The evolution of ecological specialization generates biological diversity and may lead to speciation. Genetic architecture can either speed or retard this process. If resource use and mate choice have a common genetic basis through pleiotropy or close linkage, the resulting genetic correlations can promote the joint evolution of specialization and reproductive isolation, facilitating speciation. Here we present a model of the role of genetic correlations in specialization and speciation, and test it by analysing the genetic architecture of key traits in two highly specialized host races of the pea aphid (Acyrthosiphon pisum pisum; Hemiptera : Aphididae). We found several complexes of pleiotropic or closely linked quantitative trait loci (QTL) that affect key traits in ways that would promote speciation: QTL with antagonistic effects on performance on the two hosts are linked to QTL that produce asortative mating (through habitat choice). This type of genetic architecture may be common in taxa that have speciated under divergent natural selection.