Summary and conclusions

One goal of this chapter, shared with all the other parts of this book, was to emphasize the ubiquity of reticulate evolution as an agent of genetic and evolutionary change. Another goal, unique to the topic in this chapter, was to address the hypothesis that genetic exchange events have impacted greatly the evolutionary trajectory of organisms that parasitize and kill humans. The first of these goals, I hope, is accomplished as this chapter is considered in the context of the previous seven. The second objective, I believe, is obvious from the examples. Thus, the organisms that make up the lion-share of the pathogens that maim and kill humans are represented in the examples discussed in the various instances. Furthermore, the degree to which many of these organisms have impacted H. sapiens has been inferred to be due to their hybrid constitution. In some cases—for example, the bacterial pathogens—the mechanism of genetic exchange has been the uptake of DNA from other, very divergent, lineages. In other organisms such as the insect vectors of many of our diseases, the genetic transfer has been through sexual reproduction and introgression. Yet, regardless of the mechanism and even the ultimate effect of the genetic transfers on the recipient's fitness, it is clear that the organisms that cause so much pain and suffering for humans are mosaics of divergent genomes.

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