Genes involved in the immune response show signals of rapid evolution, with the precise evolutionary mode varying among components of the immune system. Extant populations harbour tremendous genetic and phenotypic variation in resistance, providing the substrate upon which selection acts. Examination of both evolutionar-ily ancient and current patterns has only rarely been performed. The most complete example is from the ref(2)P locus in D. melanogaster, which is polymorphic for the ability to permit or restrict Sigma virus infection. In natural populations, this locus shows evidence for elevated polymorphism, partial selective sweeps, and spatial heterogeneity in allele frequencies, all of which reflect an on-going battle between host and pathogen. These polymorphisms also often become fixed, driving long-term adaptive amino acid evolution. Other parts of the immune system could be equivalently studied, such as a polymorphic locus in the mosquito A. gambiae that confers resistance to malaria. In general, characterization of forces that facilitate or inhibit the spread of host resistance through populations, combined with genome-scale comparisons between species, will allow the linkage of short-term and long-term patterns to fully define the lability and constraint on adaptive evolution across the immune system.

Understanding the factors that influence the evolution of the immune response has important ramifications for diverse fields of study. Evaluation of the feasibility of applications such as the proposed engineering of transgenic disease-vector insects to control transmission and the use of pathogens to implement biological control of pest populations benefits from the most complete understanding possible of how resistance arises and propagates through natural populations. These are inherently evolutionary biological questions. The evolutionary dynamics of insect-pathogen interactions also has clinical importance in so far as insects can serve as model hosts for humans. Evolutionary inferences about how pathogens interact and interfere with different components of the immune system inform studies in molecular immunology. Advances in immunology, in turn, will test these predictions and identify new sets of genes and pathways in a wider range of organisms, further broadening the field of evolutionary genetics.

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