[W]e have estimated that natural selection drives twice as much change in immune-related proteins as in proteins with no immune function. Interestingly, the rate of adaptation is also more variable among immunity genes than among other genes in the genome, with a small subset of immunity genes evolving under intense natural selection. We suggest that these genes may represent hotspots of host–parasite coevolution within the genome.

Obbard, D., Welch, J., Kim, K., & Jiggins, F. (2009). Quantifying Adaptive Evolution in the Drosophila Immune System PLoS Genetics, 5 (10) DOI: 10.1371/journal.pgen.1000698

(This is particularly interesting to me because I’m trying to look at co-evolution between pathogens and immunity myself.  I’ve been tentatively suggesting that adaptive immune components (co)-evolve faster than innate immune components; of course, Drosophila only have innate immunity, so this paper suggests that the innate immune system also evolves rapidly.  That’s not unexpected, and doesn’t disprove my hypothesis, but it’s interesting anyway.  Also, there are some techniques in here I might be able to make use of.)