Aging and evolution

If you don't agree with evolutionary theory- don't bother with this node.

What does evolution theory have to say about ageing? How can it ever be evolutionarily desirable for an organism to age? Why are tortoises so long lived? Why do cats live longer than dogs?

Ok. The normally accepted theory is as follows:

First, consider that organisms will tend to have an average lifespan due to accident, disease, famine etc. in their normal environment; in the absence of any ageing effect. People simply do get run over, they die of some contagious disease, they shoot each other; whatever it is this imposes a natural, statistical lifespan that very few individual can exceed; currently for humans it's several hundred years.

Now consider ageing, consider that most diseases and conditions of old age, such as cancer are caused by cumulative damage, genetic damage in the case of cancer; heart disease is due to blood vessel damage in many cases; strokes are similarly due to blood vessel issues. So most of the leading causes of death are directly caused by insufficient repair/maintenance over time; and all visible signs of ageing are too. This is what we believe ageing to be; lack of perfect repair.

This repair is believed to require extra energy (food) to repair and more complex genetic programming.

The theory goes that unless the necessary changes to the genes to do the repair are required by the environment, i.e. it become possible for an organism to live longer, any genes for this will not be selected for, and it will not happen.

What's the point in genes for living ten years if you usually get run over in a fortnight? These genes do you no good, and will not aid your survival; and will be lost without significant penalty due to trivial mutations. They may even do harm- doing too much repair may require more resources.

However, let us suppose that a species or part of a species suddenly becomes almost invulnerable except for ageing. What will happen? Well in a lot of cases the genetically lucky, longer lived members will end up having more children, because they have more time to do so; and their better repair processes will mean they look better for longer, and hence are more easily able to attract a mate.

Their descendents will have more of the genes that are conducive to a longer life; and hence these longer lived genes tend to spread though the population. This will keep increasing the lifespan until the species starts to die of accidents more than old age, when these genetic changes will slow and stop and a new equilibrium is reached.

Ok, is there any actual evidence for this theory?

Yes. Plenty! The best defended creatures live considerably longer than you would normally expect them to. Tortoises have excellent protection from their shell- they can live for centuries. Birds are usually able to fly away from danger- they live for decades, whereas mice which are fairly similar sizes and can't fly away from cats and predators only last for sixth months or a year. Cats are very well armed, dogs less so. Cats can live for 20 or more years, dogs usually die far younger. Humans are quite good at protecting themselves; and so we live a very long time by most animals standards.

There's even experimental support. Some worms have been bred for doubled lifespan. This was done by breeding only from the older members.

For humans, this means that, as our societies (now) contain individuals that essentially only die of old age; it is to be expected that evolution will slow down ageing over the coming few thousand generations; if nothing else were to happen.

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