The number of host species a parasite infects is known as “host specificity”. This fundamental parasite phenotype can vary dramatically among parasite species and is critical for understanding disease epidemics in humans and wildlife. Despite substantial research into the determinants of host specificity, little is known about its evolution, especially in vector-transmitted parasites like those that cause malaria. This project is designed to reveal the evolution and genetic basis of host specificity in haemosporidian parasites of birds, commonly known as avian malaria parasites. We will generate genomic sequences from a clade of avian malaria parasites using a targeted sequence capture approach and paired-end Illumina sequencing and use those sequences to construct a phylogeny. We will then test for phylogenetic signal in host specificity, model transitions from generalists to specialist parasites and vice-versa, and test for genes and parasite lineages under selection and determine whether specialists and generalists experience similar or different selective pressures. We will also generate genomic sequence data from generalist parasites across their hosts from a local site in southern Sweden to determine if generalists can become genetically differentiated on their hosts (either through drift or host-specific selection pressures), a possible path to parasite speciation and specialization. These two objectives will provide an unprecedented look into the evolution and genetic basis of host specificity and will be useful to basic research in disease biology and applied work in conservation and disease mitigation. The project will also result in genomic resources that will facilitate future comparative work into the evolution of other haemosporidians including those that cause malaria in humans.