Molecular and morphological divergence in a pair of bird species and their ectoparasites.

In an evolutionary context, parasites tend to be morphologically conservative relative to their hosts. However, the rate of neutral molecular evolution across many parasite lineages is faster than in their hosts. Although this relationship is apparent at the macroevolutionary scale, insight into the processes underpinning it may be gained through investigations at the microevolutionary scale. Birds and their ectoparasitic lice have served as important natural experiments in co-evolution. Here, we compared mitochondrial and morphological divergence in 2 recently diverged avian host lineages and their parasites. Galapagos hawks (Buteo galapagoensis) are phenotypically divergent from their closest mainland relatives, the Swainson's hawk (Buteo swainsoni). Both species are host to a feather louse species of Craspedorrhynchus (Insecta: Phthiraptera: Ischnocera, Philopteridae). We sequenced the 59 end of the mitochondrial gene cytochrome oxidase c subunit I (COI) from a set of hawks and lice. Although this fragment allowed unambiguous identification of host and parasite lineages on the islands and the mainland, only a single variable site was present in the 2 hosts, but 2 major Craspedorrhynchus clades divergent by ,10% were recovered that sorted perfectly with host species. We found significant population genetic structure within the Galapagos Craspedorrhynchus lineage. While the host species are highly differentiated phenotypically, the 2 Craspedorrhynchus louse lineages are phenotypically overlapping, although subtle but significant morphological differences exist. Across species, morphological evolution tends to be more conservative in parasites than their hosts (Klassen, 1992). This tendency, coupled with the fact that some parasites are vertically transmitted across ecological and evolutionary time (Page, 2003; Nieberding and Olivieri, 2007), led parasitologists to use parasite distributions to infer host genealogical relationships (Klassen, 1992). In contrast to phenotypic change, the rate of neutral molecular evolution among lice tends to be faster than in their hosts (Hafner et al., 1994; Page et al., 1998). Parasites and their hosts could be compelling systems in which to test the prediction that rates of morphological evolution in co-evolved hosts and parasites are independent of genetic changes in those same lineages. Given that the rate of neutral evolution in the parasites is expected to exceed that in the host, and that parasites tend to be morphologically conservative with respect to their hosts, we expect less genetic divergence at homologous loci in 2 recently derived hosts than in their parasites, and more phenotypic divergence in the hosts than in the parasites. The feather louse (Insecta: Phthiraptera) fauna of birds inhabiting the Galapagos Islands has a special and controversial role in the history of co-evolutionary biology (Kellogg and Kuwana, 1902; Klassen, 1992; Choudhury et al., 2002). Claims of rampant horizontal transfer of lice among Galapagos birds are noteworthy (reviewed in Palma, 1994). The endemic birds of Galapagos were central to the development of Darwin's theory of