There is a book out this year that seems related to this discussion, called Survival of the Sickest by Sharon Moalem, a medical student with PhD.'s in neurogenetics and evolutionary biology. He writes this book in a conversational style fairly understandable for general audiences. I recall his describing endogenous retroviruses in the human genome and reverse transcriptase as a mechanism. His main argument seems to be that a number of hereditary diseases like sickle cell anemia, diabetes, and hemachromatosis (a problem in regulating iron absorption) are hereditary because they conveyed some advantage to survival in the past, such as resistance to bubonic plague or malaria. He also touches on research in non-coding DNA and transposons ('jumping genes') and epigenetics (on variables affecting whether genes show up actively in the phenotype or remain dormant in the genotype.) I found the book in Orange County Public Library. A website: http://www.survivalofthesickestthebook.com/ has excerpts,reviews, and the author's blog. Johnathan Prince, a professional writer, is listed as co-author, presumably helping to make the book understandable to general readers. That includes me, and I found it fascinating and educational.