Divergence in Thermal Reaction Norms of Life-History Traits Among Populations of Daphnia magna
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- Institutt for biologi 
Patterns of phenotypic and genetic variance in thermal reaction norms allow predicting evolutionary response for a population. The ability of phenotypic traits to respond to selection depends on their genetic variance, and we expect a positive correlation between population divergence and the amount of genetic variance. In the presence of genetic by environment interaction, the genetic variance of a single trait may change with the environment, showing zones of canalization with low genetic variance or inversely zones with high genetic variance. Whether and how this variation in genetic variance translate into variation in population divergence remains unknown. To have a better understanding of the interplay between within-population genetic variation and phenotypic divergence among-populations in the presence of genetic by environment interaction, I studied thermal reaction norms of three life- history traits in three populations of Daphnia magna over a broad latitudinal range in Europe. Exposing the populations to a common garden environment allowed quantifying evolved (genetic) differences. The results demonstrate high divergence in population-specific thermal reaction norms, particularly between the northernmost population and the two more southern populations. These patterns are consistent with adaptation to low temperatures in the northernmost population. For two of the three life-history traits there was a negative correlation between within- and among-population genetic variance. This suggests that low within-population variation at specific temperature does not constrain the among-population divergence at these temperatures, but rather that genetic variance may be lost as a result of selection.