The evidence supporting the idea that variability in immune response driven by parasites can be a factor favoring speciation of host lineages is mostly correlational, but is supported by a growing body of literature. One general factor favoring this is the development of strong local immunological accommodations of particular host populations to the distinctive parasites they encounter, such that across a broader host range different host populations differ significantly with respect to the nature of their responses (Wheatley, 1980; Blais et al., 2007; Scharsack et al., 2007; Matthews et al., 2010) . For example, malaria might be encountered in some but not all parts of the host range where appropriate mosquitoes were present, or different suites of parasites might be encountered in different foraging habitats such as rivers or lakes (Eizaguirre et al., 2011) . Differences among populations of the same host species with respect to their immune defenses have been noted in Drosophila against parasitoids (Kraaijeveld and Godfray, 1999) , Darwin's finches (Lindstrom et al., 2004) , sticklebacks coping with eye flukes (Kalbe and Kurtz, 2006) , and marine amphipods infected with trematodes (Thomas et al., 2000; Bryan-Walker et al., 2007) . Differentiation resulting from spatially variable antagonistic interactions with parasites would in this case provide the substrate for further diversification of host lineages (Thompson, 2005 (Thompson, , 2009 .