While the source of expanding populations might be preadapted to cold winters, expanding spiders also have to cope with cooler and more humid summers in northern Europe. The environmental isolation of expanding populations could be explained by divergent selection between the newly colonized and native environments. The time lag from the initial establishment in the mid‐20th century to the current expansion might be explained by the necessary waiting time for adaptations to emerge (Clements and Ditommaso 2011). Such lag phases are commonly observed in biological invasions and range expansions (Sexton et al. 2009). An adaptation to novel climatic regimes has been shown to evolve quickly in spiders, even in the face of high gene flow (Tanaka 1996; Krehenwinkel and Tautz 2013). Apart from direct climate tolerance, a particular phenotype in northern European populations could be found in their reduced body size. Yellow sac spiders are an annual species. They grow from spring till summer and then reproduce. A reduced body size might thus be an adaptation to a shorter growing season in cooler climates and a necessity for earlier reproduction in more northern latitudes. Latitudinal size gradients have been described for many arthropod species and can evolve quickly (Huey et al. 2000; Krehenwinkel and Tautz 2013). However, more experimental data, for example, from reciprocal transplant experiments, common garden studies and whole genome sequencing, will be necessary to support the assumption of an adaptive divergence of the studied populations. Based on our data, we currently cannot exclude an involvement of phenotypic plasticity in the observed differences. Most phenotypes are known to be affected by environmental context, and plasticity plays an important role in species responses to contemporary climate change (Gienapp et al. 2008; Merilä and Hendry 2014).