One of the less realized implications of natural selection has to do with the traits that are expressed at a certain age. It is possible for nature to select for a healthy baby, a healthy child, a healthy adult. However, after reproductive maturity there is no way for nature to select for a healthy senior citizen. Natural selection can only select for the fitness of an organism and its ability to survive to reproductive maturity - but not afterwards.

Aging itself is seen as evolutionary pressure for beneficial genes to express themselves in earlier stages of life. One example of this is earlier puberty (the age of puberty is slowly growing older in areas with longer life expectancies). The pressure is for the organism to to reach puberty and become independent. However, there is no pressure for an organism to live longer. The traits that escape this early expression may indeed be harmful late-onset traits.

There are a number of diseases that strike after reproductive maturity such as Huntington's disease. Huntington's disease is a genetic mutation near the end of chromosome 4. This disease typically strikes after the age of 50, well after most people have had children if they are going to do so. At this point, they have passed down the genes for the disease to the next generation when they were reproductively fit.

By far, the most well known disease that typically affects people after reproductive maturity is that of cancer. While genetics is not the only role in causing cancer, it does play a part. Returning back to the aging issue, women who reach puberty earlier (historically a good thing for survival of the species) are more susceptible to cancer. Likewise, male hormones cause baldness and prostrate cancer later in life (it is believed, though unconfirmed, that eunuchs have a longer and healthier life).

There have been several experiments to test this including the demonstration that selection for longevity in fruit flies is closely linked to sexual maturity - the longer the life span, the later the sexual maturity. Likewise, a population of opossum that lived on an island with no predators had a longer life expectancy (comparing the elasticity of collagen) and later sexual maturity because they were not forced to reproduce early or not at all by the predators.

Extreme cases of aging can be seen in species that only reproduce once such as the salmon that die after spawning and squid that likewise die after spawning. Both of these species (and others like them) show an extreme case of 'aging' - after reproducing there is simply no reason for them to live.

Another factor in aging related diseases is the accumulation of stress upon the organisim. This ranges from exposure to radiation (read radiation exposure to get a fairly complete list of various sources and realize that many of these (such as elevation, home construction, food, and such)) are not new to the 20th century. These stresses slowly build up errors in the DNA over decades. Diseases such as Alzheimer's are not things that happen suddenly but rather build up over many years with the slow accumulation of malformed proteins on neurons.

The point that I hope to get across with this is that it is alarmist to claim that microwave ovens, power lines or any other amenities of modern day life are to universally blame. In many instances, the traits just haven't had the opportunity to be expressed before modern medicine.