Abstract
Biological invasions may expose populations to strong selection for local adaptation along geographical gradients in climate. However, evolution during contemporary timescales can be constrained by low standing genetic variation and genetic correlations among life-history traits. We examined limits to local adaptation associated with northern migration of the invasive wetland plant purple loosestrife (Lythrum salicaria) using a selection model incorporating a trade-off between flowering time and size at reproduction, and common garden experiments of populations sampled along a latitudinal transect of approximately 1200 km in eastern North America. A strong trade-off between flowering time and size at reproduction caused early-flowering plants to be smaller with reduced seed production in northern populations. Northward spread was associated with a decline in genetic variance within populations and an increase in genetic skew for flowering time and size, with limited genetic variation for small, early-flowering genotypes. These patterns were predicted by our selection model of local adaptation to shorter growing seasons and were not consistent with expectations from non-adaptive processes. Reduced fecundity may limit population growth and rates of spread in northern populations. Identifying genetic constraints on key life-history traits can provide novel insights into invasion dynamics and the causes of range limits in introduced species.