Genes
Some of our genes are perpetuated despite the fact that they cause disease. Some of their effects are “quirks” that were harmless
when we lived in a more natural environment. For instance, most of the genes that predispose to heart disease were harmless until we began overindulging on fatty diets. The genes that cause nearsighted- ness cause problems only in cultures where children do close work
early in life. Some of the genes that cause aging were subject to little selection when average life spans were shorter.
Many other genes that cause disease have actually been selected for because they provide benefits, either to the bearer or to other individuals with the gene in other combinations. For instance, the gene that causes sickle-cell disease also prevents malaria. In addition to this well-known example, many others are discussed in later chap- ters, including sexually antagonistic genes that benefit fathers at the expense of mothers or vice versa.
Our genetic code is constantly being disrupted by mutations. On very rare occasions these changes in DNA are beneficial, but much more commonly they create disease. Such damaged genes are con- stantly being eliminated or kept to a minimum by natural selection. For this reason defective genes with no compensating benefit are not a common cause of disease.
