Evolution of Senescence

antibiotics and other interventions that put added pressure on the organisms. Bacteria also compete with one another, which affects the pathogen genetics.

As mentioned, bacteria must first overcome the innate immune system. This largely means overcoming barrier systems but it also must overcome the inflammatory responses in humans as well as the cells that indiscriminately kill off pathogens without the need for antibody responses. Humans use competition between healthy bacteria on the skin and in the GI tract that prevent pathogens from taking hold. Mucus and the blood-brain barrier keep pathogens out of delicate areas of the body.

Some bacteria make enzymes that break down mucus to ensure a greater chance of infection. Other pathogens target areas of the GI tract that have less mucus. There are bacteria that break the cell to cell junctions and bacteria that break up the protective skin barriers.

Humans have a way to sequester iron needed by microorganisms. This has led to the evolution of organisms that can scavenge for iron as part of their pathogenicity. There are also bacteria that steal iron from human proteins.

The greatest selective pressure that pathogens have placed on humans in gaining the advantage in diseases is the development of antibiotic resistance. It can happen very quickly and can cross to different species of bacteria. Bacteria have evolved to expel antibiotics, avoid attaching to antibiotics, and inactivate antibiotics. A big problem now is the development of multi-drug resistance. Less effective but still a problem is the elimination of rival bacteria by secreting toxic proteins that kill beneficial bacteria.

EVOLUTION OF SENESCENCE

As an organism ages, their ability to function, fight off disease, and succeed reproductively goes down. All organisms die at some point and there are several hypotheses as to how this occurs. It was originally believed that aging was a mechanism of making way for the next generation as a necessary part of evolution. This theory has largely been discarded since the beginning of the 20th century.

Another theory is that aging is a matter of neglect because of the highly competitive aspect of life on earth. Animals in the wild die from predation, accidents or disease; this lowers the average age at the time of death. It means that there is no evolutionary advantage to being fit as an older organism.

The theory depends on the idea that there are random mutations that accumulate over time, leading to the damage seen in older individuals. While this may be true, there are scientific reasons why this might not completely explain senescence and death.

Since then, there has been the theory of antagonistic pleiotropy, which means that one single mutation can have both beneficial and deleterious effects on the host. The idea is that having certain genes is beneficial early in life but becomes damaging to the host later in life. This is the theory that exists today but it is mostly the prevailing theory because a better theory has not been discovered. If it was entirely true, breeding animals for long life would mean they would have reduced reproduction but actually the opposite has been found to be true.

Another theory is called the disposable soma theory, which means that excess energy is spent in old age for repair of the body, leading to a lack of energy available for maintenance so the body deteriorates. This theory depends on limited resources available to the organism. The problem with this is that organisms faced with calorie restriction actually live longer than those with more resources.

Still another theory is that DNA damage is the main cause of aging. The ability to repair DNA decreases when cells are not dividing as much, which would lead to more DNA damage. This further leads to a loss of tissue and cell function over time. Free radicals are believed to be behind the destruction of the DNA and the cell. Organisms that use oxygen to the greatest degree as part of metabolism will die faster.

A final theory is that there is programmed cell death for all cells. Species that live longer can offset the damage caused by free radicals, oxidation, and the shortening of end segments called telomeres on DNA fragments. Organisms that reach sexual maturity earlier are less likely to be able to repair damaged DNA so they have a shorter lifespan.

There are theories about natural selection that may help to explain aging. Remember that evolution acts on populations and not on individuals so that populations that survive better as a whole will live longer if it is beneficial. In species where older post-reproductive members of the population help to raise the young, these species have a longer lifespan. This is true for dolphins and pilot whales.

Evolution necessitates the death of aged species so that they die off rather than dominate the gene pool. If too many older organisms contribute to the gene pool, the population will not evolve to its greatest degree. In this case, evolvability itself contributes to senescence.

There are two kinds of mortality. Intrinsic mortality is aging due to internal factors, while extrinsic mortality is aging due to environmental factors. Organisms with less predators have a greater degree of intrinsic mortality, while organisms with a lot of predators have a greater degree of extrinsic mortality. Predatory animals live longer than prey animals. Humans have a lower intrinsic mortality because their intelligence has helped to overcome it.

There are a couple of diseases that are related to accelerated senescence and death. Progeria is a single-gene disease that accelerates aging in childhood so that they die early in life. Werner syndrome is another single-gene disease that prevents growth at puberty and an early onset of death in the person s twenties or thirties.