Studies of ecology and evolution have become largely integrated, and increasing attention is paid to the role of ecology for speciation and post speciation divergence. In this thesis I have applied an in-depth approach studying the ecology of a butterfly species pair; the morphologically virtually identical sister-species, the Wood white (Leptidea sinapis) and Reál’s wood white (Leptidea reali). PAPER I showed a quite deep between-species division in sequence data from mitochondrial DNA. The reuniting in secondary contact zones might in contrast be quite recent, as males of L. sinapis and L. reali cannot distinguish between con- and heterospecific females (PAPER II) and since the between-species niche separation is incomplete (PAPER III, IV, V). Furthermore, the two species have partitioned their niches in different directions in different European regions as the two species shift habitat generalist and specialist roles throughout their joint distribution (PAPER III). However, the local niche partitioning has resulted in species-specific adaptations in terms of propensity to enter diapause (PAPER III, V, VI), host plant acceptance (PAPER V), and in ability to use host plant as cue for the decision to enter diapause or direct development (PAPER VI). The habitat separation is decoupled from host plant preference, at least in south central Sweden (PAPER IV), which implies that selection for niche partitioning has acted on habitat preferences directly and not via divergent selection on host plant preference. Finally, there is a high cost of appearing at a site where the other species is in the majority as much time (PAPER VII) and energy (PAPER II) are devoted to court heterospecific females or being courted by heterospecific males (PAPER VII). Hence, selection likely favours habitat specialisation in the rarest species in each region, and the direction of niche separation might simply be decided by which species that reached an area first. The species that first colonises an area would then most likely become a generalist filling up all suitable habitats, whereas the second invader might be forced to specialise, as the cost of being rare is too large everywhere but in the core population. This thesis highlights the role of ecology, and especially of local processes, for post-speciation selection and character displacement.