
Ancestral properties can influence patterns of evolutionary diversification, but ancestors can rarely be observed
directly. We examined variation and sexual dimorphism of morphological traits in an anadromous threespine
stickleback population representing the ancestral form for resident postglacial stickleback populations in the area.
A combination of traditional and geometric morphometric methods were used to study variation over multiple years
in an anadromous population that breeds in Rabbit Slough, Cook Inlet, Alaska. Major armor anomalies were
extremely rare but their occurrence at measurable frequencies suggests that significant standing variation for
armor phenotypes exists in anadromous populations. Sexual dimorphism was a major source of variation, and most
traits differed significantly between sexes, particularly head length, length of the pelvic girdle, and body shape.
Consequently, some degree of sexual dimorphism appears to be the ancestral condition for many traits in derived
resident freshwater stickleback radiations. Morphological variation among years, especially in body shape, was
significant in both sexes, but the magnitude of annual variation was always less than variation due to other factors.
Phenotypic means were relatively stable over short time scales. Postglacial stickleback radiations are among the
most enlightening cases of adaptive radiation, and our detailed study of variation in an anadromous stickleback