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.
http://www.senescence.info/
http://spauldingrehab.mgh.harvard.edu/mcmenemy/facinghd.html