COUNTERGRADIENT VARIATION IN GROWTH AMONG NEWLY-HATCHED FUNDULUS-HETEROCLITUS - GEOGRAPHIC DIFFERENCES REVEALED BY COMMON-ENVIRONMENT EXPERIMENTS

1. Experiments were designed to reveal geographic differentiation, and its evolutionary significance, in early life-history characters of Fu ndulus heteroclitus. The inherited capacity for growth was studied for five populations, from South Carolina to Maine, USA, at three tempera tures representative of field conditions. Two generations of newly hat ched fish were reared and tested. The second generation of fish was pr oduced from parents that were themselves reared in the laboratory, in order to control for maternally transmitted environmental effects. 2. Over the first three weeks post-hatch, individuals from northern popul ations grew more rapidly than those from southern populations at highe r temperatures (21 and 28 degrees C) whereas there were generally no d ifferences among populations at the lowest temperature (17 degrees C). 3. The results do not conform to thermal adaptation models, in which northern populations are expected to show higher growth rates than sou thern populations at cold temperatures, and vice versa at higher tempe ratures. Instead, the higher growth rate of northern fish is interpret ed as an adaptation to a short growing season. This represents an exam ple of countergradient variation, in that genetic influences favouring rapid growth at high latitudes oppose the environmental effect of a s horter growing season. The dine of growth rates is concordant with mor phological, behavioural and genetic contrasts between subspecies of Fu ndulus heteroclitus.